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Structure and Senses of Fish | FN0044 |
Fisheries Victoria, Melbourne
Updated: March 2007
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How do fish cope with their environment? How do they breathe, why don't they sink, do they feel pain, do they distinguish colours, how do they see the world? Every fisherman has asked questions like these at one time or another. This Information Note is designed to answer some of these questions, and to explain how fish survive so well in what appears to be a harsh and hostile environment.
It is difficult to describe a "typical" fish. More than half of all species of vertebrates (animals with backbones) are fish and they have taken on a variety of shapes, some of them bizarre to say the least.
The herring, Clupea harengus has been chosen for description because it is one of the most abundant food fish in the world, lives in the open ocean (as do a large percentage of the world's fish species) and has a comparatively simple shape.
The external structure offers little resistance to the water. The body is spindle shaped, somewhat heavier toward the front than the rear and the cross-section is elliptical. The head is integral with the body. Also the body is generally free of projections that might offer resistance. The eyes are smooth and do not extend beyond the contours of the head; the gill opening is covered by a smooth flap (operculum); and the scales lie closely against the body surface.
Resistance is lessened further by an overall coating of slime (which also forms a barrier to bacterial infections). Of course the fins protrude but these stabilise the fish. They can be folded along the body during rapid swimming and act as brakes when erected.
Fins are of two general types, paired or unpaired (and median). The paired pectoral and pelvic (ventral) fins correspond to the fore and hind limbs of terrestrial vertebrates.
The unpaired fins are the dorsal (back), caudal (tail) and anal (belly) fins. Fish never have more than two pairs of paired fins (some only have one pair) but the number of dorsal and anal fins varies widely.
In the herring the fins are supported by 'soft' rays but in many species (such as the perches which are more representative of Australian fish - Murray Cod, Macquarie Perch) the front part of the dorsal and anal fins and the outer parts of paired fins are supported by bony spines. These spines give the fins greater rigidity and also might be used for defence or offence.
Skin
The streamlining of the fish is carried over to the skin, which probably fits more closely than the skins of other vertebrates. The skin is a relatively impervious, tough and elastic protective covering. This protection is even more effective because of the scales characteristic of fish (the absence of scales in some species can be regarded as a special development). Scientists can age some species of fish by counting the 'annulus' or year-mark on scales.
The skin holds certain sense organs, numerous glands, and colour cells responsible for the intricate patterns many fish display. Also a waste product called guanin is deposited on the skin and because it can reflect light produces white, silver or occasionally iridescent effects.
Skeletal System
The skeleton of a fish may consist of actual bones or may be cartilage. The major divisions are: the backbone and associated structures (ribs, unpaired fins and tail); the girdles (pectoral and pelvic) and attached paired fins; and the skull, including the supporting structure of the gill cover.
The skeleton supports the body, affords protection (the cranium protects the brain and the vertebrae protect the spinal cord), offers surfaces for attachment of muscles, and provides leverage for movement. Because of the supporting effect of water the last two functions are of less significance among fish than among terrestrial vertebrates. | |
Musculature
The absence of such complicated appendages as legs and wings allows fish to have a primitive arrangement of muscles down each side of the body in a series of definite and similar segments. In most fish these vertical segments are divided into dorsal (upper) and ventral (lower) sections by the lateral line.
Fish also have various specialised muscles such bas those which move the jaws, gillcovers and fins. Also there are so-called 'smooth' muscles that are essentially part of certain organs (the wall of the digestive tract for instance), and the cardiac muscles of the heart.
Respiration
Most fish get their necessary oxygen through gills. Each of the gill filaments, which are attached to the outer curve of the gill arches, is richly supplied with blood vessels.
As the water passes over the gills, carbon dioxide and other wastes are discharged from the blood and oxygen dissolved in the water is absorbed into the blood stream through the delicate membrane of the filaments.
The swim-bladder may have developed originally as an organ of respiration; it still has that function in some primitive fish. However the swim-bladder in most fish helps them stay afloat. Sharks, which do not have a swim-bladder, sink to the bottom if they stop swimming.
Nervous System
In comparison with higher vertebrates a fish's nervous system is poorly developed. The brain is extremely small in relation to body size. The lack of 'grey matter' is particularly appalling in the bony fish (such as herring and perch) because in this group the cerebrum, traditional centre of thought and reason, is almost totally lacking.
Also fish have relatively few, poorly-developed nerves. Therefore their ability to experience sensations such as pain would be diminished in comparison to higher vertebrates.
Circulation
In the higher vertebrates two chambers of the heart (one auricle and one ventricle) are concerned with pumping blood from the heart to the lungs and two with distribution of the oxygenated blood to the various body parts.
However fish get by with just a ventricle and auricle. The blood is pumped forward by the heart to the base of the gills, passes through the capillaries of the gill filaments, and is then distributed to the body tissues through arteries and capillaries. Blood collected by other capillaries returns to the heart through the veins.
Digestive Tract
The digestive system consists of the mouth, gullet, stomach, intestines, pancreas and liver. Size and position of the mouth vary widely with the feeding habits of the fish. Bottom-feeding fish have a mouth turned downward. When the main food is found in the open water the mouth usually is terminal (pointing directly forward). If a fish feeds mainly on the surface the mouth may slope sharply upward (as in the spotted barramundi or saratoga). Shape and spacing of the teeth also vary. Predatory fish usually have numerous, strong teeth on the jaws, as well as other parts of the mouth and pharynx. Some species have teeth shaped for crushing or grinding and others have no teeth at all.
Collection of food is helped in some species by the gill rakers (attached to the inside curve of the gill arches) which are modified to form a comb-like structure that strains small particles from the water.
The rest of the alimentary tract is straightforward, except for the tube-like sacs attached to the stomach near its exit. These are called pyloric caeca but their exact function is unknown. Some fish have none at all which others have considerable numbers (the mackerel has nearly 200). | |
Reproduction
The ovaries (one or more commonly two) in the female fish lie in the upper part of the body cavity, more or less parallel to the kidneys. In most fish the eggs are first discharged into a hollow central cavity of the ovary and then passed to the exterior through special ducts.
In certain fish which bear live young (sharks, for instance) the terminal position of the ducts may be expanded to accommodate the developing offspring. In other viviparous fish (such as mosquito fish) the young develop in the ovary itself.
The number and size of eggs vary enormously from species to species. Pelagic fish that spawn in the open sea produce the most. Nest builders usually produce fewer eggs than 'wild spawners', while in viviparous species the number may be small (only 4 to 14 eggs for one of the rays). Eggs of the pelagic fish are naturally small but in some sharks they are bigger than ostrich eggs.
The testes of the male fish are in a similar position in the body cavity to the ovaries of the female and like them have special ducts to take the sex products from the body. Males of viviparous species have special organs (developed from the pelvic or anal fin) for internal fertilisation of the eggs.
Smell
The olfactory organs consist of deep pits lined with special tissue. The size and position on the head of the these organs vary widely. The use of this olfactory sense varies not only with the species but also with the conditions. English experiments showed that pollock that were not particularly hungry usually smelt food before taking it.
However when ravenous the same fish bolted down clams soaked in such obnoxious substances as turpentine and chloroform.
Sight
General structure of a fish's eye is similar to that of other vertebrates. However there are certain modifications for seeing under water. The outer wall of the eye is flatter in fish than in land vertebrates. The lens itself is much more rounded.
Fish focus their eyes not by changing the shape of the lens (as terrestrial vertebrates do) but rather by shifting its position. There is evidence that fish are comparatively near-sighted but experiments have proved they can distinguish colours. Eyes tend to be small and inefficient in species that live regularly turbid water and may be lacking altogether in fish in underground waters.
Hearing
In fish and other vertebrates the ear is an organ of equilibrium as well as hearing. The part concerned with hearing lacks the intricate internal structure found in higher vertebrate. This and supporting experimental evidence suggest fish do not hear at all in the ordinary sense. Their "hearing" probably consists of little more than the detection of vibrations in the water. In many fish the ear is connected to the swim bladder by a tubelike growth from the latter or by a series of small bones.
It is possible this intensifies the impulses from vibrations in the water. Another structure that might help is the lateral line organ which experiments indicate might be capable of detecting low-frequency vibrations (about six a second).
Taste
Little is known about the sense of taste in fish. In fact there is some question as whether this sense exists in most species. Many of the tasting functions are performed by organs distributed over the body or on barbels.
Touch
Touch is probably the most highly developed sense fish possess. Sense organs in the form of buds or small pits and in contact with nerves are distributed over the entire body. They are particularly numerous in strategic positions such as the surface of feelers and barbels. The extent to which fish feel pain has long been debated. Although no one will ever know how a fish feels when hooked there is ample evidence that the experience is not disturbing enough to halt feeding activities.
It is not uncommon for fish that have escaped before being landed, or released, to take the hook again immediately afterwards. In Australia the tailor is a prime example. Specimens have been caught with two gleaming sets of hooks already in their jaws.
Acknowledgements
This Information Note was developed by Charles Barnham PSM, based largely on a paper written by well-known American biologist Ralph Hile, US Bureau of Fisheries and National Marine Fisheries Service 1930-1970. The original paper appeared in the Marine Fisheries Review (Vol. 35, nos. 5-6) .The article reproduced here was published in Australian Fisheries, March 1976. The previous version of this Information Note was published in March 1998.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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