Shellfish Breeding and Hatchery Management (2+1)

7.1.2 Larval development

The zoea larva undergoes 5 successive moults to become the megalopa larva. Thus there are 5 zoeal stages. For each moult, it takes 3-4 days. Thus to develop a megalopa from a zoea it takes 15-20 days. Megalopa develops into juvenile crab after 8-11 days.

A newly hatched zoea larva of crab measures 1.2 mm and it consists of a cephalothorax and a 5 segmented abdomen and telson. The carapace bears 4 spines, one dorsal spine bent backwards, rostral spine bent forwards and 2 lateral spine closely pressed against the sides of the body. The eyes are not stalked. The first maxilla consists of coax, basis and endopodite (each with 4,5 and 6 setae respectively), a second maxilla has coax and basis bilobed, with 1-1 and 2-3 setae respectively, the endopodite is having 3 rudimentary setae at its tip where as exopodite is with 4 rudimentary setae. The first maxilliped also has coax, basis and exopodite and endopodite, but the endopodite is 3 segmented, and it has only 3 rudimentary setae. Second and third segment of abdomen has a pair of knobs. Telson is rudimentary and forks are not well defined.

As it passes through 5 zoel stages, growth in body size, appendages etc take place. Zoea is 3.5 mm in length. From zoea II onwards the eyes are stalked (for distinguishing characteristics of larva stages of crab).

The megalopa larva has a crab like appearance. The cheliped is well developed. 5 pairs of pleopods are present which becomes locomotory in function. Chelipeds are used to catch prey.