4. Identification of sexes
• Sexes can be morphologically distinguished based on
the shape of the abdominal flap.
• Though the shape of this flap is similar in both immature
and mature male, it is different in mature and immature
female.
• In mature female the shape of abdominal flap is half-
round, while in immature female it is broad and
triangular.
5.
6. Sexual dimorphism
• The sexes are separate and there is distinct sexual
dimorphism in crabs:
• Distinction between male & female based on
morphological characters.
Male Female
1. Abdominal flap which is folded
firmly against the ventral side is
narrow (triangle shape) of
cephalothorax is slender and
triangular.
2. Appendages are present only on 1st
and 2nd
abdominal segments and the
same are modified to copulatory
organs.
3. The claws are comparatively larger.
1. Abdominal flap folded against the
ventral side of body is broad (Round
shape) and berried females.
2. There are 4 pairs of abdominal
appendages present from 2nd
to
5th
segment and the same are used
for carrying the eggs.
3. The claws are smaller.
7.
8. Size at maturity
• The size range at first maturity in males and females
of S. tranquebarica and S. serrata is presented in the
Table below
• Size range at first maturity in S. tranquebarica and S.
serrata
Species Sex
Size range at first
maturity
(Carapace width in mm)
S.tranquebarica
S.serrata
Male
Female
Male
Female
125-133
129-135
80-89
85-96
10. Mating
• The size at first sexual maturity is 120 mm (carapace
width) for larger species (S.tranquebarica) and 83 mm
(carapace width) for S. serrata.
• The early maturing ovary is bright orange in colour where
as, in matured – ready to spawn female, it is deep yellow
in colour. Copulation takes place between a hard shelled
male and a freshly moulted, soft bodied female.
• The courtship is initiated by a “premating embrace”
between hard shelled male and hard shelled female which
lasts for 2-3 days.
11.
12. Spawning
• The ova are extruded by the female and the same are
fertilized by the sperm stored in the spematophores.
• The fertilized eggs are attached to the ovigerous setae of
the abdominal appendages.
• In S. tranquebarica, the nos. of eggs may be 2-3 million per
female of body weight whereas in S. serrata it is 0.5-2.5
million such as females with eggs attached to the abdomen
are called berried.
14. Incubation & hatching
• The berried females carry their eggs for 2 weeks, during
which period the embryos develop.
• The eggs undergo change of colour from orange to grey /
brown.
• Just before releasing the larvae, the eggs become grey
black.
• After the incubation period, the larvae hatch out.
• The hatched out larvae need feed for their growth.
17. Introduction
• In general, the crustacean eggs hatch to produce nauplius
larva which is free swimming.
• It is not always true as in some cases the nauplius stage is
passed within the eggs so that there is no free swimming
nauplius larva produced after hatching, E.g. freshwater
prawns, cray fishes, crabs, etc.
• In such animals, the freshly hatched out larvae are called
Zoea and these larvae will be taking food immediately after
hatching and their development is faster than other stages.
• But the risk is that in the absence of feed, they may
become canibals or die of starvation.
18.
19. 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.
20. • 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.
• Telson is rudimentary and forks are not well defined.
21. • Second and third segment of abdomen has a pair of knobs.
• 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.
23. Identification of larval stages of crabs
(Scylla serrata)
Larval stages Important morphological characters
Zoea I Eyes sessile ; 5 abdominal segments; telson with 3 + 3 spines.
Zoea II Eyes stalked ; 5 abdominal segments; telson with 4 + 4 spines.
Zoea III 6 abdominal segments
Zoea IV Rudiments of remaining thoracic appendages, abdominal
segments with buds of pleopod.
Zoea V Remaining thoracic appendages developed; pleopods on
abdominal segments with setae; telson with 5 + 5 spines.
Megalopa Carapace longer than wider; abdomen with 5 pairs of pleopods ; a
pair of cheliped; 4 pairs of legs.
First crab instar Carapace with 9 anterolateral teeth on either side; 3 pairs of
walking legs; last pair of legs with paddle shaped dactylus;
resembles parent.
24.
25.
26.
27. References
• Pillay.T.V.R., Thomas.P. (2003), BREEDING AND SEED
PRODUCTION OF FINFISH & SHELL FISH, Daya Publishing
House, Delhi. pp 258-263.
• Report on the Seminar on the Mud crab culture and trade held
at Surat Thani, Thailand, Nov. 5 – 8 1991 . (Ed: C.A. Angell) –
BOBP Publn
