What is the oval shaped bone in the heads of some fishes?

What is the oval shaped bone in the heads of some fishes?

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I had a dish of fish and found 2 oval shaped nearly 1cm long bones in its head.What help does it do to the fish? what is its name? I have clicked the picture myself .

I am a professional fish biologist and those are otoliths; I have aged thousands of them. They are primarily used in maintaining equilibrium, hearing, and balance. The details of the shape of the otolith can vary between species but generally look very similar. They are located in the skull of the fish, usually at the back.

Here is an image of other otoliths: Photo source: Examples of common fish otoliths found at archaeological sites in Florida. Images from the Florida Fish and Wildlife Conservation Commission

Otoliths are calcium based structures located within otolithic organs (the sacule, lagena, and utricle) and make up the inner ear of boney (teleost) fishes. They come in pairs and there are three of them: Sagittal (within the saccule), lapillali otoliths (within the utricle) and asterisci (within the lagena). They are made of calcium carbonate and are generally composed in two different crystalline formations, aragonite and vaterite; crystalline form and amount depend on the otolith and stress induce metabolic processes. Otoliths are formed as the fish grows. In periods of fast growth (warmer months) the otolith forms faster and in periods of slow growth (colder months) the otolith forms more slowly. Given the cyclical pattern of warm and cold months in the year, the otolith form annuli (growth rings); annuli can be used to age fish just like rings on a tree. Additionally, they are metabolically inert and cannot be reabsorbed as calcium reserves like other boney structures in the fish; thus, they are a permanent record of the fish's age. This permanent record can also be used to evaluate the life history (i.e., movement patterns) using trace element analysis.

Specifically, how do they work/help the fish? Within the sacule, lagena, and utricle the otoliths are suspended in endolymphatic fluid within their respective organs. The organs have hair-like sensors in them that detect vibration or movement of the otoliths within, triggering an associated nervous stimuli. The nervous stimuli are then interpreted as movement and orientation.

Here are some sources:

This book is also an excellent resource for a much more in-depth look:


The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female’s vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but are shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species are born sexually mature, although they are not fully grown.

Some fishes are hermaphroditic—an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

Successful reproduction and, in many cases, defense of the eggs and the young are assured by rather stereotypical but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in the gravel beds where they themselves hatched (anadromous fishes). Some, such as the freshwater eels (family Anguillidae), live and grow to maturity in fresh water and migrate to the sea to spawn (catadromous fishes). Other fishes undertake shorter migrations from lakes into streams, within the ocean, or enter spawning habitats that they do not ordinarily occupy in other ways.

Bony fish

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Bony fish, (superclass Osteichthyes), any member of the superclass Osteichthyes, a group made up of the classes Sarcopterygii (lobe-finned fishes) and Actinopterygii (ray-finned fishes) in the subphylum Vertebrata, including the great majority of living fishes and virtually all the world’s sport and commercial fishes. The scientific term Pisces has also been used to identify this group of fishes. Osteichthyes excludes the jawless fishes of the class Agnatha (hagfishes and lampreys) and the cartilaginous fishes constituting the class Chondrichthyes (sharks, skates, and rays) but includes the 29,000 species and more than 400 families of modern bony fishes (infraclass Teleostei) of the world, as well as a few primitive forms. The primary characteristic of bony fishes is a skeleton at least partly composed of true bone (as opposed to cartilage). Other features include, in most forms, the presence of a swim bladder (an air-filled sac to give buoyancy), gill covers over the gill chamber, bony platelike scales, a skull with sutures, and external fertilization of eggs.

Bony fishes occur in all freshwater and ocean environments, including caves, deep-sea habitats, and thermal springs and vents. The variety of shapes and behavioral habits is remarkable. Their body sizes range from tiny species such as the pygmy goby (Pandaka pygmaea 12 mm [0.5 inch]) to the enormous marlins and swordfishes (family Istiophoridae) with lengths up to 4.5 metres (15 feet) and the ocean sunfish ( Mola mola), which may weigh over 900 kg (1 ton).

What Causes Bone Lucencies?

According to the University of Washington School of Medicine, bone lucencies can be caused by a variety of factors, such as cysts, cancer, benign tumors or infection. Healio goes on to point out that fractures also cause bone lucencies.

Dr. Joseph Accurso on HealthTap describes lucencies as areas on an X-ray where the bone is less dense, which can be a sign of bone lesions. Many other factors can cause bone lucencies. The University of Washington School of Medicine explains that one of these major factors is tumors, whether benign or malignant. Some of the most common bone tumors are caused by osteoblastoma, myeloma, chondroblastoma, hemangloma and enchondroma.

When bone lucencies are caused by fractures, Healio makes the correlation between these bone breaks and a Vitamin D deficiency.

Wikipedia says that osteoblastoma, one of the causes of bone lucency, occurs when abnormal bone-like tissue grows around the bones, creating tumors. Although these tumors are considered benign, they can interfere with the body's movement depending on their placement, such as if they are located on the spinal cord or nerve roots. This cancer is heralded by bone pain, swelling and tenderness. It is a very rare disease and even more rarely fatal. Doctors mostly treat it with surgery.

Mycteroperca microlepis

Gag Grouper. Photo courtesy NOAA

This oblong fish has small eyes over a pointed snout, and a continuous dorsal fin to a square caudal fin. Their colors vary by age and gender, but they range from gray to light brown with darker blotches or squiggles, giving it a faint marble pattern, or darker to a camouflage pattern. Like many groupers, they start out as females, and then change into males at a certain age or size after a few spawning seasons. They prefer rocky or grassy bottoms of coastal waters in the Western Atlantic, where they can hunt crustaceans and smaller fish, and grow to well over 4 feet long.

Order: Perciformes Family: Serranidae Genus: Mycteroperca Species: microlepis

Common Names

English language common names include gag grouper, charcoal belly, gag, gag-velvet rockfish and velvet rockfish. Other common names are abadejo (Spanish), aguají (Spanish), badèche baillou (French), badejo (Portuguese), badejo brando (Portuguese), badejo-bicudo (Portuguese), badejo-branco (Portuguese), badejo-da-areia (Portuguese), badejo-saltão (Portuguese), badejo-sapateiro (Portuguese), cuna (Spanish), Cuna aguají (Spanish), fløjlsbars (Danish), garoupa (Portuguese), and serigado-badejo (Portuguese).

Importance to Humans

Gag grouper caught by a recreational fisherman (Gulf coast of Florida). Photo © Sean Morey

The gag grouper provides important recreational and commercial fisheries. It is caught with hook and line and the flesh is marketed fresh.

Danger to Humans

There have been reported cases of ciguatera from human consumption of gag grouper. Ciguatera poisoning is caused by dinoflagellates (microalgae) found on dead corals or macroalgae. By feeding on these corals and macroalgae, herbivorous fishes accumulate a toxin generated by these dinoflagellates. Largely a phenomenon of tropical marine environments, ciguatoxin accumulates still further in snappers and other large predatory reef species that feed on these herbivorous fishes. If accumulated levels of the toxin are great enough they can cause poisoning in humans whom consume the flesh of these fishes. Poisoned people report having gastrointestinal problems for up to several days, and a general weakness in their arms and legs. It is very rare to be afflicted with ciguatera poisoning.


Gag grouper in deep water. Photo courtesy NOAA

Ranked among the most valuable fisheries in the southeastern US, the gag grouper is sought after both recreationally and commercially. Due to fishing pressure, this species is vulnerable to overfishing. It is especially vulnerable when fish aggregate for spawning. The population of mature males has declined dramatically due to the targeting of larger fish, leading to concerns of insufficient males for reproductive efforts. Young gag grouper are often taken as bycatch in the shrimp fishery over seagrass beds.

This species was last assessed in 1996. A taxon is considered vulnerable by IUCN when it is not critically endangered or endangered but is facing a high risk of extinction in the wild in the medium-term future. In the latest report of stock assessment, the gag grouper was not considered overfished (NMFS 2004), however it was indicated that a reduction in mortality was necessary because this species is considered to be experiencing overfishing. The IUCN is a global union of states, governmental agencies, and non-governmental organizations in a partnership that assesses the conservation status of species.

Geographical Distribution

World distribution map for the gag grouper

The gag grouper is found in the western Atlantic Ocean from North Carolina (US) south to the Yucatan Peninsula (Mexico). It is scarce in waters surrounding Bermuda. Juveniles have been recorded as far north as Massachusetts. There have also been records of gag grouper occurring off the coasts of Bermuda, Cuba, and eastern Brazil.


Residing in brackish to marine waters, the gag grouper is found offshore on rocky bottom as well as inshore on rocky or grassy bottoms to depths of 500 feet (152 m). It is common on rocky ledges along the eastern Gulf of Mexico.

Adult gag grouper school in groups of 5-50 individuals or may be found solitary. Recordings have been made of adult gag grouper producing thumping sounds through the swim bladder by vibrations resulting from the contraction of associated musculature. These sounds are produced during times when the fish is under duress.


Gag grouper. Illustration courtesy FAO

Distinctive Features
The gag grouper is typical among the groupers with an oblong-shaped elongate body. The head is long while the mouth is large with a protruding lower jaw. The bases of the dorsal and anal fins are covered with scales and thick skin. The caudal fin is large and has a slightly concave margin. This grouper is often confused with the black grouper (M. bonaci), however it is has some distinguishing characteristics. These include the shape of the caudal fin – the gag grouper has a slightly concave margin along the posterior edge of the caudal fin while the black grouper has square-shaped caudal fin. The preopercle (a boomerang-shaped bone whose edges form the posterior and lower margins of the cheek region the most anterior of the bones comprising the gill cover) is angular and slightly notched with a distinct lobe. This characteristic also helps to distinguish the gag grouper from black grouper which has a gently rounded preopercle.

Juvenile gag grouper. Photo © George Burgess

Body color of the gag grouper is dependent upon the sex and age of the fish. Juveniles and mature females are pale to brown-gray with dark blotches and worm-shaped markings resulting in a marbled appearance. The caudal, anal, and pelvic fins have dark black-blue outer margins. Inactive individuals sometimes display a camouflaged pattern with dark brown “saddles” separated by white bars just below the dorsal fin. Large mature males are pale to medium gray in color with barely visible reticulations below the dorsal fin. The ventral surface is darker gray to black in color. The soft dorsal fin, caudal fin, pectoral, and pelvic fins are also dark gray to black while the margins of the anal and caudal fins are white. Individuals may exhibit a darker phase in which the posterior of the body, penduncle, soft dorsal fin, and anal fins are black in color.

Gag grouper are often confused with the black grouper (M. bonaci) above. Photo © George Ryschkewitsch

The gag grouper is often confused with the black grouper, however it may be distinguished based on the color of the fin margins. The caudal fin of the gag grouper has white margins on the anal and caudal fins while the black does not.

There are two well-developed canine teeth present anteriorly in each jaw. These are quite effective for holding prey items.

Size, Age, and Growth
Gag grouper reach a maximum total length of 4.75 feet (1.45 m) and a maximum weight of 80.5 lbs (36.5 kg). Males reach maturity at approximately 8 years of age and a correlating total length of 39 inches (98 cm) while females mature at 5-6 years of age and 26-30 inches (67-75 cm) total length. This species is believed to have a lifespan of 16 years.

Gag grouper over a rocky reef bottom. Photo courtesy U.S. Geological Survey

Food Habits
Adult gag grouper primarily feed on fishes, crabs, shrimps, and cephalopods while juveniles measuring less than 8 inches (20 cm) in length feed on crustaceans residing in shallow grass beds.

Similar to other serranids, gag grouper are protogynous hermaphrodites. They begin life as female, however after a few years of spawning as a female, some gag groupers change sex, becoming functional males. This transition generally occurs at 10-11 years of age corresponding to lengths of 37-39 inches (95-100 cm).

This juvenile gag grouper may eventually grow to a total length of over 4 feet. Photo © George Burgess

Juvenile gag grouper may fall prey to cannibalism as well as to large fishes. Sharks and other large fishes are known predators of adult gag grouper.

Nematodes, including Porrocaecum sp. and Procamallanus pereirai, are known parasites of gag grouper. Another reported parasite of this grouper is the isopod, Exocorallana tricornis.


This grouper was first described as Trisotropis microlepis by Goode and Bean in 1879. However, this name was later changed by taxonomists to the currently valid Mycteroperca microlepis (Goode and Bean, 1879). The genus name, Mycteroperca, is derived from the Greek, “mykter, -eros” meaning nose and “perke” meaning perch. The species name, microlepis, is derived from the Greek “micro” small and “lepis” meaning scale, in reference to the small scales covering the body of this fish.


In many respects, fish anatomy is different from mammalian anatomy. However, it still shares the same basic body plan from which all vertebrates have evolved: a notochord, rudimentary vertebrae, and a well-defined head and tail. [5] [6]

Fish have a variety of different body plans. At the broadest level, their body is divided into head, trunk, and tail, although the divisions are not always externally visible. The body is often fusiform, a streamlined body plan often found in fast-moving fish. They may also be filiform (eel-shaped) or vermiform (worm-shaped). Fish are often either compressed (laterally thin) or depressed (dorso-ventrally flat).

There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body. The skeleton of the fish is made of either cartilage (cartilaginous fishes) or bone (bony fishes). The fins are made up of bony fin rays and, except for the caudal fin, have no direct connection with the spine. They are supported only by the muscles. The ribs attach to the spine.

Bones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue. Bones come in a variety of shapes and have a complex internal and external structure. They are lightweight, yet strong and hard, in addition to fulfilling their many other biological functions.

Fish are vertebrates. All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal (vertebral column or notochord), [7] with a hollow tube of nervous tissue (the spinal cord) above it and the gastrointestinal tract below. In all vertebrates, the mouth is found at, or right below, the anterior end of the animal, while the anus opens to the exterior before the end of the body. The remaining part of the body beyond the anus forms a tail with vertebrae and the spinal cord, but no gut. [8]

The defining characteristic of a vertebrate is the vertebral column, in which the notochord (a stiff rod of uniform composition) found in all chordates has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs, derived embryonically and evolutionarily from the notochord). However, a few fish have secondarily [ clarification needed ] lost this anatomy, retaining the notochord into adulthood, such as the sturgeon. [9]

The vertebral column consists of a centrum (the central body or spine of the vertebra), vertebral arches which protrude from the top and bottom of the centrum, and various processes which project from the centrum or arches. An arch extending from the top of the centrum is called a neural arch, while the haemal arch or chevron is found underneath the centrum in the caudal vertebrae of fish. The centrum of a fish is usually concave at each end (amphicoelous), which limits the motion of the fish. In contrast, the centrum of a mammal is flat at each end (acoelous), a shape that can support and distribute compressive forces.

The vertebrae of lobe-finned fishes consist of three discrete bony elements. The vertebral arch surrounds the spinal cord, and is broadly similar in form to that found in most other vertebrates. Just beneath the arch lies the small plate-like pleurocentrum, which protects the upper surface of the notochord. Below that, a larger arch-shaped intercentrum protects the lower border. Both of these structures are embedded within a single cylindrical mass of cartilage. A similar arrangement was found in primitive tetrapods, but in the evolutionary line that led to reptiles, mammals and birds, the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became the bony vertebral body. [10]

In most ray-finned fishes, including all teleosts, these two structures are fused with and embedded within a solid piece of bone superficially resembling the vertebral body of mammals. In living amphibians, there is simply a cylindrical piece of bone below the vertebral arch, with no trace of the separate elements present in the early tetrapods. [10]

In cartilaginous fish such as sharks, the vertebrae consist of two cartilaginous tubes. The upper tube is formed from the vertebral arches, but also includes additional cartilaginous structures filling in the gaps between the vertebrae, enclosing the spinal cord in an essentially continuous sheath. The lower tube surrounds the notochord and has a complex structure, often including multiple layers of calcification. [10]

Lampreys have vertebral arches, but nothing resembling the vertebral bodies found in all higher vertebrates. Even the arches are discontinuous, consisting of separate pieces of arch-shaped cartilage around the spinal cord in most parts of the body, changing to long strips of cartilage above and below in the tail region. Hagfishes lack a true vertebral column, and are therefore not properly considered vertebrates, but a few tiny neural arches are present in the tail. [10] [11] Hagfishes do, however, possess a cranium. For this reason, the vertebrate subphylum is sometimes referred to as "Craniata" when discussing morphology. Molecular analysis [ specify ] since 1992 has suggested that the hagfishes are most closely related to lampreys, [12] and so also are vertebrates in a monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of Craniata. [13]

The head or skull includes the skull roof (a set of bones covering the brain, eyes and nostrils), the snout (from the eye to the forward-most point of the upper jaw), the operculum or gill cover (absent in sharks and jawless fish), and the cheek, which extends from the eye to the preopercle. The operculum and preopercle may or may not have spines. In sharks and some primitive bony fish the spiracle, a small extra gill opening, is found behind each eye.

The skull in fishes is formed from a series of only loosely connected bones. Jawless fish and sharks only possess a cartilaginous endocranium, with the upper and lower jaws of cartilaginous fish being separate elements not attached to the skull. Bony fishes have additional dermal bone, forming a more or less coherent skull roof in lungfish and holost fish. The lower jaw defines a chin.

In lampreys, the mouth is formed into an oral disk. In most jawed fish, however, there are three general configurations. The mouth may be on the forward end of the head (terminal), may be upturned (superior), or may be turned downwards or on the bottom of the fish (subterminal or inferior). The mouth may be modified into a suckermouth adapted for clinging onto objects in fast-moving water.

The simpler structure is found in jawless fish, in which the cranium is represented by a trough-like basket of cartilaginous elements only partially enclosing the brain and associated with the capsules for the inner ears and the single nostril. Distinctively, these fish have no jaws. [14]

Cartilaginous fish such as sharks also have simple, and presumably primitive, skull structures. The cranium is a single structure forming a case around the brain, enclosing the lower surface and the sides, but always at least partially open at the top as a large fontanelle. The most anterior part of the cranium includes a forward plate of cartilage, the rostrum, and capsules to enclose the olfactory organs. Behind these are the orbits, and then an additional pair of capsules enclosing the structure of the inner ear. Finally, the skull tapers towards the rear, where the foramen magnum lies immediately above a single condyle, articulating with the first vertebra. Smaller foramina for the cranial nerves can be found at various points throughout the cranium. The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper. [14]

In the ray-finned fishes, there has also been considerable modification from the primitive pattern. The roof of the skull is generally well formed, and although the exact relationship of its bones to those of tetrapods is unclear, they are usually given similar names for convenience. Other elements of the skull, however, may be reduced there is little cheek region behind the enlarged orbits, and little if any bone in between them. The upper jaw is often formed largely from the premaxilla, with the maxilla itself located further back, and an additional bone, the sympletic, linking the jaw to the rest of the cranium. [14]

Although the skulls of fossil lobe-finned fish resemble those of the early tetrapods, the same cannot be said of those of the living lungfishes. The skull roof is not fully formed, and consists of multiple, somewhat irregularly shaped bones with no direct relationship to those of tetrapods. The upper jaw is formed from the pterygoid bones and vomers alone, all of which bear teeth. Much of the skull is formed from cartilage, and its overall structure is reduced. [14]

The head may have several fleshy structures known as barbels, which may be very long and resemble whiskers. Many fish species also have a variety of protrusions or spines on the head. The nostrils or nares of almost all fishes do not connect to the oral cavity, but are pits of varying shape and depth.

Skull of Tiktaalik, a genus of extinct sarcopterygian (lobe-finned "fish") from the late Devonian period

Jaw Edit

The vertebrate jaw probably originally evolved in the Silurian period and appeared in the Placoderm fish which further diversified in the Devonian. Jaws are thought to derive from the pharyngeal arches that support the gills in fish. The two most anterior of these arches are thought to have become the jaw itself (see hyomandibula) and the hyoid arch, which braces the jaw against the braincase and increases mechanical efficiency. While there is no fossil evidence directly to support this theory, it makes sense in light of the numbers of pharyngeal arches that are visible in extant jawed animals (the gnathostomes), which have seven arches, and primitive jawless vertebrates (the Agnatha), which have nine. [ citation needed ]

It is thought that the original selective advantage garnered by the jaw was not related to feeding, but to increase respiration efficiency. The jaws were used in the buccal pump (observable in modern fish and amphibians) that pumps water across the gills of fish or air into the lungs of amphibians. Over evolutionary time, the more familiar use of jaws in feeding was selected for and became a very important function in vertebrates.

Linkage systems are widely distributed in animals. The most thorough overview of the different types of linkages in animals has been provided by M. Muller, [15] who also designed a new classification system which is especially well suited for biological systems. Linkage mechanisms are especially frequent and various in the head of bony fishes, such as wrasses, which have evolved many specialized aquatic feeding mechanisms. Especially advanced are the linkage mechanisms of jaw protrusion. For suction feeding a system of connected four-bar linkages is responsible for the coordinated opening of the mouth and 3-D expansion of the buccal cavity. Other linkages are responsible for protrusion of the premaxilla.

Eyes Edit

Fish eyes are similar to terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rod cells and cone cells (for scotopic and photopic vision), and most species have colour vision. Some fish can see ultraviolet and some can see polarized light. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. [16] The ancestors of modern hagfish, thought to be protovertebrate, [17] were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators and where it is advantageous to have a convex eyespot, which gathers more light than a flat or concave one. Unlike humans, fish normally adjust focus by moving the lens closer to or further from the retina. [18]

Gills Edit

The gills, located under the operculum, are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. They are not usually visible, but can be seen in some species, such as the frilled shark. The labyrinth organ of Anabantoidei and Clariidae is used to allow the fish to extract oxygen from the air. Gill rakers are finger-like projections off the gill arch which function in filter feeders to retain filtered prey. They may be bony or cartilaginous.

Skin Edit

The epidermis of fish consists entirely of live cells, with only minimal quantities of keratin in the cells of the superficial layer. It is generally permeable. The dermis of bony fish typically contains relatively little of the connective tissue found in tetrapods. Instead, in most species, it is largely replaced by solid, protective bony scales. Apart from some particularly large dermal bones that form parts of the skull, these scales are lost in tetrapods, although many reptiles do have scales of a different kind, as do pangolins. Cartilaginous fish have numerous tooth-like denticles embedded in their skin in place of true scales.

Sweat glands and sebaceous glands are both unique to mammals, but other types of skin glands are found in fish. Fish typically have numerous individual mucus-secreting skin cells that aid in insulation and protection, but may also have venom glands, photophores, or cells that produce a more watery serous fluid. [19] Melanin colours the skin of many species, but in fish the epidermis is often relatively colourless. Instead, the colour of the skin is largely due to chromatophores in the dermis, which, in addition to melanin, may contain guanine or carotenoid pigments. Many species, such as flounders, change the colour of their skin by adjusting the relative size of their chromatophores. [19]

Scales Edit

The outer body of many fish is covered with scales, which are part of the fish's integumentary system. The scales originate from the mesoderm (skin), and may be similar in structure to teeth. Some species are covered instead by scutes. Others have no outer covering on the skin. Most fish are covered in a protective layer of slime (mucus).

There are four principal types of fish scales.

    , also called dermal denticles, are similar to teeth in that they are made of dentin covered by enamel. They are typical of sharks and rays. are flat, basal-looking scales that cover a fish's body with little overlapping. They are typical of gar and bichirs. are small, oval-shaped scales with growth rings like the rings of a tree. Bowfin and remora have cycloid scales. are similar to cycloid scales, also having growth rings. They are distinguished by spines that cover one edge. Halibut have this type of scale.

Another less common type of scale is the scute, which may be an external, shield-like bony plate a modified, thickened scale that is often keeled or spiny or a projecting, modified (rough and strongly ridged) scale. Scutes are usually associated with the lateral line, but may be found on the caudal peduncle (where they form caudal keels) or along the ventral profile. Some fish, such as pineconefish, are completely or partially covered in scutes.

Lateral line Edit

The lateral line is a sense organ used to detect movement and vibration in the surrounding water. For example, fish can use their lateral line system to follow the vortices produced by fleeing prey. In most species, it consists of a line of receptors running along each side of the fish.

Photophores Edit

Photophores are light-emitting organs which appear as luminous spots on some fishes. The light can be produced from compounds during the digestion of prey, from specialized mitochondrial cells in the organism called photocytes, or from symbiotic bacteria. Photophores are used for attracting food or confusing predators.

Fins Edit

Fins are the most distinctive features of fish. They are either composed of bony spines or rays protruding from the body with skin covering them and joining them together, either in a webbed fashion as seen in most bony fish, or similar to a flipper as seen in sharks. Apart from the tail or caudal fin, fins have no direct connection with the spine and are supported by muscles only. Their principal function is to help the fish swim. Fins can also be used for gliding or crawling, as seen in the flying fish and frogfish. Fins located in different places on the fish serve different purposes, such as moving forward, turning, and keeping an upright position. For every fin, there are a number of fish species in which this particular fin has been lost during evolution. [ citation needed ]

Spines and rays Edit

In bony fish, most fins may have spines or rays. A fin may contain only spiny rays, only soft rays, or a combination of both. If both are present, the spiny rays are always anterior. Spines are generally stiff, sharp and unsegmented. Rays are generally soft, flexible, segmented, and may be branched. This segmentation of rays is the main difference that distinguishes them from spines spines may be flexible in certain species, but never segmented.

Spines have a variety of uses. In catfish, they are used as a form of defense many catfish have the ability to lock their spines outwards. Triggerfish also use spines to lock themselves in crevices to prevent them being pulled out.

Lepidotrichia are bony, bilaterally-paired, segmented fin rays found in bony fishes. They develop around actinotrichia as part of the dermal exoskeleton. Lepidotrichia may have some cartilage or bone in them as well. They are actually segmented and appear as a series of disks stacked one on top of another. The genetic basis for the formation of the fin rays is thought to be genes coding for the proteins actinodin 1 and actinodin 2. [20]

Types of fin Edit

    : Located on the back of the fish, dorsal fins serve to prevent the fish from rolling and assist in sudden turns and stops. Most fishes have one dorsal fin, but some fishes have two or three. In anglerfish, the anterior of the dorsal fin is modified into an illicium and esca, a biological equivalent to a fishing rod and lure. The two to three bones that support the dorsal fin are called the proximal, middle, and distalpterygiophores. In spinous fins, the distal pterygiophore is often fused to the middle or not present at all.
  • Caudal/Tail fins: Also called the tail fins, caudal fins are attached to the end of the caudal peduncle and used for propulsion. The caudal peduncle is the narrow part of the fish's body. The hypural joint is the joint between the caudal fin and the last of the vertebrae. The hypural is often fan-shaped. The tail may be heterocercal, reversed heterocercal, protocercal, diphycercal, or homocercal.
    • Heterocercal: vertebrae extend into the upper lobe of the tail, making it longer (as in sharks)
    • Reversed heterocercal: vertebrae extend into the lower lobe of the tail, making it longer (as in the Anaspida)
    • Protocercal: vertebrae extend to the tip of the tail the tail is symmetrical but not expanded (as in lancelets)
    • Diphycercal: vertebrae extend to the tip of the tail the tail is symmetrical and expanded (as in the bichir, lungfish, lamprey and coelacanth). Most Palaeozoic fishes had a diphycercal heterocercal tail. [21]
    • Homocercal: vertebrae extend a very short distance into the upper lobe of the tail tail still appears superficially symmetric. Most fish have a homocercal tail, but it can be expressed in a variety of shapes. The tail fin can be rounded at the end, truncated (almost vertical edge, as in salmon), forked (ending in two prongs), emarginate (with a slight inward curve), or continuous (dorsal, caudal, and anal fins attached, as in eels).
    • "Cephalic fins": The "horns" of manta rays and their relatives, sometimes called cephalic fins, are actually a modification of the anterior portion of the pectoral fin.

    Intestines Edit

    As with other vertebrates, the intestines of fish consist of two segments, the small intestine and the large intestine. In most higher vertebrates, the small intestine is further divided into the duodenum and other parts. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine may be used instead of duodenum. [24] In bony fish, the intestine is relatively short, typically around one and a half times the length of the fish's body. It commonly has a number of pyloric caeca, small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food. There is no ileocaecal valve in teleosts, with the boundary between the small intestine and the rectum being marked only by the end of the digestive epithelium. [19] There is no small intestine as such in non-teleost fish, such as sharks, sturgeons, and lungfish. Instead, the digestive part of the gut forms a spiral intestine, connecting the stomach to the rectum. In this type of gut, the intestine itself is relatively straight, but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns. This fold creates a valve-like structure that greatly increases both the surface area and the effective length of the intestine. The lining of the spiral intestine is similar to that of the small intestine in teleosts and non-mammalian tetrapods. [19] In lampreys, the spiral valve is extremely small, possibly because their diet requires little digestion. Hagfish have no spiral valve at all, with digestion occurring for almost the entire length of the intestine, which is not subdivided into different regions. [19]

    Pyloric caeca Edit

    The pyloric caecum is a pouch, usually peritoneal, at the beginning of the large intestine. It receives faecal material from the ileum, and connects to the ascending colon of the large intestine. It is present in most amniotes, and also in lungfish. [25] Many fish in addition have a number of small outpocketings, also called pyloric caeca, along their intestine despite the name they are not homologous to the caecum of amniotes. Their purpose is to increase the overall surface area of the digestive epithelium, therefore optimizing the absorption of sugars, amino acids, and dipeptides, among other nutrients. [25] [26]

    Stomach Edit

    As with other vertebrates, the relative positions of the esophageal and duodenal openings to the stomach remain relatively constant. As a result, the stomach always curves somewhat to the left before curving back to meet the pyloric sphincter. However, lampreys, hagfishes, chimaeras, lungfishes, and some teleost fish have no stomach at all, with the esophagus opening directly into the intestine. These fish consume diets that either require little storage of food, no pre-digestion with gastric juices, or both. [27]

    Kidneys Edit

    The kidneys of fish are typically narrow, elongated organs, occupying a significant portion of the trunk. They are similar to the mesonephros of higher vertebrates (reptiles, birds, and mammals). The kidneys contain clusters of nephrons, serviced by collecting ducts which usually drain into a mesonephric duct. However, the situation is not always so simple. In cartilaginous fish, there is also a shorter duct which drains the posterior (metanephric) parts of the kidney, and joins with the mesonephric duct at the bladder or cloaca. Indeed, in many cartilaginous fish, the anterior portion of the kidney may degenerate or cease to function altogether in the adult. [28] Hagfish and lamprey kidneys are unusually simple. They consist of a row of nephrons, each emptying directly into the mesonephric duct. [28] Like the Nile tilapia, the kidney of some fish shows its three parts head, trunk, and tail kidneys. [29] Fish do not have a discrete adrenal gland with distinct cortex and medulla, similar to those found in mammals. The interrenal and chromaffin cells are located within the head kidney [30]

    Spleen Edit

    The spleen is found in nearly all vertebrates. It is a non-vital organ, similar in structure to a large lymph node. It acts primarily as a blood filter, and plays important roles in regards to red blood cells and the immune system. [31] In cartilaginous and bony fish it consists primarily of red pulp and is normally a somewhat elongated organ as it actually lies inside the serosal lining of the intestine. [32] The only vertebrates lacking a spleen are the lampreys and hagfishes. Even in these animals, there is a diffuse layer of haematopoietic tissue within the gut wall, which has a similar structure to red pulp, and is presumed to be homologous to the spleen of higher vertebrates. [32]

    Liver Edit

    The liver is a large vital organ present in all fish. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion. It is very susceptible to contamination by organic and inorganic compounds because they can accumulate over time and cause potentially life-threatening conditions. Because of the liver's capacity for detoxification and storage of harmful components, it is often used as an environmental biomarker. [33]

    Heart Edit

    Fish have what is often described as a two-chambered heart, [34] consisting of one atrium to receive blood and one ventricle to pump it, [35] in contrast to three chambers (two atria, one ventricle) of amphibian and most reptile hearts and four chambers (two atria, two ventricles) of mammal and bird hearts. [34] However, the fish heart has entry and exit compartments that may be called chambers, so it is also sometimes described as three-chambered, [35] or four-chambered, [36] depending on what is counted as a chamber. The atrium and ventricle are sometimes considered "true chambers", while the others are considered "accessory chambers". [37]

    The four compartments are arranged sequentially:

      : A thin-walled sac or reservoir with some cardiac muscle that collects deoxygenated blood through the incoming hepatic and cardinal veins. [verification needed] [35]
  • Atrium: A thicker-walled, muscular chamber that sends blood to the ventricle. [35]
  • Ventricle: A thick-walled, muscular chamber that pumps the blood to the fourth part, the outflow tract. [35] The shape of the ventricle varies considerably, usually tubular in fish with elongated bodies, pyramidal with a triangular base in others, or sometimes sac-like in some marine fish. [36]
  • Outflow tract (OFT): Goes to the ventral aorta and consists of the tubular conus arteriosus, bulbus arteriosus, or both. [36] The conus arteriosus, typically found in more primitive species of fish, contracts to assist blood flow to the aorta, while the bulbus anteriosus does not. [37][38]
  • Ostial valves, consisting of flap-like connective tissues, prevent blood from flowing backward through the compartments. [36] The ostial valve between the sinus venosus and atrium is called the sino-atrial valve, which closes during ventricular contraction. [36] Between the atrium and ventricle is an ostial valve called the atrioventricular valve, and between the bulbus arteriosus and ventricle is an ostial valve called the bulbo-ventricular valve. [36] The conus arteriosus has a variable number of semilunar valves. [37]

    The ventral aorta delivers blood to the gills where it is oxygenated and flows, through the dorsal aorta, into the rest of the body. (In tetrapods, the ventral aorta is divided in two one half forms the ascending aorta, while the other forms the pulmonary artery). [32]

    The circulatory systems of all vertebrates are closed. Fish have the simplest circulatory system, consisting of only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single cycle circulation. [39]

    In the adult fish, the four compartments are not arranged in a straight row, instead forming an S-shape with the latter two compartments lying above the former two. This relatively simpler pattern is found in cartilaginous fish and in the ray-finned fish. In teleosts, the conus arteriosus is very small and can more accurately be described as part of the aorta rather than of the heart proper. The conus arteriosus is not present in any amniotes, presumably having been absorbed into the ventricles over the course of evolution. Similarly, while the sinus venosus is present as a vestigial structure in some reptiles and birds, it is otherwise absorbed into the right atrium and is no longer distinguishable. [32]

    Swim bladder Edit

    The swim bladder or gas bladder is an internal organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth, ascend, or descend without having to waste energy in swimming. The bladder is found only in the bony fishes. In the more primitive groups like some Leuciscinae, bichirs and lungfish, the bladder is open to the esophagus and doubles as a lung. It is often absent in fast swimming fishes such as the tuna and mackerel families. Fish with bladders open to the esophagus are called physostomes, while fish with the bladder closed are called physoclists. In the latter, the gas content of the bladder is controlled through a rete mirabilis, a network of blood vessels affecting gas exchange between the bladder and the blood. [40]

    Weberian apparatus Edit

    Fishes of the superorder Ostariophysi possess a structure called the Weberian apparatus, a modification which allows them to hear better. This ability may explain the marked success of ostariophysian fishes. [41] The apparatus is made up of a set of bones known as Weberian ossicles, a chain of small bones that connect the auditory system to the swim bladder of fishes. [42] The ossicles connect the gas bladder wall with Y-shaped lymph sinus that is next to the lymph-filled transverse canal joining the saccules of the right and left ears. This allows the transmission of vibrations to the inner ear. A fully functioning Weberian apparatus consists of the swim bladder, the Weberian ossicles, a portion of the anterior vertebral column, and some muscles and ligaments. [42]

    Fish reproductive organs include testes and ovaries. In most species, gonads are paired organs of similar size, which can be partially or totally fused. [43] There may also be a range of secondary organs that increase reproductive fitness. The genital papilla is a small, fleshy tube behind the anus in some fishes from which the sperm or eggs are released the sex of a fish often can be determined by the shape of its papilla. [ citation needed ]

    Testes Edit

    Most male fish have two testes of similar size. In the case of sharks, the testis on the right side is usually larger. The primitive jawless fish have only a single testis located in the midline of the body, although even this forms from the fusion of paired structures in the embryo. [32]

    Under a tough membranous shell, the tunica albuginea, the testis of some teleost fish, contains very fine coiled tubes called seminiferous tubules. The tubules are lined with a layer of cells (germ cells) that from puberty into old age, develop into sperm cells (also known as spermatozoa or male gametes). The developing sperm travel through the seminiferous tubules to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly created sperm cells mature (see spermatogenesis). The sperm move into the vas deferens, and are eventually expelled through the urethra and out of the urethral orifice through muscular contractions.

    However, most fish do not possess seminiferous tubules. Instead, the sperm are produced in spherical structures called sperm ampullae. These are seasonal structures, releasing their contents during the breeding season and then being reabsorbed by the body. Before the next breeding season, new sperm ampullae begin to form and ripen. The ampullae are otherwise essentially identical to the seminiferous tubules in higher vertebrates, including the same range of cell types. [44]

    In terms of spermatogonia distribution, the structure of teleost testes have two types: in the most common, spermatogonia occur all along the seminiferous tubules, while in Atherinomorpha, they are confined to the distal portion of these structures. Fish can present cystic or semi-cystic spermatogenesis [ definition needed ] in relation to the release phase of germ cells in cysts to the lumen of the seminiferous tubules. [43]

    Ovaries Edit

    Many of the features found in ovaries are common to all vertebrates, including the presence of follicular cells and tunica albuginea There may be hundreds or even millions of fertile eggs present in the ovary of a fish at any given time. Fresh eggs may be developing from the germinal epithelium throughout life. Corpora lutea are found only in mammals, and in some elasmobranch fish in other species, the remnants of the follicle are quickly resorbed by the ovary. [44] The ovary of teleosts is often contains a hollow, lymph-filled space which opens into the oviduct, and into which the eggs are shed. [44] Most normal female fish have two ovaries. In some elasmobranchs, only the right ovary develops fully. In the primitive jawless fish and some teleosts, there is only one ovary, formed by the fusion of the paired organs in the embryo. [44]

    Fish ovaries may be of three types: gymnovarian, secondary gymnovarian or cystovarian. In the first type, the oocytes are released directly into the coelomic cavity and then enter the ostium, then through the oviduct and are eliminated. Secondary gymnovarian ovaries shed ova into the coelom from which they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the oviduct. [45] Gymnovaries are the primitive condition found in lungfish, sturgeon, and bowfin. Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct. [43] Secondary gymnovaries are found in salmonids and a few other teleosts.

    Central nervous system Edit

    Fish typically have quite small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. [46] However, some fish have relatively large brains, most notably mormyrids and sharks, which have brains about as massive relative to body weight as birds and marsupials. [47]

    Fish brains are divided into several regions. At the front are the olfactory lobes, a pair of structures that receive and process signals from the nostrils via the two olfactory nerves. [46] Similar to the way humans smell chemicals in the air, fish smell chemicals in the water by tasting them. The olfactory lobes are very large in fish that hunt primarily by smell, such as hagfish, sharks, and catfish. Behind the olfactory lobes is the two-lobed telencephalon, the structural equivalent to the cerebrum in higher vertebrates. In fish the telencephalon is concerned mostly with olfaction. [46] Together these structures form the forebrain.

    The forebrain is connected to the midbrain via the diencephalon (in the diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs functions associated with hormones and homeostasis. [46] The pineal body lies just above the diencephalon. This structure detects light, maintains circadian rhythms, and controls color changes. [46] The midbrain or mesencephalon contains the two optic lobes. These are very large in species that hunt by sight, such as rainbow trout and cichlids. [46]

    The hindbrain or metencephalon is particularly involved in swimming and balance. [46] The cerebellum is a single-lobed structure that is typically the biggest part of the brain. [46] Hagfish and lampreys have relatively small cerebella, while the mormyrid cerebellum is massive and apparently involved in their electrical sense. [46]

    The brain stem or myelencephalon is the brain's posterior. [46] As well as controlling some muscles and body organs, in bony fish at least, the brain stem governs respiration and osmoregulation. [46]

    Vertebrates are the only chordate group to exhibit a proper brain. A slight swelling of the anterior end of the dorsal nerve cord is found in the lancelet, though it lacks the eyes and other complex sense organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalisation. [8] The central nervous system is based on a hollow nerve tube running along the length of the animal, from which the peripheral nervous system branches out to innervate the various systems. The front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain) then further differentiated in the various vertebrate groups. [48] Two laterally placed eyes form around outgrows from the midbrain, except in hagfish, though this may be a secondary loss. [49] [50] The forebrain is well developed and subdivided in most tetrapods, while the midbrain dominates in many fish and some salamanders. Vesicles of the forebrain are usually paired, giving rise to hemispheres like the cerebral hemispheres in mammals. [48] The resulting anatomy of the central nervous system, with a single, hollow ventral nerve cord topped by a series of (often paired) vesicles is unique to vertebrates. [8]

    Cerebellum Edit

    The circuits in the cerebellum are similar across all classes of vertebrates, including fish, reptiles, birds, and mammals. [51] There is also an analogous brain structure in cephalopods with well-developed brains, such as octopuses. [52] This has been taken as evidence that the cerebellum performs functions important to all animal species with a brain.

    There is considerable variation in the size and shape of the cerebellum in different vertebrate species. In amphibians, lampreys, and hagfish, the cerebellum is little developed in the latter two groups, it is barely distinguishable from the brain-stem. Although the spinocerebellum is present in these groups, the primary structures are small paired nuclei corresponding to the vestibulocerebellum. [44]

    The cerebellum of cartilaginous and bony fishes is extraordinarily large and complex. In at least one important respect, it differs in internal structure from the mammalian cerebellum: The fish cerebellum does not contain discrete deep cerebellar nuclei. Instead, the primary targets of Purkinje cells are a distinct type of cell distributed across the cerebellar cortex, a type not seen in mammals. In mormyrids (a family of weakly electrosensitive freshwater fish), the cerebellum is considerably larger than the rest of the brain put together. The largest part of it is a special structure called the valvula, which has an unusually regular architecture and receives much of its input from the electrosensory system. [53]

    Most species of fish and amphibians possess a lateral line system that senses pressure waves in water. One of the brain areas that receives primary input from the lateral line organ, the medial octavolateral nucleus, has a cerebellum-like structure, with granule cells and parallel fibers. In electrosensitive fish, the input from the electrosensory system goes to the dorsal octavolateral nucleus, which also has a cerebellum-like structure. In ray-finned fishes (by far the largest group), the optic tectum has a layer—the marginal layer—that is cerebellum-like. [51]

    Identified neurons Edit

    A neuron is "identified" if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has one and only one neuron with the same set of properties. [54] In vertebrate nervous systems, very few neurons are "identified" in this sense (in humans, there are believed to be none). In simpler nervous systems, some or all neurons may be thus unique. [55]

    In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish. [56] Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and one on the right. Each Mauthner cell has an axon that crosses over, innervating neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes. The synapses generated by a Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a C-shape, then straightens, thereby propelling itself rapidly forward. Functionally, this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not the only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response all by itself, in the context of ordinary behavior, other types of cells usually contribute to shaping the amplitude and direction of the response.

    Mauthner cells have been described as command neurons. A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior all by itself. [57] Such neurons appear most commonly in the fast escape systems of various species—the squid giant axon and squid giant synapse, used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of a command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances. [58]

    Immune organs vary by type of fish. [59] In the jawless fish (lampreys and hagfish), true lymphoid organs are absent. These fish rely on regions of lymphoid tissue within other organs to produce immune cells. For example, erythrocytes, macrophages and plasma cells are produced in the anterior kidney (or pronephros) and some areas of the gut (where granulocytes mature). They resemble primitive bone marrow in hagfish.

    Cartilaginous fish (sharks and rays) have a more advanced immune system. They have three specialized organs that are unique to chondrichthyes the epigonal organs (lymphoid tissues similar to mammalian bone) that surround the gonads, the Leydig's organ within the walls of their esophagus, and a spiral valve in their intestine. These organs house typical immune cells (granulocytes, lymphocytes and plasma cells). They also possess an identifiable thymus and a well-developed spleen (their most important immune organ) where various lymphocytes, plasma cells and macrophages develop and are stored.

    Chondrostean fish (sturgeons, paddlefish and bichirs) possess a major site for the production of granulocytes within a mass that is associated with the meninges, the membranes surrounding the central nervous system. Their heart is frequently covered with tissue that contains lymphocytes, reticular cells and a small number of macrophages. The chondrostean kidney is an important hemopoietic organ it is where erythrocytes, granulocytes, lymphocytes and macrophages develop.


    Distinguishing features of the teleosts are mobile premaxilla, elongated neural arches at the end of the caudal fin and unpaired basibranchial toothplates. [4] The premaxilla is unattached to the neurocranium (braincase) it plays a role in protruding the mouth and creating a circular opening. This lowers the pressure inside the mouth, sucking the prey inside. The lower jaw and maxilla are then pulled back to close the mouth, and the fish is able to grasp the prey. By contrast, mere closure of the jaws would risk pushing food out of the mouth. In more advanced teleosts, the premaxilla is enlarged and has teeth, while the maxilla is toothless. The maxilla functions to push both the premaxilla and the lower jaw forward. To open the mouth, an adductor muscle pulls back the top of the maxilla, pushing the lower jaw forward. In addition, the maxilla rotates slightly, which pushes forward a bony process that interlocks with the premaxilla. [5]

    The pharyngeal jaws of teleosts, a second set of jaws contained within the throat, are composed of five branchial arches, loops of bone which support the gills. The first three arches include a single basibranchial surrounded by two hypobranchials, ceratobranchials, epibranchials and pharyngobranchials. The median basibranchial is covered by a toothplate. The fourth arch is composed of pairs of ceratobranchials and epibranchials, and sometimes additionally, some pharyngobranchials and a basibranchial. The base of the lower pharyngeal jaws is formed by the fifth ceratobranchials while the second, third and fourth pharyngobranchials create the base of the upper. In the more basal teleosts the pharyngeal jaws consist of well-separated thin parts that attach to the neurocranium, pectoral girdle, and hyoid bar. Their function is limited to merely transporting food, and they rely mostly on lower pharyngeal jaw activity. In more derived teleosts the jaws are more powerful, with left and right ceratobranchials fusing to become one lower jaw the pharyngobranchials fuse to create a large upper jaw that articulates with the neurocranium. They have also developed a muscle that allows the pharyngeal jaws to have a role in grinding food in addition to transporting it. [6]

    The caudal fin is homocercal, meaning the upper and lower lobes are about equal in size. The spine ends at the caudal peduncle, the base of the caudal fin, distinguishing this group from those in which the spine extends into the upper lobe of the caudal fin, such as most fish from the Paleozoic (541 to 252 million years ago). The neural arches are elongated to form uroneurals which provide support for this upper lobe. [5] In addition, the hypurals, bones that form a flattened plate at the posterior end of the vertebral column, are enlarged providing further support for the caudal fin. [7]

    In general, teleosts tend to be quicker and more flexible than more basal bony fishes. Their skeletal structure has evolved towards greater lightness. While teleost bones are well calcified, they are constructed from a scaffolding of struts, rather than the dense cancellous bones of holostean fish. In addition, the lower jaw of the teleost is reduced to just three bones the dentary, the angular bone and the articular bone. [8]

    External relationships Edit

    The teleosts were first recognised as a distinct group by the German ichthyologist Johannes Peter Müller in 1845. [9] The name is from Greek teleios, "complete" + osteon, "bone". [10] Müller based this classification on certain soft tissue characteristics, which would prove to be problematic, as it did not take into account the distinguishing features of fossil teleosts. In 1966, Greenwood et al. provided a more solid classification. [9] [11] The oldest teleost fossils date back to the late Paleozoic, evolving from fish related to the bowfins in the clade Holostei. During the Mesozoic and Cenozoic they diversified, and as a result, 96 percent of all known fish species are teleosts. The cladogram shows the relationship of the teleosts to other bony fish, [12] and to the terrestrial vertebrates (tetrapods) that evolved from a related group of fish. [13] [14] Approximate dates are from Near et al., 2012. [12]

    Internal relationships Edit

    The phylogeny of the teleosts has been subject to long debate, without consensus on either their phylogeny or the timing of the emergence of the major groups before the application of modern DNA-based cladistic analysis. Near et al. (2012) explored the phylogeny and divergence times of every major lineage, analysing the DNA sequences of 9 unlinked genes in 232 species. They obtained well-resolved phylogenies with strong support for the nodes (so, the pattern of branching shown is likely to be correct). They calibrated (set actual values for) branching times in this tree from 36 reliable measurements of absolute time from the fossil record. [12] The teleosts are divided into the major clades shown on the cladogram, [15] with dates, following Near et al. [12]

    Evolutionary trends Edit

    The first fossils assignable to this diverse group appear in the Early Triassic, [16] after which teleosts accumulated novel body shapes predominantly gradually for the first 150 million years of their evolution [16] (Early Triassic through early Cretaceous).

    The most basal of the living teleosts are the Elopomorpha (eels and allies) and the Osteoglossomorpha (elephantfishes and allies). There are 800 species of elopomorphs. They have thin leaf-shaped larvae known as leptocephali, specialised for a marine environment. Among the elopomorphs, eels have elongated bodies with lost pelvic girdles and ribs and fused elements in the upper jaw. The 200 species of osteoglossomorphs are defined by a bony element in the tongue. This element has a basibranchial behind it, and both structures have large teeth which are paired with the teeth on the parasphenoid in the roof of the mouth. The clade Otocephala includes the Clupeiformes (herrings) and Ostariophysi (carps, catfishes and allies). Clupeiformes consists of 350 living species of herring and herring-like fishes. This group is characterised by an unusual abdominal scute and a different arrangement of the hypurals. In most species, the swim bladder extends to the braincase and plays a role in hearing. Ostariophysi, which includes most freshwater fishes, includes species that have developed some unique adaptations. [5] One is the Weberian apparatus, an arrangement of bones (Weberian ossicles) connecting the swim bladder to the inner ear. This enhances their hearing, as sound waves make the bladder vibrate, and the bones transport the vibrations to the inner ear. They also have a chemical alarm system when a fish is injured, the warning substance gets in the water, alarming nearby fish. [17]

    The majority of teleost species belong to the clade Euteleostei, which consists of 17,419 species classified in 2,935 genera and 346 families. Shared traits of the euteleosts include similarities in the embryonic development of the bony or cartilaginous structures located between the head and dorsal fin (supraneural bones), an outgrowth on the stegural bone (a bone located near the neural arches of the tail) and caudal median cartilages located between hypurals of the caudal base. The majority of euteleosts are in the clade Neoteleostei. A derived trait of neoteleosts is a muscle that controls the pharyngeal jaws, giving them a role in grinding food. Within neoteleosts, members of the Acanthopterygii have a spiny dorsal fin which is in front of the soft-rayed dorsal fin. [18] This fin helps provide thrust in locomotion [19] and may also play a role in defense. Acanthomorphs have developed spiny ctenoid scales (as opposed to the cycloid scales of other groups), tooth-bearing premaxilla and greater adaptations to high speed swimming. [5]

    The adipose fin, which is present in over 6,000 teleost species, is often thought to have evolved once in the lineage and to have been lost multiple times due to its limited function. A 2014 study challenges this idea and suggests that the adipose fin is an example of convergent evolution. In Characiformes, the adipose fin develops from an outgrowth after the reduction of the larval fin fold, while in Salmoniformes, the fin appears to be a remnant of the fold. [20]

    Diversity Edit

    There are over 26,000 species of teleosts, in about 40 orders and 448 families, [21] making up 96% of all extant species of fish. [22] Approximately 12,000 of the total 26,000 species are found in freshwater habitats. [23] Teleosts are found in almost every aquatic environment and have developed specializations to feed in a variety of ways as carnivores, herbivores, filter feeders and parasites. [24] The longest teleost is the giant oarfish, reported at 7.6 m (25 ft) and more, [25] but this is dwarfed by the extinct Leedsichthys, one individual of which has been estimated to have a length of 27.6 m (91 ft). [26] The heaviest teleost is believed to be the ocean sunfish, with a specimen landed in 2003 having an estimated weight of 2.3 t (2.3 long tons 2.5 short tons), [27] while the smallest fully mature adult is the male anglerfish Photocorynus spiniceps which can measure just 6.2 mm (0.24 in), though the female at 50 mm (2 in) is much larger. [25] The stout infantfish is the smallest and lightest adult fish and is in fact the smallest vertebrate in the world the females measures 8.4 mm (0.33 in) and the male just 7 mm (0.28 in). [28]

    Open water fish are usually streamlined like torpedoes to minimize turbulence as they move through the water. Reef fish live in a complex, relatively confined underwater landscape and for them, manoeuvrability is more important than speed, and many of them have developed bodies which optimize their ability to dart and change direction. Many have laterally compressed bodies (flattened from side to side) allowing them to fit into fissures and swim through narrow gaps some use their pectoral fins for locomotion and others undulate their dorsal and anal fins. [29] Some fish have grown dermal (skin) appendages for camouflage the prickly leather-jacket is almost invisible among the seaweed it resembles and the tasselled scorpionfish invisibly lurks on the seabed ready to ambush prey. Some like the foureye butterflyfish have eyespots to startle or deceive, while others such as lionfish have aposematic coloration to warn that they are toxic or have venomous spines. [30]

    Flatfish are demersal fish (bottom-feeding fish) that show a greater degree of asymmetry than any other vertebrates. The larvae are at first bilaterally symmetrical but they undergo metamorphosis during the course of their development, with one eye migrating to the other side of the head, and they simultaneously start swimming on their side. This has the advantage that, when they lie on the seabed, both eyes are on top, giving them a broad field of view. The upper side is usually speckled and mottled for camouflage, while the underside is pale. [31]

    Some teleosts are parasites. Remoras have their front dorsal fins modified into large suckers with which they cling onto a host animal such as a whale, sea turtle, shark or ray, but this is probably a commensal rather than parasitic arrangement because both remora and host benefit from the removal of ectoparasites and loose flakes of skin. [32] More harmful are the catfish that enter the gill chambers of fish and feed on their blood and tissues. [33] The snubnosed eel, though usually a scavenger, sometimes bores into the flesh of a fish, and has been found inside the heart of a shortfin mako shark. [34]

    Some species, such as electric eels, can produce powerful electric currents, strong enough to stun prey. Other fish, such as knifefish, generate weak varying electric fields to detect their prey they swim with straight backs to avoid distorting their electric fields. These currents are produced by modified muscle or nerve cells. [17]

    Teleosts are found worldwide and in most aquatic environments, including warm and cold seas, flowing and still freshwater, and even, in the case of the desert pupfish, isolated and sometimes hot and saline bodies of water in deserts. [35] [36] Teleost diversity becomes low at extremely high latitudes at Franz Josef Land, up to 82°N, ice cover and water temperatures below 0 °C (32 °F) for a large part of the year limit the number of species 75 percent of the species found there are endemic to the Arctic. [37]

    Of the major groups of teleosts, the Elopomorpha, Clupeomorpha and Percomorpha (perches, tunas and many others) all have a worldwide distribution and are mainly marine the Ostariophysi and Osteoglossomorpha are worldwide but mainly freshwater, the latter mainly in the tropics the Atherinomorpha (guppies, etc.) have a worldwide distribution, both fresh and salt, but are surface-dwellers. In contrast, the Esociformes (pikes) are limited to freshwater in the Northern Hemisphere, while the Salmoniformes (salmon, trout) are found in both Northern and Southern temperate zones in freshwater, some species migrating to and from the sea. The Paracanthopterygii (cods, etc.) are Northern Hemisphere fish, with both salt and freshwater species. [36]

    Some teleosts are migratory certain freshwater species move within river systems on an annual basis other species are anadromous, spending their lives at sea and moving inland to spawn, salmon and striped bass being examples. Others, exemplified by the eel, are catadromous, doing the reverse. [38] The fresh water European eel migrates across the Atlantic Ocean as an adult to breed in floating seaweed in the Sargasso Sea. The adults spawn here and then die, but the developing young are swept by the Gulf Stream towards Europe. By the time they arrive, they are small fish and enter estuaries and ascend rivers, overcoming obstacles in their path to reach the streams and ponds where they spend their adult lives. [39]

    Teleosts including the brown trout and the scaly osman are found in mountain lakes in Kashmir at altitudes as high as 3,819 m (12,530 ft). [40] Teleosts are found at extreme depths in the oceans the hadal snailfish has been seen at a depth of 7,700 m (25,300 ft), and a related (unnamed) species has been seen at 8,145 m (26,720 ft). [41] [42]

    The Basic Body Shape of Fish and How They Move

    Like all animals, the fish’s body is a result of specialization in its environment. Water is about 800 times thicker than air and an aquatic life has its own difficulties, such as buoyancy, drag and the amount of effort needed to move through such a dense medium.

    While most fishes share common features of streamlining for easy movement through the water, their exact forms vary greatly depending on whether they are predators or prey, how they feed and what measures they take for attack or defense. Every fish is optimized for survival.

    The bony fish are the most evolved and show the greatest body specialization. Every feature is developed to exploit their underwater environment. Some have flat bodies and sucker-style mouths ideal for resisting strong currents and moving along rocks, feeding on algae -- such as the common plec -- while others have streamlined forms adapted to quick, constant movement and upturned mouths to suck insects from the water’s surface, like the zebra danio.

    The problem of buoyancy has also led to some interesting forms, like the colorful, lively mbuna. Popular among fishkeepers, these fishes are maneuverable and can ‘hover’ in place thanks to their adjustable air sac (swim-bladder) and highly-developed pectoral and pelvic paired fins. They have traded streamlining and speed for this ability, so generally move slower. Fishes like this have two types of muscles: brown and white. The brown muscle is continually supplied with oxygen and has good blood circulation, so is used for continuous activity. The white muscle (called ‘anaerobic’ muscle because it quickly builds up oxygen-debt) is powerful and gives a short-term boost of emergency speed.

    In contrast, fishes that swim constantly in midwater, like tuna and mackerel, are much more streamlined and frequently lack the swim-bladder. They counteract the possibility of sinking with muscular effort reduced by decreasing drag and having a thinner cross-section -- both offered by the absence of the buoyancy device. Their muscle is mostly brown to facilitate constant swimming and their fins are usually retracted as they are only used for turning.

    Bottom-feeders are generally much more sedentary. They have limited locomotory requirements, as can be seen in examples such as the suckermouth and whiptail catfish. They tend to be compressed dorso-ventrally and, since they live on the bottom of their environment, have no need for a swim-bladder. Their specialization comes in the forms of camouflage, feeding and defense rather than quick movement.

    Differences between bony fish and cartilaginous fish

    The sharks , rays and chimeras (deep-sea fish, also called rat fish) of this class (from the Greek chondros = cartilage + ichthys = fish) are the most primitive living vertebrates with complete and separate vertebrae, movable jaws and even fins.

    This group is ancient and represented by numerous fossil remains. They belong to some of the largest and most efficient marine predators. All have a cartilaginous skeleton, specialized teeth that are renewed throughout life and a skin thickly covered by tooth-shaped scales.

    Almost all are marine, although there are species of sharks and rays that regularly penetrate estuaries and rivers, and, in tropical regions, freshwater species.

    All cartilaginous fish are predators, although the phytoplankton also ingest phytoplankton. In this case there are rigid projections of the gill arcs, which function as filters. Much of their diet is made up of live prey, although they eat corpses when available.

    The Bone Fish

    Bony fish are the largest group (corresponding to 9 out of 10 species) and diverse fishes present. These animals inhabit all types of water, sweet, brackish, salty, hot or cold (although most are limited to temperatures between 9 and 11ºC). This is the most recent class from a phylogenetic point of view as well as considered more evolved. The taxonomy within this class has often been altered, due to the discovery of new species, as well as of new relations between the already known ones.

    Typically the bony fish are not larger than 1 m in length but there are reduced forms (certain gobies are only 10 mm long) and gigantic (swordfish with 3.70 m, sturgeon with 3.80 m and 590 kg of weight or fish -Water with 900 kg weight).

    They have adapted to live in sometimes difficult conditions, such as lakes at high altitude, polar zones, hydrothermal vents, puddles with high salinity or poor in oxygen, etc.

    Many fish periodically migrate from site to site or from deep water to the surface, both for spawning and feeding.

    Its main features include a body, taller than wide and oval in silhouette, which facilitates movement through the water.

    The head extends from the tip of the muzzle to the opening of the operculum, the trunk from there to the anus, behind which is the tail. The body has a strong segmental musculature – myomeros -, separated by delicate connective septa.

    The skeleton is formed by true bones, although some species may have cartilaginous bones (sturgeon, for example), with numerous distinct vertebrae, although notochord is persistent in the intervertebral spaces.

    The skeleton has 3 main parts: spine , skull and rays of the fins . The ribs and the pectoral girdle (there is no pelvic girdle, connecting these fins by means of tendons, without attachment to the spinal column). Numerous other small bones support the rays of the fins.

    The main differences between bony fish and cartilaginous fish

    We can classify the fish into two large groups that are quite different from one another: condrictes (cartilaginous fish) and osteitis (bony fish). Despite some glaring differences, it is common to make mistakes by differentiating the two groups. The following are the main differences between a cartilaginous fish and a bony fish. It is worth noting that we can find some representatives that do not follow the rules below.

    First we can differentiate the two groups by the skeleton. Cartilaginous fish have a skeleton made up entirely of cartilage, while bone fish have a skeleton made up of bones.

    Another striking difference is the gills. The bony fish have a membrane that covers the gill slits, while the cartilaginous fish have their gills exposed, without any protection.

    Scales can also be used to differentiate these two groups. While cartilaginous fish have placoid scales and dermal and epidermal origin, bone fish have scales of exclusively dermal origin.

    By observing the mouth, you can also see a difference. While the cartilaginous fish have a ventral mouth, the bony fish present their mouth in the anterior region of the body.

    The bony fish present, among other characteristics, the presence of operculum

    Reproduction is also an important factor. While bony fish have external fertilization, the cartilaginous ones have a structure called the clasper, which acts to aid in internal reproduction. The clasper is a modified pelvic fin which helps in the introduction of spermatozoa. Besides this difference, we can highlight the fact that in the cartilaginous fish there is no larvae appearing, whereas in the bony fish there is a larva that later develops and forms the fry.

    We can also notice that cartilaginous fish have cloaca, differently from bony fish.

    Another difference concerns the swim bladder, a structure that assists in the flotation of the fish. This structure is found only in bony fish.

    We can mention as examples of cartilaginous fish the shark, ray and the cation. Among the bony fish, we can mention the catfish, painted and carp.

    Diplobacilli (diplo-bacilli): This is the name given to rod-shaped bacteria that remain in pairs following cell division. They divide by binary fission and are joined end to end.

    Diplobacteria (diplo-bacteria): Diplobacteria is the general term for bacteria cells that are joined in pairs.

    Diplobiont (diplo-biont): A diplobiont is an organism, such as a plant or fungus, that has both haploid and diploid generations in its life cyle.

    Diploblastic (diplo-blastic): This term refers to organisms that have body tissues that are derived from two germ layers: the endoderm and ectoderm. Examples include cnidarians: jellyfish, sea anemones, and hydras.

    Diplocardia (diplo-cardia): Diplocardia is a condition in which the right and left halves of the heart are separated by a fissure or groove.

    Diplocardiac (diplo-cardiac): Mammals and birds are examples of diplocardiac organisms. They have two separate circulatory pathways for blood: pulmonary and systemic circuits.

    Diplocephalus (diplo-cephalus): Diplocephalus is a condition in which a fetus or conjoined twins develop two heads.

    Diplochory (diplo-chory): Diplochory is a method by which plants disperse seeds. This method involves two or more distinct mechanisms.

    Diplococcemia (diplo-cocc-emia): This condition is characterized by the presence of diplococci bacteria in the blood.

    Diplococci (diplo-cocci): Spherical or oval-shaped bacteria that remain in pairs following cell division are called diplococci cells.

    Diplocoria (diplo-coria): Diplocoria is a condition that is characterized by the occurrence of two pupils in one iris. It may result from eye injury, surgery, or it may be congenital.

    Diploe (diploe): Diploe is the layer of spongy bone between the inner and outer bone layers of the skull.

    Diploid (diplo-id): A cell that contains two sets of chromosomes is a diploid cell. In humans, somatic or body cells are diploid. Sex cells are haploid and contain one set of chromosomes.

    Diplogenic (diplo-genic): This term means producing two substances or having the nature of two bodies.

    Diplogenesis (diplo-genesis): The double formation of a substance, as seen in a double fetus or a fetus with double parts, is known as diplogenesis.

    Diplograph (diplo-graph): A diplograph is an instrument that can produce double writing, such as embossed writing and normal writing at the same time.

    Diplohaplont (diplo-haplont): A diplohaplont is an organism, such as algae, with a life cycle that alternates between fully developed haploid and diploid forms.

    Diplokaryon (diplo-karyon): This term refers to a cell nucleus with double the diploid number of chromosomes. This nucleus is polyploid meaning that it contains more than two sets of homologous chromosomes.

    Diplont (diplo-nt): A diplont organism has two sets of chromosomes in its somatic cells. Its gametes have a single set of chromosomes and are haploid.

    Diplopia (diplo-pia): This condition, also known as double vision, is characterized by seeing a single object as two images. Diplopia can occur in one eye or both eyes.

    Diplosome (diplo-some): A diplosome is a pair of centrioles, in eukaryotic cell division, that aids in spindle apparatus formation and organization in mitosis and meiosis. Diplosomes are not found in plant cells.

    Diplozoon (diplo-zoon): A diplozoon is a parasitic flatworm that fuses together with another of its kind and the two exist in pairs.

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