Brood stock management and larval rearing of mud crab scylla serrata-Gayatri R. Kachh

2.  Scylla serrata (mud crab or mangrove crab or black crab)
 Economically important species of crab - estuaries and
mangroves of Africa, Australia and Asia
 The shell colour - deep, mottled green to very dark
brown.
 Classification
 Kingdom : Animalia
 Phylum : Arthropoda
 Subphylum : Crustacea
 Class : Malacostraca
 Order : Decapoda
 Family : Portunidae
 Genus : Scylla
 Species : S. serrata

3. • The natural range of Scylla serrata is in the Indo-Pacific.
• It is found from South Africa, around the coast of the Indian
Ocean to the Malay Archipelago, as well as from southern
Japan to south-eastern Australia, and as far east
as Fiji and Samoa.
• The species has also been introduced to Hawaii and Florida.

4.  A study on tidal flats in Deception Bay in Queensland found juvenile crabs
(20–99 mm or 0.8–3.9 in carapace width) were resident in
the mangrove zone, remaining there during low tide, while subadults
(100–149 mm or 3.9–5.9 in) migrated into the intertidal zone to feed at
high tide and retreated to subtidal waters at low tide. Adults (150 mm or
5.9 in and larger) were caught mainly below the low tide mark, with small
numbers captured in the intertidal zone at high tide.
 These crabs are highly cannibalistic in nature; when crabs undergo
moulting, other hard-shelled ones sometimes attack the moulting crabs and
devour them. The females can give birth to a million offspring which can
grow up to 3.5 kg (7.7 lb) in size and have a shell width of up to 24 cm
(9.4 in) wide.

5.  Interest in the aquaculture is high due to the high demand/price for them, high
flesh content, rapid growth rates in captivity, high tolerance to both nitrate and
ammonia
 Very active and will eat sinking pellets; some small fish pieces and vegetable
matter.
 Generally cooked with their shells on, when they moult their shells, they can be
served as one of many types of soft shell crab.
 Tastiest of crab species, and demand for them is large
in South and Southeast Asian countries, where they are often bought alive in the
markets.
 In the northern states of Australia and especially Queensland, mud crabs are
relatively common and generally prized above other seafood by the general public.
 Mud crabs can be killed by placing them in a freezer for up to two hours before
cooking.

6.  Broodstock : Maintenance of male and female Mud crabs for
controlled breeding purposes.
 Broodstock can be sourced from the wild, from pond-reared
animals or from domesticated improved broodstock.
 Spawning occurs throughout the year especially in warmer
countries.
 Spawners could be collected from the commercial catches in
berried stages and maintained in hatchery as brood stock.
 Maintenance of salinity around 33 ppt is a must for gonadal
maturation in brood stock management.

7. There are essential condition for raising the broodstock:
 Steady salinity close to 33 ppt,
 Daily water exchange with a continuous flow through
system
 Supply of nutritive clam meat as feed
 Maintenance of water temperature around 28 - 31°C

8.  In S. serrata, the first stage of maturation for a male occurs from 90–110
mm, while from 140–160 mm males develop their characteristic “large-
claw” and mating scars on their sternum and front walking legs become
apparent.
 A mature female mud crab produces from 1 to 6 million eggs
 The crabs are examined for ovarian maturity by looking through the
transparent membrane between the junction of the first abdominal segment
and carapace. Mature ovaries are dark orange.
 The crabs are held in a concrete tank with sand substrate and PVC pipes
(20cm diameter x 30 cm length) as shelters.

9.  They are fed mussels, squid and fish at 10-15% of body weight
daily and a SEAFDEC-formulated diet at 2%. Live marine annelids
are offered to crabs once every 1-2 weeks as a supplement.
 Water depth in the tanks is maintained at about 30 cm.
 The seawater used for the crab breeders and larvae is pre-treated in
a reservoir with 10-20 ppm calcium hypochlorite and then
neutralized with sodium thiosulfate after 12-24 h.
 The water in the tank is changed daily before feeding

10.  Females of 7 - 8 cm gathered from commercial catches are stocked at 1 seed /
m2 for fattening in fenced grow-out ponds.
 The stock is fed intensively with clam meat as high as 15 - 20% of body weight.
 When gravid females are allowed to grow continuously together with matured
males, the mated females spawn and become ovigerous or berried.
Female Scylla serrata spawning eggs onto sand in sand tray in
broodstock tank

11. The duration of egg development is decreased as temperature is
increased
As the nutrition of mud crabs is of paramount importance, they should
be fed a good-quality fresh diet of mixed seafood (squid, fish, bivalves,
marine worms)
Sufficient lipids and fatty acids are needed in broodstock diets to
enhance gonad development, hatching and larval metamorphosis .
It has been found that an artificial diet with a total lipid content of 10
percent, in combination with a natural diet, can lead to improvement in
larval production and quality.

12. Tank for holding mud crab broodstock with an aerated sand pit for crab spawning

13. Examination of ovary color between the junction of the first abdominal segment and
carapace

14. Mud crab larvae have been found to eat more at light levels of 1000–6 000 lux,
while below 1 000 lux, larvae both eat less and have increased mortality rates
As crab larvae exposed to 24-hour light exhibit decreased survival, compared with a
12-hour light/dark cycle, natural lighting should be the primary light source for larval
rearing areas
When tanks are not being examined, they can be covered with plastic or similar
sheeting. This reduces temperature fluctuation in tanks, in addition to controlling
aerosol sprays.

15. An egg mass (or sponge) of Scylla serrata; black colour indicates hatching
is imminent

16. Larval rearing tanks include circular tanks with a conical
base, hemispherical round tanks, parabolic tanks and
rectangular tanks
The total number and size of larval tanks required for a
hatchery depends on a number of parameters, including;
expected survival rates between each of the five zoeal
stages, the megalopa stage, through the final metamorphosis
to crablet; the number of batches per year required; the
number of crablets per batch; and the annual target for
production of crablets

17. Parameters to be monitored and recorded for each
larval rearing tank include:
Temperature :temperature range of 28–30 oC is
recommended
Salinity :varies from 22 to 35 ppt
Prophylaxis
oThe use of oxytetracycline (OTC) at 25–50 ppm has been
used with some success to control bacteria in mud crab
larval rearing systems
o In China, the antibiotics florfenicol and enrofloxacin at
1.4–2 ppm, as well as probiotics, have been used to inhibit
the proliferation of pathogen bacteria in mud crab
aquaculture systems

18. To control fungal infections in mud crab larviculture, treatment with
Treflan™ (a microtubule-disrupting chemical) (every 2 days) has been used
Maintaining larval water quality:
Chlorination can be used, followed either by chemical de-chlorination, or
aeration for 2–3 days to remove residues
Other water treatment options for mud crab larval rearing water include
ozone treatment (followed by carbon filtration), UV sterilization,
microfiltration and microbial conditioning
Another approach to maintain water quality in larval rearing tanks is to
establish a recirculating system, which screens and treats water in tanks.

19. Net cages set in brackiswater pond for the culture of megalopa to crab stage

20. A recirculating mud crab broodstock tank