Every fish culturist should know the main characteristics of fish and their way of life. The knowledge enables to understand the relations existing between the fish and the environment and the way they can reap advantage from it. The fish will be first studied and then its environment will be described.
THE FISH
First anatomy, i.e. the main parts of its body, and fish biology, i.e. the way it is living, will be successively studied.
FISH ANATOMY – The Basic Structure of a Fish
Like most animals, the fish has a body which includes the head, the trunk and the limbs. The body has generally an elongated shape. The head, the trunk and the tail follow each other without any separation, which enables the fish to wriggle easily in the water. Limbs are fins. The body is covered with the skin. The skin itself is covered by a layer of mucus which forms a protective sheath.
If a fish is caught and handled with dry hands or roughly, mucus is removed and the skin remains exposed and may be injured by parasites, always numerous in the water. Fish, therefore should always be handled gently, with care and as little as possible. Most fish, except for instance catfish (Siluriformes), have scales under their skin, which cover each other like the tiles on a roof. The number of the scales covering the body of a fish varies only a little and can be used for identifying the different species. Running along the
side of the body, it a line of scales which have little pits in them; this line is called the lateral line. The pits are sensory organs and are responsive to changes in pressure in the surrounding water. In some fish there is a continuous line from the gill cover to the tail, in others it is interrupted.
Others, again (Cichlid fish) have two lines running along each side of the body: an upper line going backward from the gill cover to about three quarters of the way, and a lower line running forward for a quarter from the center of the tail. The body of a fish is supported by a skeleton which is a bony frame. The skeleton keeps up the shape of the fish, acts as a muscle-holder and protects the internal organs. It is made of the skull bones which keep up the head, the backbone which keeps up the body and the tail, the fin-bones and the arch-shaped bones carrying the gills.
The Head
In the front part of the head is the mouth, of which the shape and features differ according to the feeding habits of the fish. Some fish have large mouths with sharp teeth for seizing prey: insects of fishes, while others have small mouths on the under surface of the head, suitable for scrapping up algae from the bottom. The shape of the teeth depends also on the feeding habits of the fish
. Predatory fish, such as the Hydrocynus are well supplied with sharp teeth. Vegetation eating fish may have teeth, each one with many more or less rounded points, arranged in bands along the side of the jaws (Tilapia rendalli). The teeth of some omnivorous fish (Alestes) are quite similar to the molars of man. In many fishes, there is, in addition, a bony plate bearing teeth in the back of the throat which are called pharyngeal teeth (Tilapia, Carp). On the snout, above the mouth, are the nostrils.
The nostrils are not used for breathing but only for smelling. Some fish have filaments along the side of their mouth which can be longer, than the head itself (catfish) and which are called barbels. Their number can vary within the range of 2 to 8. They are sensory organs which help the fish to find its food. At the hind end of the head are two bony flaps called opercula or gills covers, which can be lifted; underneath are the gills.
Every gill is made of a bony arch carrying long red filaments on one side called the gill filaments and short teeth like, or longer comb like projections on the other side called gill rakers. The gill filaments are the breathing organs of the fish, the gill rakers are used as a strainer to sieve out food particles from the water (fig. 3).
The trunk
The trunk is the part of the body in which are located a number of organs: the air bladder, the stomach, the intestine, the liver, the kidneys, the ovaries, the testicles. It starts from the head and includes the ventral cavity.
The tail – caudal fin
It is located behind the anus and ends with the caudal fin. An anal fin and sometimes a part of the dorsal fin can be found on the tail.
The fins
When identifying a fish, the fins are the first things, which should be examined. The number of fins, their types, sizes, situtation on the body and position in relation to each other, are most important.
The fins are similar to paddles made out of rays which would be joined together by a web. The rays can be either spiny or soft, ramified into a paint-brush and are then called soft rays. Fins can be paired or unpaired. The paired fins correspond to the fore and hind legs of the land vertebrates. There are four of them generally: two pectoral fins, one on each side of the body, behind the gill covers, and two pelvic fins which are on the ventral part of the body
either near the front, under the pectoral fins, or at the back. The unpaired fins are the dorsal fin on the upper part of the body, the caudal fin at the hind of the tail and the anal fin just behind the anus. In some fish, several fins can be missing while in others two dorsal fins, one at the back of the other, can be found. To move through the water, the fish uses mainly its caudal fin as a paddle. The other fins enable the fish to keep its balance.
The caudal fin or tail of most fish is lobed, which means that it is forked and has the upper and lower lobes attenuated to points. But in some fish, it is round, pointed or truncated. I.e. having the appearance of being cut off squarely at the end. The number of spines and/or rays in the dorsal and anal fins is generally the consistent character in a species and is seldom the same in two different species; hence these fins are very important for purpose of identification. Each fin is made up of a number of rays, which are usually bony and flexible and are either simple or branched; i.e. they divide or subdivide part of the way along the ray. The number of simple and branched rays are given separately in a description. In many fish, some or all of the rays, especially of the dorsal fin are replaced.
Some species of fish have two dorsal fins, the second of which is often an adipose fin, composed only of soft, fleshy tissue and usually without rays of any kind. The size and shape of this fin is often as a clue to the identity of a specie.
FORM
Commonly, the fish body is torpedo-shaped (fusiform), and most often slightly to strongly ovoid in cross section. In free swimming species, the body approximates the theoretically perfect streamline form in which the greatest cross section is located close to 36 percent of the length back from the anterior tip (the entering wedge) and the contours sweep back gently in the tail race. Many fishes depart moderately to completely from the foregoing generalized shape.
These departures range from globe shapes (globiform – e.g. puffers, tetraodontidae) through serpentine (a guilliform – e.g. eels, anguillidae), to threadlike in outline (filiform e.g. snipe eels, Nemichthyidae). Some are strongly flattened from side to side (compressed – butterfly fishes, chaetodontidae) and flounders, pleuronectidae) others, flattened but greatly elongated (trachipteriform, ribbonfishes, Trachipteridae) and still others, flattened from top to bottom (depressed – the skates, rajidae, and the batfishes, ogcocephalidae).
The ground plan of body organization in all fishes is bilateral symmetry i.e. the left and night halves of the body are basically mirror images of one another.
FISH ADAPTATION TO AQUATIC LIFE.
Shape – streamlined, fusiform
- rounding edge of such shapes reduces resistance
- tapering of posterior part minimizes drag
- mucus cover further reduce drag, smoothens
- Laterally compressed body prevails in quiet water body of relatively dense
- Depressed body prevail among bottom dwellers
- Dorso-ventral flattening body characterized stream
Fins – used by fishes to achieve all forms of locomotion, stabilization, balancing, change of direction and breaking in their aquatic environment
- use as gliding organ in some flying fishes
- use for terrestrial locomotion in some catfishes
- as crawling organisation use of some modified fins g. paired like pectoral, pelvic fins
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