prawns

Introduction

The technique or method of breeding to induce the fish through application of certain hormones by injection or pellet implantation in the bodies of both sexes. Such application of hormones accelerates early ovulation than the time required for natural or normal breeding process. This concept is called induced breeding.

In case of freshwater prawn induced breeding means spawning of prawn without inject hormone but create controlled environment.

About 200 species have been described, almost all of which live in freshwater at least for part of their life. Bangladesh has very rich source of prawns in the Bay of Bengal, estuaries and freshwater. A total of 56 species is reported, of which 37 are salt water, 12 are brackish water, and 7 are freshwater in habitats. About 125 species of Macrobrachium are distributed in the tropical and subtropical rejoin of the world out of which 49 species are of fisheries importance and 15 species are of cultural significance.  The Macrobrachium rosenbergii is a commercially important species of crustacean cultured extensively throughout Southeast Asia. Giant fresh water prawn (Macrobrachium rosenbergii) is a species which plays an important role in the aquaculture and fisheries industry.

      

Description

The body of postlarval and adult prawns consists of the cephalothorax (‘head’) and the abdomen (‘tail’). The bodies of freshwater prawns are divided into twenty segments (known as somites). There are 14 segments in the head, which are fused together and invisible under a large dorsal and lateral shield, known as the carapace. The carapace is hard and smooth, except for two spines on either side; one (the antennal spine) is just below the orbit and the other (the hepatic spine) is lower down and behind the antennal spine. The carapace ends at the front in a long beak or rostrum, which is slender and curved upwards. The rostrum extends further forward than the antennal scale and has 11 14 teeth on the top and 8-10 underneath. The first two of the dorsal (top side) teeth appear behind the eye socket (orbit).

The front portion of the cephalothorax, known as the cephalon, has six segments and includes the eyes and five pairs of appendages. The final three of these six segments can be seen if the animal is turned upside down and the appendages of the thorax (see below) are moved aside. The cephalon segments therefore support, from the front of the animal:

• The stalked eyes;
• The first antennae, which each have three-segment peduncles (stalks) from which three tactile flagella emerge;
• The second antennae, which each have five-segment peduncles and a single, long flagellum;
• The mandibles, which are short and hard and are used to grind food;
• The first maxillae, which are plate-like (lamelliform), hidden below the second maxillae, and used to transfer food into the mouth; and
• The second maxillae, which are similar to the first maxillae but have an additional function. Part of these appendages are constantly beating, thus producing a current of water through the gill chamber to promote the respiratory function of the latter.

The two pairs of antennae are the most important sites of sensory perception; the peduncles of the first antennae contain a statocyst, which is a gravity receptor. The mandibles and first and second maxillae form part of the six sets of mouthparts.

The rear portion of the cephalothorax, known as the thorax, consists of 8 fused segments which have easily visible pairs of appendages. These appendages consist of 3 sets of maxillipeds and 5 pairs of pereiopods, as follows:

• The first and second maxillipeds are similar to the first and second maxillae and function as mouthparts (see above);
• The third maxillipeds, which are also mouthparts but look rather like legs;
• The first and second legs (pereiopods), which have pincers (chelae). These pincer-ended legs are also called chelipeds. The first legs are slender but the second pair bears numerous small spines and is much stronger than any other leg. The second chelipeds are used for capturing food, as well as in mating and agonistic (fighting) behaviour; and
• The third, fourth and fifth legs (pereiopods), which are much shorter than the second cheliped, have simple claws (not pincers), and are sometimes called walking legs. Eggs are extruded from oval gonopores in the base of the third pereiopods of females, which are covered with a membrane. In males, sperm is extruded from gonopores which are covered by flaps, situated in the base of the fifth pereiopods.

The pereiopods include chemoreceptor cells, which are sensitive to aqueous extracts of food and to salts (and may therefore be involved in migratory and reproductive processes). The left and right second legs (chelipeds) of M. rosenbergii are equal in size, unlike some other Macrobrachium spp. In adult males they become extremely long and reach well beyond the tip of the rostrum.

The tail (abdomen) is very clearly divided into 6 segments, each bearing a pair of appendages known as pleopods or swimmerets (as this name implies, they are used for swimming, in contrast to the walking legs). The first five pairs of swimmerets are soft. In females they have attachment sites for holding clusters of eggs within the brood chamber. In males, the second pair of swimmerets is modified for use in copulation. This spinous projection is known as the appendix masculina. The sixth pair of swimmerets, known as uropods, is stiff and hard. The telson is a central appendage on the last segment and has a broad point with two small spines which project further behind the point. The telson and the uropods form the tail fan, which can be used to move the prawn suddenly backwards.