Fish anatomy

***Fish anatomy*** is the study of the form or ***morphology*** of ***fishes.***

It can be contrasted with ***fish physiology,*** which is the study of how the component parts of ***fish*** function together in the living fish.

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 denser than air, holds a relatively small amount of dissolved ***oxygen***, and absorbs more light than air does.

The body of a fish is divided into a head, trunk and tail, although the divisions between the three are not always externally visible.

The skeleton, which forms the support structure inside the fish, is either made of cartilage, in ***cartilaginous fish***, or bone in ***bony fish.***

The main skeletal element is the vertebral column, composed of articulating ***vertebrae***which are lightweight yet strong.

The ribs attach to the spine and there are no***limbs*** or limb girdles.

The main external features of the fish, the ***fins,*** are composed of either bony or soft spines called rays which, with the exception of the ***caudal fins***, have no direct connection with the spine.

They are supported by the muscles which compose the main part of the trunk.

The heart has two chambers and pumps the blood through the respiratory surfaces of the ***gills***and on round the body in a single circulatory loop.

The eyes are adapted for seeing underwater and have only local vision.

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, and these respond to nearby movements and to changes in water pressure.

Sharks and rays are***basal*** fish with numerous ***primitive*** anatomical features similar to those of ancient fish, including skeletons composed of cartilage.

Their bodies tend to be *dorso-ventrally* flattened, they usually have five pairs of gill slits and a large mouth set on the underside of the head.

The dermis is covered with separate dermal ***placoid scales***.

They have a ***cloaca*** into which the urinary and genital passages open, but not a ***swim bladder.***

Cartilaginous fish produce a small number of large, ***yolky***eggs.

Some species are ***ovoviviparous*** and the young develop internally but others are ***oviparous*** and the larvae develop externally in egg cases.

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.

## **Body:**

In many respects fish anatomy is different from humans and mammals, yet it shares the same basic vertebrate ***body plan*** from which all vertebrates have evolved: a ***notochord,*** rudimentary ***vertebrae***, and a *well-defined **head*** and ***tail.***

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*).

Also, fish are often either compressed (*laterally thin)* or depressed (*dorso-ventrally flat)*.

## **Skeleton:**

**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 main features of the fish, the fins, are 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 **functions.**

## **Vertebrae:**

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*)**, 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 continuing aft of the anus forms a ***tail*** with vertebrae and spinal cord, but no gut.

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 lost this anatomy, retaining the notochord into adulthood, such as the***sturgeon.***

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 ***hemal arch*** or ***chevron*** is found underneath the centrum in the ***caudal**(tail*) vertebrae of fish.

The centrum of a fish is usually concave at each end (*amphicoelous)*, which limits the motion of the fish.

This can be contrasted with the centrum of a mammal, which is flat at each end (*acoelous*), shaped in a manner 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 of broadly similar form to that found in most other vertebrates.

Just beneath the arch lies a small *plate-like* ***pleurocentrum***, which protects the upper surface of the ***notochord***, and below that, a larger *arch-shaped* *intercentrum* to protect 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 (*and hence, also to mammals and birds*), the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became the bony vertebral body.

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.

In ***cartilagenous fish***, such as***sharks***, the vertebrae consist of two cartilagenous tubes.

The upper tube is formed from the vertebral arches, but also includes additional cartilagenous structures filling in the gaps between the vertebrae, and so 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.***

***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.

Hagfishes do, however, possess a ***cranium.***

For this reason, the vertebrate subphylum is sometimes referred to as "***Craniata**"* when discussing morphology.

Molecular analysis since 1992 has suggested that the hagfishes are most closely related to lampreys, and so also are vertebrates in a ***monophyletic*** sense.

Others consider them a sister group of vertebrates in the common taxon of Craniata.

## **Head:**

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 preopercle.

The operculum and preopercle may or may not have spines.

In sharks and some primitive ***bony fish*** a ***spiracle***, 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.***

***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.***

There are, in addition, at various points throughout the cranium, smaller ***foramina*** for the cranial nerves.

The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper.

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 symplectic, linking the jaw to the rest of the cranium.

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 ***pterygoids*** and ***vomers*** alone, all of which bear teeth.

Much of the skull is formed from ***cartilage***, and its overall structure is reduced.

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.

## **External organs:**

### **Main article: *Fish jaw.***

**See also**:***Shark tooth.***

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 (*the **Gnathostomes**)*, which have seven arches, and primitive jawless vertebrates *(the **Agnatha**)*, which have nine.

It is thought that the original selective advantage garnered by the jaw was not related to feeding, but to increased 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 in the case of amphibians.

Over evolutionary time the more familiar use of jaws (*to humans*), 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, who also designed a new classification system, which is especially well suited for biological systems.

Linkage mechanisms are especially frequent and manifold in the head of bony fishes, such as ***wrasses,*** which have ***evolved*** many specialized ***feeding mechanisms.***

Especially advanced are the linkage mechanisms of ***jaw protrusion.***

For ***suction feeding*** a system of linked *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:**

**See also**:***Vision in fish.***

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.***

The ancestors of modern hagfish, thought to be the protovertebrate 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 *eye-spot*, 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.***

### **Gills:**

**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 ***frilled shark.***

The***labyrinth organ***of ***Anabantoidei***and ***Clariidae*** is used to allow the fish to extract oxygen from the air.

***Gill rakers*** are bony or cartilaginous, *finger-like* projections off the gill arch which function in *filter-feeders* to retain filtered prey.

### **Skin:**

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 ***poison glands,*** ***photophores***, or ***cells***that produce a more watery, ***serous*** fluid.

***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.***

### **Scales:**

**Main article**: ***Fish scale.***

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.

1. Placoid 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.

2. Ganoid scales are flat, *basal-looking* scales that cover a fish body with little overlapping.

They are typical of gar and bichirs.

3.Cycloid scales are small oval-shaped scales with growth rings.

Bowfin and remora have cycloid scales.

4. Ctenoid scales are similar to the cycloid scales, with 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 is:

* An external *shield-like* bony plate, or
* a modified, thickened scale that often is keeled or spiny, or
* a projecting, modified (*rough and strongly ridged*) scale, usually associated with the lateral line, or on the caudal peduncle forming caudal keels, or along the ventral profile.

Some fish, such as pineconefish, are completely or partially covered in scutes.

### **Lateral line:**

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 appears 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 associated with symbiotic bacteria, and are used for attracting food or confusing predators.

### **Fins:**

Main article: Fish fin.

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*frog-fish.*

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.

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.

Types of fin

* Dorsal fins are located on the back.

Most fishes have one dorsal fin, but some fishes have two or three.

The dorsal fins serve to protect the fish against rolling, and assists in sudden turns and stops.

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 bones that support the dorsal fin are called *Pterygiophore*.

There are two to three of them: "*proximal*", "*middle*", and "*distal"*.

In spinous fins the distal is often fused to the middle, or not present at all.

* The caudal fin is the tail fin, located at the end of the caudal peduncle and is used for propulsion.

The caudal peduncle is the narrow part of the fish's body to which the caudal or tail fin is attached.

The hypural joint is the joint between the caudal fin and the last of the vertebrae.

The hypural is often *fan-shaped*.

The tail is called:

* *Heterocercal* if the vertebrae extend into the upper lobe of the tail, making it longer (*as in sharks).*
* *Reversed heterocercal* if the vertebrae extend into the lower lobe of the tail, making it longer (*as in the Anaspida).*
* *Protocercal* if the vertebrae extend to the tip of the tail and the tail is symmetrical but not expanded (*as in amphioxus).*
* *Diphycercal* if the vertebrae extend to the tip of the tail and the tail is symmetrical and expanded (*as in the bichir, lungfish, lamprey and coelacanth).*
* *Most Palaeozoic fishes had a diphycercal heterocercal tail.*
* Most fish have a *homocercal* tail, where the fin appears superficially symmetric but the vertebrae extend for a very short distance into the upper lobe of the fin.

This can be expressed in a variety of shapes.

The tail fin can be:

* Rounded at the end.
* Truncated: or end in a *more-or-less* vertical edge, such as in salmon.
* Forked: or end in two prongs.
* Emarginate: or with a slight inward curve.
* Continuous: with dorsal, caudal and anal fins attached, such as in eels.
* The anal fin is located on the ventral surface behind the anus.

This fin is used to stabilize the fish while swimming.

* The paired pectoral fins are located on each side, usually just behind the operculum, and are homologous to the forelimbs of tetrapods.

A peculiar function of pectoral fins, highly developed in some fish, is the creation of the dynamic lifting force that assists some fish, such as sharks, in maintaining depth and also enables the "*flight"* for *flying-fish*.

In many fish, the pectoral fins aid in walking, especially in the *lobe-like* fins of some anglerfish and in the mudskipper.

Certain rays of the pectoral fins may be adapted into *finger-like* projections, such as in sea robins and flying gurnards.

The "*horns*" of manta rays and their relatives are called *cephalic fins*; this is actually a modification of the anterior portion of the pectoral fin.

* The paired pelvic or ventral fins are located ventrally below the pectoral fins.

They are homologous to the hindlimbs of tetrapods.

The pelvic fin assists the fish in going up or down through the water, turning sharply, and stopping quickly.

In gobies, the pelvic fins are often fused into a single sucker disk.

This can be used to attach to objects.

* The adipose fin is a soft, fleshy fin found on the back behind the dorsal fin and just forward of the caudal fin.

It is absent in many fish families, but is found in Salmonidae, characins and catfishes.

Its function has remained a mystery, and is frequently clipped off to mark *hatchery-raised* fish, though data from 2005 showed that trout with their adipose fin removed have an 8% higher tailbeat frequency.

Additional research published in 2011 has suggested that the fin may be vital for the detection of and response to stimuli such as touch, sound and changes in pressure.

Canadian researchers identified a neural network in the fin, indicating that it likely has a sensory function, but are still not sure exactly what the consequences of removing it are.

* Some types of f*ast-swimming* fish have a horizontal caudal keel just forward of the tail fin.

Much like the keel of a ship, this is a lateral ridge on the caudal peduncle, usually composed of scutes (*see below*), that provides stability and support to the caudal fin.

There may be a single paired keel, one on each side, or two pairs above and below.

* Finlets are small fins, generally between the dorsal and the caudal fins also between the anal fin and the caudal fin *(in bichirs, there are only finlets on the dorsal surface and no dorsal fin)*.

In some fish such as tuna or sauries, they are rayless, *non-retractable,* and found between the last dorsal and/or anal fin and the caudal fin.

## **Internal organs:**

***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.

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.

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.

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.

### **Pyloric caeca:**

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.

Many fish in addition have a number of small outpocketings, also called pyloric caeca, along their intestine; despite the name they are not homologous with 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.

### **Stomach:**

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, or no *pre-digestion* with gastric juices, or both.

### **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.

***Hagfish*** and ***lamprey*** kidneys are unusually simple.

They consist of a row of nephrons, each emptying directly into the mesonephric duct.

### **Spleen:**

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 regard to***red blood cells*** and the***immune system.***

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.***

The only vertebrates lacking a spleen are the lampreys and hagfishes.

Even in these animals, there is a diffuse layer of haematopoeitic tissue within the gut wall, which has a similar structure to red pulp, and is presumed to be ***homologous*** with the spleen of higher vertebrates.

### **Liver:**

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.***

### **Heart:**

Fish have what is often described as a *two-chambered* ***heart,*** consisting of one ***atrium*** to receive blood and one ***ventricle*** to pump it, 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.

However, the fish heart has entry and exit compartments that may be called chambers, so it is also sometimes described as *three-chambered*or *four-chambered*, depending on what is counted as a chamber.

The atrium and ventricle are sometimes considered “*true chambers*”, while the others are considered “*accessory chambers*”.

The four compartments are arranged sequentially:

* ***Sinus venosus***, a *thin-walled* sac or reservoir with some cardiac muscle that collects deoxygenated blood through the incoming ***hepatic*** and ***cardinal veins***.
* Atrium, a *thicker-walled*, muscular chamber that sends blood to the ventricle.
* Ventricle, a *thick-walled*, muscular chamber that pumps the blood to the fourth part, the outflow tract.

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.

* The outflow tract (*OFT*) to the ventral aorta, consisting of the tubular ***conus arteriosus, bulbus arteriosus,*** or both.

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.

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

The ostial valve between the sinus venosus and atrium is called the *sino-atrial* valve, which closes during ventricular contraction.

Between the atrium and ventricle is an ostial valve called the *atrio-ventricular* valve, and between the bulbus arteriosus and ventricle is an ostial valve called the *bulbo-ventricular* valve.

The conus arteriosus has a variable number of semilunar valves.

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 has divided in two; one half forms the **ascending aorta**, while the other forms the **pulmonary artery***).

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.

In the adult fish, the four compartments are not arranged in a straight row but, instead form 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.

### **Swim bladder:**

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 minnows, 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.

The condition of a bladder open to the esophagus is called physostome, the closed condition physoclist.

In the latter, the gas content of the bladder is controlled through a rete mirabilis, a network of blood vessels effecting gas exchange between the bladder and the blood.

### **Weberian apparatus:**

Fishes of the superorder Ostariophysi possess a structure called the Weberian apparatus, a modification which allow them to hear better.

This ability which may well explain the marked success of otophysian fishes.

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.

The ossicles connect the gas bladder wall with *Y-shaped* lymph sinus that abuts the *lymph-filled* transverse canal joining the sacculi 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.

## **Reproductive organs:**

**Main article: Fish reproduction.**

**See also: Reproductive processes in fish.**

Fish reproductive organs include testes and ovaries.

In most species, gonads are paired organs of similar size, which can be partially or totally fused.

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.

### **Testes:**

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.

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.

In terms of spermatogonia distribution, the structure of teleosts testes has two types: in the most common, spermatogonia occur all along the seminiferous tubules, while in Atherinomorph fish they are confined to the distal portion of these structures.

Fish can present cystic or semi-cystic spermatogenesis in relation to the release phase of germ cells in cysts to the seminiferous tubules lumen.

### **Ovaries:**

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.

The ovary of teleosts is often contains a hollow, lymph-filled space which opens into the oviduct, and into which the eggs are shed.

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.

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.

Gymnovaries are the primitive condition found in lungfish, sturgeon, and bowfin.

Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct.

Secondary gymnovaries are found in salmonids and a few other teleosts.

### **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.

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.

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.

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.

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.

The pineal body lies just above the diencephalon.

This structure detects light, maintains circadian rhythms, and controls color changes.

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.

The hindbrain or metencephalon is particularly involved in swimming and balance.

The cerebellum is a single-lobed structure that is typically the biggest part of the brain.

Hagfish and lampreys have relatively small cerebellae, while the mormyrid cerebellum is massive and apparently involved in their electrical sense.

The brain stem or myelencephalon is the brain's posterior.

As well as controlling some muscles and body organs, in bony fish at least, the brain stem governs respiration and osmoregulation.

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.

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), further differentiated in the various vertebrate groups.

Two laterally placed eyes form around outgrows from the midbrain, except in hagfish, though this may be a secondary loss.

The forebrain is well developed and subdivided in most tetrapods, while the midbrain dominate in many fish and some salamanders.

Vesicles of the forebrain are usually paired, giving rise to hemispheres like the cerebral hemispheres in mammals.

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.

### **Cerebellum:**

The circuits in the cerebellum are similar across all classes of vertebrates, including fish, reptiles, birds, and mammals.

There is also an analogous brain structure in cephalopods with well-developed brains, such as octopuses.

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.

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 mormyrid fish (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.

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.

### **Identified neurons**:

A neuron is called *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.

In vertebrate nervous systems very few neurons are "identified" in this sense—in humans, there are believed to be none—but in simpler nervous systems, some or all neurons may be thus unique.

In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish.

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.

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.

## **Immune system:**

See also: Fish diseases and parasites.

Immune organs vary by type of fish.

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 tissue 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 (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; where erythrocytes, granulocytes, lymphocytes and macrophages develop.

Like chondrostean fish, the major immune tissues of bony fish (or teleostei) include the kidney (especially the anterior kidney), which houses many different immune cells.

In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (e.g. in the skin, gills, gut and gonads).

Much like the mammalian immune system, teleost erythrocytes, neutrophils and granulocytes are believed to reside in the spleen whereas lymphocytes are the major cell type found in the thymus.

In 2006, a lymphatic system similar to that in mammals was described in one species of teleost fish, the zebrafish.

Although not confirmed as yet, this system presumably will be where naive (unstimulated) T cells accumulate while waiting to encounter an antigen.

**Gallery *Slide-Show*Images:**

*** External anatomy of a bony fish (Hector's lantern-fish) 1. Operculum (gill cover). 2. Lateral line. 3. Dorsal fin. 4. Adipose fin, 5. Caudal peduncle. 6. Caudal fin. 7. Anal fin. 8. Photophores. 9. Pelvic fins (paired). 10. Pectoral fins (paired).
* Internal anatomy of a bony fish.
* Anatomical directions and axes.
* Skeleton of a bony fish.
* Skeletal structure of a bass showing the vertebral column running from the head to the tail.
* Skeletal structure of an Atlantic cod.
* The X-ray tetra (Pristella maxillaris) has a visible backbone. The spinal cord is housed within its backbone.
* A vertebra (diameter 5 mm) of a small ray-finned fish.
* Skull bones as they appear in a seahorse.
* Positions of fish mouths: (a) - terminal, (b) - superior, (c) - subterminal, inferior.
* Barbels.
* Skull of a northern pike.
* Skull of Tiktaalik, a genus of extinct sarcopterygian (lobe-finned "fish") from the late Devonian period.
* Jaws of great white shark.
* 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.
* Zenion hololepis is a small deep water fish with large eyes.
* The deep sea half-naked hatchetfish has eyes which look overhead where it can see the silhouettes of prey.
* Gill of a rainbow trout.
* Cycloid scales covering rohu.
* The lateral line is clearly visible as a line of receptors running along the side of this Atlantic cod.
* The haddock, a type of cod, is ray-finned. It has three dorsal and two anal fins.
* Types of caudal (tail) fin: (A) - Heterocercal, (B) - Protocercal, (C) - Homocercal, (D) - Diphycercal.
* Sharks possess a heterocercal caudal fin. The dorsal portion is usually larger than the ventral portion.
* The high performance big-eye tuna is equipped with a homocercal caudal fin and finlets and keels.
* Internal organs of a male yellow perch A = gill, B = heart atrium, C: heart ventricle, D: liver (cut), E: stomach, F = pyloric caeca, G = swim bladder, H = intestine, I = testis, J = urinary bladder.
* The black swallower is a species of deep sea fish with an extensible stomach which allows it to swallow fish larger than itself.
* Internal organs of a female Atlantic cod 1. Liver, 2. Gas bladder, 3. ovary, 4. Pyloric caeca, 5. Stomach, 6. Intestine.
* Blood flow through the heart: sinus venosus, atrium, ventricle, and outflow tract.
* Cardiovascular cycle in a fish.
* The swim bladder of a rudd.
* 7 = testicles or ovaries.
* Dorsal view of the brain of the rainbow trout.
* Cross-section of the brain of a porbeagle shark, with the cerebellum highlighted.
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