Mating habits, Ovulation, Cleavage and Development of eggs in Prawns

Copulation takes place in the prawns only in the night. Prior to copulation the prawn creep about at the bottom, only one male following a female. In a few minutes the female moults becoming soft, the male remaining hard without molting. After moulting, the male advances and embraces the female on the ventral side and they swim with their bodies inclined. Copulation lasts for 3 or 4 minutes after which they separate without caring for each other. During copulation the male ejects the spermatophore containing the sperm through the petasma located on the fifth pereiopods into the thelycum of the female and the spermatophores finally reach the seminal vesicle. The same male cannot copulate twice in the same night because all the spermatophores are deposited in one female.

Spawning takes place at night while swimming leisurely in the water, one to two feet above the bottom. It begins when eggs have matured and the germinal vesicle of the egg melts. During spawning the pereiopods are held tightly to the body and pleopods are moved to and fro vigorously. Due to this the eggs are scattered behind.

Nearly 700,000 eggs are shed in 3 to 4 minutes by a prawn of size 20 cm long. The eggs are slightly heavier than water and if the water is disturbed sinking of eggs is delayed. Spawning does not weaken the prawn. While the eggs are extruded the spermatozoa from the spermatophores lodged in the seminal vesicle also extrude out into the sea water through a minute pair of openings at the base of the fourth pair of pereopods of the female, and fertilization takes place in the sea.

The egg inside the ovary is covered with a layer of follicle cells and possesses 2 or 3 nucleoli. There is no vitelline membrane. When egg grows it absorbs the follicle cells and in a mature egg the latter grow smaller becoming a thin membrane, The nucleoli also becomes smaller and increasing in number. After, all the follicle cells are absorbed, a jelly-like substance is found on the surface cytoplasm. The egg is an irregular sphere before fertilization discharging radially the whitish, transluscent,jelly-like substance from within. The latter becomes granular and separates from the egg 7 minutes after spawning, but a little may remain even after the formation of the fertilization membrane. By the time the first segmentation is over the jelly may completely disappear and the egg becomes spherical.

During fertilization a few spermatozoa, reaches the egg surface one minute after spawning, and after the jelly is almost disappeared. At the place of contact of the spermatozoa on the egg, entrance cones appear according to the number of spermatozoa that have reached the egg. The cytoplasm of a cone protrudes up to the head of the spermatozoon and into this the head sinks. Soon the cytoplasm shrinks and drags the spermatozoan into the egg through the egg surface. Only one spermatozoan is able to pass into the egg even if more sperms have reached the surface. The others are discarded because the cytoplasms in their respective entrance cones do not reach up to the heads. When the sperm is entering the egg the first polar body is extruded before the fertilization membrance is formed. After this the fertilization membrance is formed around the egg and the second polar body emerges at the same place as the first polar body. Finally, the first polar body remains above the membrance, and the second below it before the first cleavage.

An egg soon after it is spawned

The first cleavage is total and equal and takes place 30 to 40 minutes after spawning. After this further segmentation takes place every 12 to 15 minutes. After the second division four cells are formed and when by further cleavage 64 to 128 cells are formed an invagination occurs at the vegetative cells of the embryo. Soon an embryonic membrane surrounds the embryo. After about 2 hours and it gradually becomes oval.

The rudiments of the antenna first appears as a swelling in the middle part of the embryo. The root of the mandible appears below that of the antenna followed by the swelling of the antennule above it. When the three pairs of appendages have well developed the embryo become a nauplius . A dark red ocellus appears on the anterior end of the body which is ventrally placed .The embryo begins to show signs of movement within the egg-membrane and in 13 to 14 hours after spawning the nauplius emerges from it.

The newly emerged nauplius LARVA has usual 3 pairs of appendages: the anterior uniramous antennules, middle antennae and posterior mandibles, both biramous. It swims by means of the appendages, the antennae being the most active, and the antennules coming next. They are attracted to light but avoid direct sunlight. There are six naupliar stages, each stage formed after the moulting of the previous stage, their size ranging from 0.34 mm in the first to 0.51 in the sixth stage. After each moult body size increases and structural complexities take place. When at rest, the nauplius keeps its dorsal side down, remaining in water in a perpendicular position with the three pairs of appendages slanting upwards. While hatching the colour is dark yellow, later by moulting the colour becomes whitish and semi-transparent. Appendages get some reddish- brown specks. Nauplii are not feeding stages, there being yolk in the body for nourishment. When the protozoea stage is formed the yolk is almost completely absorbed

4th Stage

5th stage

6th stage

The sixth nauplius moults in about 36 to 37 hours and the first protozoea emerges. The body is elongated and changes are seen in the appearance and swimming behaviour. The endopodites and exopodites of mandibles fall off, their function now being mastication. The first and second antennae are the chief organs of locomotion and they are also aided in swimming by the first and second maxillipeds which have developed well. It is photopositive but shuns very bright light. The carapace covers the anterior side of the body up to the eighth segment. The compound eyes make their appearance towards the end of this stage. The uncovered eyes make their appearance towards the end of this stage. The uncovered lower part has six segments which belong to the thoracic region. Later five abdominal segments develop with indistinct boundary lines. The telson is well developed with a semi- spherical forked end. The appendages are well developed with distinct articulations and plumose setae and spines.

There are three protozoea stages formed by moulting, and after each moult more appendages and other structures are formed. Eye protrude out in the second protozoea stage. The third protozoea moults into the mysis stage. The mysis resembles the adult prawn having the cephalic and thoracic segments united to form a cephalothorax which is covered by the carapace. Rostrum develops to a little more than half the length of the carapace. The first and second antenna cease to be natatory and become olfactory in function. The five pairs of pereopods take on the function of swimming, the three pairs of maxillipeds also assisting. They swim with their heads down and telson up, keeping the body in a slanting position. They do not show much preference to light. The body colour is pale yellow, the colour of the mouth parts, thorax, abdominal segments and tip of first antenna is reddish brown.

A zoea (after 3 molting the Zoea transforms into a Mysis)

2nd Stage

3rd Stage

The size of the mysis stages varies from about 3.10 mm length in the first to 4.52 mm in the third stage. The abdomen is a little shorter than the body. On the basal segment of the antennule with three segments the rudiment of an otolith is developed. The rudiments of the green gland appear on the protopodite of antenna. On the first three pairs of pereopods of early mysis, rudiments of arthrobranchs appear and they become clearly distinguishable in the third mysis. In this stage a rudiment of arthrobranch appears on the fourth pair of pereopods also. The five pairs of pleopods on abdomen are rudimentary and functionless. There are three mysis stages formed by moulting. After each molt the body structure change. The third mysis moults into the first postLARVAe.

A Mysis (After 3 moltings, the Mysis transforms into a Post LARVA)

2nd Stage

3rd Stage

In the postLARVA the five pairs of pleopods fully developed and functions as the chief organ for swimming, the uropods also assisting in balancing. The pereopods are used only for walking and grasping. At this stage it goes down and creeps on the sand below, which was still now swimming in the water column. Day and night they creep and burrow into the sand at intervals. After 10 or 12 moults they begin to creep on the sand as the adults. After 20 to 22 moults the shape of the body and appendages resemble those of the adults. The colour changes in the postLARVA after four or five moults to greenish-black, brownish-black, brownish-yellow etc on various part of the body. POST LARVAL STAGES.