Scylla serrata (mud crab or mangrove crab or black crab)

Economically important species of crab - estuaries andmangroves of Africa, Australia and Asia

The shell colour - deep, mottled green to very darkbrown.

Classification

Kingdom : Animalia

Phylum : Arthropoda

Subphylum : Crustacea

Class : Malacostraca

Order : Decapoda

Family : Portunidae

Genus : Scylla

Species : S. serrata

• 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.

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.

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.

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.

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

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.

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

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

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.

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

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

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.

An egg mass (or sponge) of Scylla serrata; black colour indicates hatching

is imminent

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

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

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.

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

A recirculating mud crab broodstock tank