the notochord bends upwards at its posterior end. Such type of tail is mainly found among the elasmobranches. ventral and the caudal fins. The homocercal tails are the most common types and characterize the higher bony fishes. Tail thus becomes symmetrical externally but remains asymmetrical internally. the dorsal lobe is not apparent in the fin but the ventral lobe is quite well developed.1. unsupported by the fin rays. Protoheterocercal This intermediate tail type is different from above in having separate dorsal.2. Chimera and Protopterus is very much like the protocercal tails of earlier forms but the palaeontiological and embryological evidences suggest it to be a modified secondary form of heterocercal tail. The posterior end of notochord becomes ossified into the urostyle.4. Diphycercal A diphycercal tail as found in dipnoans. while the ventral lobe becomes well developed.1. Homocercal A homocercal tail is a modified form of heterocercal tail. and ventral side of the fish. 2. both externally and internally. The large sized ventral lobe possesses a longer postero-dorsal and a shorter antero-ventral lobe.1. The notochord is continued straight upto the end of the tail and the fin lobes are disposed symmetrically above and below to it. so that the dorsal lobe of the tail is almost lacking or represented by a few spines. 2.5. The latter contributes the formation of two equal sized lobes.4..1. 2. Such a type of fin is found in cyclostomes and the living lung fishes. From this type of tail the heterocercal and diphycercal tails are said to have been derived.4.1. Heterocercal In heterocercal tail.184.108.40.206.38Fish Biology and Ecologya continuous fold. which are formed by the interruption of the continuous fold on dorsal. that provides a rigid support to the dermotrichia. Because the notochord is turned upwards. 2.1. Consequently the tail becomes asymmetrical.4.
The radials were arranged in decreasal manner on either side of the median axis. They are modified into suckers in globe fish and helps in fixing with rocks. The supporting endoskeleton of the paired fins varies greatly in different groups of fishes.. Hypocercal The hypocercal tail type is derived from the heterocercal type by bending of the hinder end of the notochord downwards. In Trigon. found in ceratodus and present in the devonian crossopterigii. Broad and long pectoral fins acts like the wings of bird in Exocoetus and fly in the air. In Periopthalaus pectoral fins are useful for walking. Various types of paired fin skeleton in the teleosts may be derived from the archiptenygium by shortening of the median axis and reduction in the number of postaxial radials that finally disappear completely. Generally pelvic fins are small in size. Torpido they extend laterally from head to tail and modify into leafy structure.1. A pair of pelvic fins are present at the ventral side of the body on either side of cloacal aperture.4.8. Paired fins Paired apprises were not present in the ancestral vertebrates and were developed during the course of early evolution. 2. giving rise to pleuroarchic or uniserial type of skeleton found in teleosts Pectoral fins are present behind the gills and horizontal to the ventro lateral portion.2. This is called a biserial arrangement of the radials and known as biserial archipterygim. In male sharks a pair of copulatory structures the claspers are present inside the pelvic fins.1.4. The original primitive condition is known as the ‘archipterygium’ and was provided with an anterior pre-axial and a porterior post-axial series of radials attached to it.40Fish Biology and Ecology2.1. The dorsal lobe thus becomes larger than the ventral lobe which is much reduced.
and for the same fish from different localities. and by itself is also an important factor affecting growth rate of fishes. Ammonia if present in high concentration. whereas slow growth rate indicates the opposite. degree of competition. Growth in fishes 3. Factors affecting growth rate The growth rate in fishes is highly variable and depends upon a number of environmental factors such as temperature. the assessment of age and growth rate are conducted together. 3. if dissoloved oxygen falls below a certain level. age and the state of maturity of the fish. The rate of growth varies in fishes from species to species. Dissolved oxygen level depends on temperature. Possibly. amount of dissolved oxygen. For example. and along with other factors. Abundant food supply and existence of other favourable conditions result in fast growth rate. as it is influenced by various factors. As the age of a fish and its growth are closely related.1. Growth rate depends on the population. Growth of an organism can be defined as a change in its size (length and weight) over a period of time. will slow down the growth rate. photoperiod. ammonia. the fish is deprived of ‘extra’ aerobic energy required for growth and reproduction. Thus. Temperature is one of the most important environmental factors. optimum food consumption for maximum growth is temperature dependent. quality of food taken. influences growth rate.42Fish Biology and Ecology3GROWTH IN FISHES AND PRAWNSGrowth has been one of the most intensively studied aspect of fish biology as it indicates the health of the individual and the population.1. in the young salmon maximum growth rate is achieved at 15°C.1. salinity.
Endopodie is quite small. Thus.3.3. Maxillulae help in the manipulation of food. Antennae The antennae lie. while exopodite is in the form of a broad and leaf-like plate. jaw). The proximal segment represents the basis.5. Free borders of coxa and basis are covered with pointed spines and project inwards as jaws or gnathobases (Gr. while exopodite forms a large expanded. lying one on either side of the mouth. 2.6. Endopodite forms a curved process bifurcated at the apex.1. and a plate-like incisor process ending in 3 teeth. which opens by a minute renal aperture on the inner margin of coxa. which is differentiated in to a proximal. Basis bears a spine. while two distal segments represent the endopodite. while the outer smooth margin bears a small spine. a stout molar process bearing 5 to 6 dental plates. The exopodite is absent.4.32Fish Biology and Ecology2. Endopodite is represented by many-jointed sensory feeler.1. Mandibles constitute the biting jaws and are masticatory in function. fan-shaped scaphognathite or baler.1.2. Outer margin of head carries a mandibular palp made to 3 segments. Mandibles The two mandibles are strong calcified bodies. the squama or scale.1. the antennae are sensory.1.1. Maxillulae These are small. just below the antennules.3. The small coxa is patially divided.1. It bears setae along its inner and distal margins. excretory and balancing in function. and a distal solid head. gnathos. The head forms two processes.3.. triangular and hollow apophysis. Squamma probably serves as a balancer during swimming. 2. the movements of which create a water. Maxillae These are also thin and leaf-like mouth appendages. The protopodite is greatly swollen due to presence of excretory organ within. one on either side. The exopodite is absent.1. thin and leaf-like appendages. Almost the entire mandible consists of coxa. 2. while the large basis forms a bifurcated gnathobase internally.