Mud crab species:
There are four kinds of mud crabs in the Philippines: the king crab (Scylla serrata), the purple crab (S. tranquebarica), the orange or red crab (S. olivacea), and the rare green mud crab (S. paramamosain). Among the mud crabs, the king crab is the most popular for its fast growth and flavor and is called an “export winner” for its high demand in the international market.
Tanks for broodstock maturation, rearing, spawning, nursing, and for holding water should be constructed. They can be made of concrete, fiberglass or wood with dimensions of 0.5 to 1 cubic meter. The number of tanks should depend on the number of larvae and crablets that you are planning to produce.
Mud crabs feed on phytoplankton and zooplankton. Thus, in any hatchery one must learn how to culture phytoplankton to ensure that there is enough food for the mud crab juveniles. The first step to phytoplankton culture is obtaining an algal starter from laboratories or institutions that are selling them.
Chlorella, Nannochloropsis, and Tetraselmis are the common algae. You can start culturing them in one-liter capacity bottles. When the phytoplankton has bloomed and the density is 3 to 5 x 105 cells/ml, transfer them to ten-liter carboys. Upon reaching the same density, they should be transferred to a 0.5 to 1 ton- tanks where they will be allowed to bloom more.
In culturing phytoplankton, maintain the water temperature from 20-25 °C for indoor culture and provide good aeration to give enough supply of carbon needed for plant growth. And use boiled or filtered seawater with a salinity of 25-30 ppt for the culture and sterilize all the containers before use.
Also, for bigger culture, use urea (21-0-0) and ammonium phosphate (16-20-0) to supply the essential nutrients needed by the algae.
Broodstock management and spawning
The major source of broodstock is the wild-caught post juveniles or half-grown crabs while adult or berried female crabs are used as spawners. A minimum weight of 200 to 300 grams for broodstock and 450 grams for spawners are desired.
The king crab is sexually matured when the width of its carapace reaches 14 cm and it weighs 450 grams while the other mud crabs can spawn even if its carapace is still below 10 cm and weigh 300 grams.
As soon as the broodstocks and spawners of the king crab arrive at the hatchery- they place them in the tanks at 300 spawners per tank and 1000 broodstocks per tank with a 10 cm sand substrate at the bottom. Feed them daily with mussel meat (Perna viridis) at 5% to 10% of their biomass. Change the water (at least 30%), scrub the sides of the tank and remove excess feed daily.
Mud crabs become mature and mate when they are 4 months old. In ponds or tanks, male and female crabs clasp for 5 to 7 days to fertilize the eggs.
There are two methods of spawning: the natural and the induced. In the natural method, the mud crab is left in the tanks until they become sexually mature and spawn. For induced spawning, the most common method used is ablation where one (unilateral ablation) or both (bilateral ablation) of the mud crab’s eye stalks are crushed or cut to induce spawning 7-13 days after fertilization.
The advantage of unilateral ablation is that the mud crab can spawn again while in bilateral ablation the mud crab dies after hatching the eggs. In both cases, observe the mud crabs closely until the eggs fill the abdominal flap.
Since ablation increases the appetite of the mud crabs, feed the mud crabs more until the eggs are hatched. Incubation ranges from 7 to 13 days. At this time, the berried (pregnant) crabs should be transferred to another tank without the substrate. Expect the eggs to hatch when the eggs turn from light orange to dark grey.
After the eggs are hatched, transfer the larvae into the large tanks filled with 5-10 tons of filtered seawater with 34 ppt salinity. Add algae and rotifer at a rate of 5 x 10 (3) cells per cubic meter and 25 ind/ml, respectively. Yeast-grown rotifer may also be added at 5 ind/ml. Give supplemental diet of 6 grams/ton/day and feed them at 6AM, 12 NN, 6 PM and 12 MN.
Rearing of larvae in nursery
Line the nursery tanks with soil (10 cm thick) inoculated with lab-lab. For large tanks, line them with mud substrate that has been seeded with lab-lab. Apply lime and chicken manure at 2 kg/ 10-ton tank and ammonium phosphate at 500 grams per tank. After fertilization, transfer the megalopae at a density of 20,000 to 30,000 per cubic meter. Apply organic fertilizer to encourage the lab-lab to bloom.
For those using hapa nets as nurseries, use hapa nets with a mesh size of 1 mm and a dimension of 1m x 1 mx 1.5 m. Install the nets in a canvass-lined earthen pond. Line the hapa net bottom with 3-5 cm thick mud substrate. Apply chicken manure and inorganic fertilizer (16-20-0) at a rate of 20 g to 500 g /cubic meter. Fill the pond with water until a depth of 20 cm to promote growth of microbenthic algae that will serve as food for the larvae. Other organisms such as bloodworms, oligochaetes could also serve as food for the larvae. Harvest by totally draining the pond.
Source: “A guide to hatchery and nursery production of mud crab (Scylla serrata) juveniles” by Romeo Fortes, Juliana Baylon, Evelyn Marasigan, Allan Failaman, Gerome Genodepaz, Sol Garibay and Gisela Ann Mamon of the College of Fisheries and Ocean Sciences at the University of the Philippines in the Visayas, Miag-ao, Ilo-ilo. Photo: www.todayaqua.com