Fish anatomy

Source: Wikipedia, the free encyclopedia.

pectoral fins (paired)
Diagram showing the internal anatomy of a generic bony fish
Internal anatomy of a bony fish

Fish anatomy is the study of the form or morphology of fish. It can be contrasted with fish physiology, which is the study of how the component parts of fish function together in the living fish.[1] In practice, fish anatomy and fish physiology complement each other, the former dealing with the structure of a fish, its organs or component parts and how they are put together, such as might be observed on the dissecting table or under the microscope, and the latter dealing with how those components function together in living fish.

The anatomy of fish is often shaped by the physical characteristics of water, the medium in which fish live. Water is much

caudal fins, have no direct connection with the spine. They are supported by the muscles which compose the main part of the trunk.[2]
The heart has two chambers and pumps the blood through the respiratory surfaces of the gills and then around the body in a single circulatory loop.[3] The eyes are adapted for seeing underwater and have only local vision.[definition needed] There is an inner ear but no external or middle ear. Low-frequency vibrations are detected by the lateral line system of sense organs that run along the length of the sides of fish, which responds to nearby movements and to changes in water pressure.[2]

oviparous and the larvae develop externally in egg cases.[4]

The bony fish lineage shows more

derived anatomical traits, often with major evolutionary changes from the features of ancient fish. They have a bony skeleton, are generally laterally flattened, have five pairs of gills protected by an operculum, and a mouth at or near the tip of the snout. The dermis is covered with overlapping scales. Bony fish have a swim bladder which helps them maintain a constant depth in the water column, but not a cloaca. They mostly spawn a large number of small eggs with little yolk which they broadcast into the water column.[4]

Body

Anatomical directions and axes

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).

Skeleton

bony fish
See also: Fish bone

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 endoskeleton of the fish is made up of two main components: the axial skeleton consisting of the skull and vertebral column, and the appendicular skeleton supporting the fins.[7] 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
.

Vertebrae

Skeletal structure of a bass showing the vertebral column running from the head to the tail
Skeletal structure of an Atlantic cod
ray-finned fish

Fish are vertebrates. All vertebrates are built along the basic

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.[9]

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.[10]

The vertebral column consists of a

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

reptiles, mammals and birds, the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became the bony vertebral body.[11]

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.[11]

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.[11]

monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of Craniata.[14]

Head

Skull bones as they appear in a seahorse
Positions of fish mouths: terminal (a), superior (b), and subterminal or inferior (c).
Illustration of barbels on the head of a fish

The head or

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

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.[15]

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

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.[15]

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

sympletic, linking the jaw to the rest of the cranium.[15]

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.[15]

The head may have several fleshy structures known as

nares
of almost all fishes do not connect to the oral cavity, but are pits of varying shape and depth.

External organs

Jaw

Moray eels have two sets of jaws: the oral jaws that capture prey and the pharyngeal jaws that advance into the mouth and move prey from the oral jaws to the esophagus
for swallowing.
Main article: Fish jaw
See also: Shark tooth

The vertebrate jaw probably originally evolved in the

gnathostomes), which have seven arches, and primitive jawless vertebrates (the Agnatha), which have nine.[citation needed
]

A great white shark partially out of the water with its mouth open
Jaws of a great white shark
External videos
video icon Video of a slingjaw wrasse catching prey by protruding its jaw
video icon Video of a red bay snook catching prey by suction feeding

It is thought that the original selective advantage garnered by the jaw was not related to feeding, but to increase

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.

protrusion
of the premaxilla.

Eyes

half-naked hatchetfish
has eyes which look overhead where it can see the silhouettes of prey.
See also: Vision in fish

Fish eyes are similar to

polarized light. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots.[17] The ancestors of modern hagfish, thought to be protovertebrate,[18] 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.[19]

Gills

Gill of a rainbow trout
Main article: Fish gill
See also:
Fish respiration

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

filter feeders
to retain filtered prey. They may be bony or cartilaginous.

Skin

The skin of the fish are a part of the integumentary system, which contains two layers: the epidermis and the dermis layer. The epidermis is derived from the ectoderm and becomes the most superficial layer that consists entirely of live cells, with only minimal quantities of keratin. It is generally permeable. The dermis is derived from the mesoderm and resembles the little connective tissue which are composed of mostly collagen fibers found in bony fish. Some fish species have scales that emerge from the dermis, penetrate the thin layer of the basement membrane that lies between the epidermis and dermis, and becomes externally visible and covers the epidermis layer.[20]

Generally, the skin also contains

sebaceous glands that are both unique to mammals, but additional types of skin glands are found in fish. Found in the epidermis, fish typically have numerous individual mucus-secreting skin cells called goblet cells that produce a slimy substance to the surface of the skin. This aids in insulation and protection from bacterial infection.[21][22] The skin colour of many mammals are often due to melanin found in their epidermis. In fish, however, the colour of the skin are 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. Some fishes may also have venom glands, photophores, or cells that produce a more watery serous fluid in the dermis.[23][21][24]

Placoid scales from a shark: A. epidermis; B. bermis; C. pulp core; D. dentine; E. basal plate; F. enamel; G. spine

Scales

Main article: Fish scale

Also part of the fish's integumentary system are the scales that cover the outer body of many jawed fish. The commonly known scales are the ones that originate from the dermis or mesoderm, and may be similar in structure to teeth. Some species are covered by scutes instead. Others may have no scales covering the outer body.

  • Singular bowfin cycloid scale
    Singular bowfin cycloid scale
  • Cycloid scales covering rohu
    Cycloid scales covering rohu
  • Bowfin cycloid scales
    Bowfin cycloid scales

There are four principal types of fish scales that originate from the dermis.[25][26]

  • Fish scales: 1. cycloid scale; 2. ctenoid scale; 3. placcoid scale; 4. ganoid scale
    Fish scales: 1. cycloid scale; 2. ctenoid scale; 3. placcoid scale; 4. ganoid scale
  • Cycloid scale
    Cycloid scale
  • Fish scales: A. ganoid; B. cycloid; C. ctenoid
    Fish scales: A. ganoid; B. cycloid; C. ctenoid

Another less common type of scale is the

pineconefish
, are completely or partially covered in scutes. Their function are intended for protection as a body armor for fish against environmental abrasions and predations from other species.

Lateral line

Main article: Lateral line
The lateral line is clearly visible as a line of receptors running along the side of this Atlantic cod.

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

Main article:
Photophores

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

photocytes, or from symbiotic bacteria
. Photophores are used for attracting food or confusing predators.

Fins

Main article: Fish fin
The haddock, a type of cod, is ray-finned. It has three dorsal and two anal fins.
caudal fin
: heterocercal (A), protocercal (B), homocercal (C), and diphycercal (D)
ventral
portion.
The high performance[definition needed] bigeye tuna is equipped with a homocercal caudal fin, finlets and keels.

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

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.[27]

Types of fin

Internal organs

urinary bladder
intestine
deep sea fish
with an extensible stomach which allows it to swallow fish larger than itself.

Intestines

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.[31]
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
ileocaecal valve in teleosts, with the boundary between the small intestine and the rectum being marked only by the end of the digestive epithelium.[24] 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.[24] 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.[24]

Pyloric caeca

Many fish have a number of small outpocketings, called pyloric caeca, along their intestine. The purpose of the caeca is to increase the overall surface area of the intestines, thereby increasing the absorption of nutrients.[32][33]

The number of pyloric caeca varies widely between species, and in some species of fish no caeca are present at all. Species with few or no caeca compensate for their lack by having longer intestines, or by have taller or more convoluted intestinal villi, thereby achieving similar levels of absorptive surface area.[32][33]

Lungfish also have a pouch located at the beginning of their intestine, which is also called a pyloric

caecum, but it has a different structure and function that the pyloric caeca of other fish species. The lungfish caecum is homologous (due to common descent) with the caecum present in most amniotes (tetrapod vertebrates that include all mammals, reptiles, and birds).[32]
In most herbivores the caecum receives partially digested food from the small intestine, and serves as a fermentation chamber to break down cellulose (such as grass or leaves) in the diet. In carnivores the caecum is often greatly reduced or missing.

Stomach

As with other vertebrates, the relative positions of 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.[34]

Kidneys

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.[35] Hagfish and lamprey kidneys are unusually simple. They consist of a row of nephrons, each emptying directly into the mesonephric duct.[35] Like the Nile tilapia, the kidney of some fish shows its three parts; head, trunk, and tail kidneys.[36] 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.[36]

Urinary bladder

This section is an excerpt from Bladder § Fish.[edit]

The gills of most teleost fish help to eliminate ammonia from the body, and fish live surrounded by water, but most still have a distinct bladder for storing waste fluid. The urinary bladder of teleosts is permeable to water, though this is less true for freshwater dwelling species than saltwater species.[37]: p. 219  In freshwater fish the bladder is a key site of absorption for many major ions[38] in marine fish urine is held in the bladder for extended periods to maximise water absorption.[38] The urinary bladders of fish and tetrapods are thought to be analogous while the former's swim-bladders and latter's lungs are considered homologous.

Most fish also have an organ called a
pilchards, and herrings are among the few types of fish in which a urinary bladder is poorly developed. It is largest in those fish which lack an air bladder, and is situated in front of the oviducts and behind the rectum.[39]

Spleen

The

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.[41]

Liver

The liver is a large

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.[42]

Heart

Blood flow through the heart: sinus venosus, atrium, ventricle, and outflow tract
Cardiovascular cycle in a fish

Fish have what is often described as a two-chambered heart,[43] consisting of one atrium to receive blood and one ventricle to pump it,[44] 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.[43] However, the fish heart has entry and exit compartments that may be called chambers, so it is also sometimes described as three-chambered,[44] or four-chambered,[45] depending on what is counted as a chamber. The atrium and ventricle are sometimes considered "true chambers", while the others are considered "accessory chambers".[46]

The four compartments are arranged sequentially:

  1. hepatic and cardinal veins.[verification needed][44]
  2. Atrium: A thicker-walled, muscular chamber that sends blood to the ventricle.[44]
  3. Ventricle: A thick-walled, muscular chamber that pumps the blood to the fourth part, the outflow tract.[44] 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.[45]
  4. Outflow tract (OFT): Goes to the ventral aorta and consists of the tubular
    conus arteriosus, bulbus arteriosus, or both.[45] 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.[46][47]

Ostial valves, consisting of flap-like connective tissues, prevent blood from flowing backward through the compartments.

semilunar valves.[46]

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).[41]

The circulatory systems of all vertebrates are

capillaries of the body tissues. This is known as single cycle circulation.[48]

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.[41]

Swim bladder

Main article: Swim bladder
rudd

The swim bladder or gas bladder is an internal organ that contributes to the ability of a fish to control its

rete mirabilis, a network of blood vessels affecting gas exchange between the bladder and the blood.[49]

Weberian apparatus

Fishes of the

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.[51]

Reproductive organs

Main article: Fish reproduction
See also: Reproductive processes in fish
Testicles or ovaries (7)

Fish reproductive organs include

haploid spermatids. During spermiogenesis, the last stage of spermatogenesis, the haploid spermatids develop into spermatozoa.[54] In the ovaries, oogonia also undergo mitosis and meiosis during oogenesis, and this gives rise to primary oocytes and then eventually the ovum. The primary oocyte divides and produces the secondary oocyte as well as a polar body, before the secondary oocyte develops into the haploid ootid.[55]

1) Sagittal view of the anus and urogenital opening. 2) Ventral view of the anus and urogenital opening.

Testes

Maturity stages of (A) ovaries and (B) testicles of the cichlid Crenicichla menezesi:[56] (a) immature; (b) maturing; (c) mature; (d) partially spent

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.[41]

Under a tough membranous shell, 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.[57]

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.[52]

Ovaries

Many of the features found in ovaries are common to all vertebrates, including the presence of

elasmobranch fish; in other species, the remnants of the follicle are quickly resorbed by the ovary.[57] The ovary of teleosts is often contains a hollow, lymph-filled space which opens into the oviduct, and into which the eggs are shed.[57] 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.[57]

Fish ovaries may be of three types: gymnovarian, secondary gymnovarian or cystovarian. In the first type, the

salmonids
and a few other teleosts.

Nervous system

Anatomical diagram showing the pairs of olfactory, telencephalon, and optic lobes, followed by the cerebellum and the myelencephalon
Dorsal view of the brain of the rainbow trout
See also: Brain § Vertebrates

Central nervous system

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.

mormyrids and sharks, which have brains about as massive relative to body weight as birds and marsupials.[60]

Fish brains are divided into several regions. At the front are the

olfaction.[59] Together these structures form the forebrain
.

The forebrain is connected to the

mesencephalon contains the two optic lobes. These are very large in species that hunt by sight, such as rainbow trout and cichlids.[59]

The hindbrain or metencephalon is particularly involved in swimming and balance.[59] The cerebellum is a single-lobed structure that is typically the biggest part of the brain.[59] Hagfish and lampreys have relatively small cerebella, while the mormyrid cerebellum is massive and apparently involved in their electrical sense.[59]

The

brain stem or myelencephalon is the brain's posterior.[59] As well as controlling some muscles and body organs, in bony fish at least, the brain stem governs respiration and osmoregulation.[59]

Vertebrates are the only chordate group to exhibit a proper brain. A slight swelling of the anterior end of the

rhombencephalon (hindbrain) then further differentiated in the various vertebrate groups.[61] Two laterally placed eyes form around outgrows from the midbrain, except in hagfish, though this may be a secondary loss.[62][63] 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.[61] 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.[9]

porbeagle shark
with the cerebellum highlighted

Cerebellum

The circuits in the cerebellum are similar across all classes of vertebrates, including fish, reptiles, birds, and mammals.[64] There is also an analogous brain structure in cephalopods with well-developed brains, such as octopuses.[65] 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

vestibulocerebellum.[57]

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

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.[66]

Most species of fish and amphibians possess a lateral line system that senses

optic tectum has a layer—the marginal layer—that is cerebellum-like.[64]

Identified neurons

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.[67] 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.[68]

In vertebrates, the best known identified neurons are the gigantic

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.[70] 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.[71]

Immune system

See also: Fish diseases and parasites

Immune organs vary by type of fish.

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

lymphocytes
, plasma cells and macrophages develop and are stored.

hemopoietic
organ; it is where erythrocytes, granulocytes, lymphocytes and macrophages develop.

Like chondrostean fish, the major immune tissues of bony fish (

naive T cells accumulate while waiting to encounter an antigen.[76]

See also

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Works cited

External links

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