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Studies on Some Aspects of Reproduction of Mud Crab, Scylla serrata (Forskal)
Thesis Abstract:
Some aspects on the reproduction of mud crab, Scylla serrata, in captive conditions were studied. By ablation of bilateral eyestalks of the female, 14 batches of eggs were spawned after five days of culture in fiberglass tanks. Spawning occurred at any time of day, month, and year. Majority of crabs spawned without undergoing molting and mating under culture condition. Crab spawners were mainly in sizes of 201-300g. Average fecundity was 1,479,992.12±eggs/crab. However, many eggs could not adhere to the pleopods of the female but fell on the bottom of the tanks. Regression between body weight of female and fecundity was significant (p < .05). In addition, the second spawning of the crabs under captivity was also observed.
Incubation period of eggs lasted for 9.71 days (9-12 days) and hatching process took 27.71 hours (8-60 hours) in conditions of water temperatures of 25-31°C, salinity of 27.5-30 ppt, DO of 6.2-6.8 ppm, and ammonia of 0.02 ppm. Duratio for incubation and hatching was significantly related to water temperature (p < .05). In salinity range of 0-40 ppt, crab eggs could hatch in salinity of 20-40 ppt but the optimal was 30 ppt. In addition, artificial incubation of eggs with incubating density from 762.4 to 48,793.6 egg/L gave hatching rate decreasing from 65.77±6.52 to 16.66±1.31 percent.
In rearing larvae of mud crabs, under photoperiods of 24 hr L:0 hr D, 18 hr L:6 hr D, 12 hr L:12 hr D, 6 hr L:18 hr D and 0 hr L:24 hr D metamorphosis, growth, and survival of crab larvae were significantly effected. Metamorphosis of larvae was significantly delayed under photoperiods of 6 hr L:18 hr D and especially 0 hr L:24 hr D in comparison with other larvae under other photoperiods conditions (p < .05). There was no significant difference in body length of larvae between treatments with exception of that in Zoea5. Larvae under photoperiods of 0 hr L: 24 hr D and 6 hr L:18 hr D all died in Zoea2 and Megalope stages, respectively. Meanwhile, under 24 hr L:0 hr D, 18 hr L:6 hr D, and 12 hr L:12 hr D, survival rate of larvae was rather high until Zoea5 but extremely dropped thereafter. Photoperiods of 24 hr L:0 hr D, 18 hr L:6 hr D, and 12 hr L:12 hr D were recommended for mud crab larval rearing.
Under different conditions of light intensity: 0 lux (complete darkness), 300- 350 lux, 1,500-2,000 lux, 4,500-6,000 lux and under transparent roof, metamorphosis were significantly delayed for larvae under complete darkness and 300-350 lux. There were no significant differences in body length of larvae under different lighting intensity. However, from Zoea5 onwards, body length of larvae under 4,500-6,000 lux and under transparent roof were significantly longer than that of larvae under other treatments. Larvae under complete darkness and 300-350 lux all died at Zoea1 and Megalope stages, respectively. Survival rate of larvae under 4,500-6,000 lux and transparent roof were found significantly higher than the one under other treatments. Lighting intensity of 4,500-6,000 lux and under transparent roof were thus suitable for larval rearing of mud crabs.
With different feeding regimes for the larvae, the results showed that larvae fed with green water only or fed with green water plus artificial plankton and prepared feed was not significantly different from the control (no feeding), and that all died at Zoea1 after three days of culture. Larvae fed with green water, rotifer, and Artemia nauplii were similar to larvae fed with green water and Artemia nauplii in terms of metamorphosis and growth, but the former achieved survival rate of Zoea better than the latter, and the latter gave survival rate of Crab1 stage better than the former. Green water and Artemia nauplii, thus, could be used satisfactory at larval stage.
In rearing crab seeds from Crab1 to Crab7 with different salinity from 30 to 0 ppt at intervals of 6 ppt, salinity of 18-30 ppt was found better for crabs in terms of molting, growth and survival rate than other salinity. Crabs could not tolerate freshwater (salinity of 0 ppt) over a period of two days.
In culture of crab seed from Crab1 to Crab6 fed with trash fish and pellets, crabs fed with pellets gave results comparable with that of crabs fed with trash fish in terms of molting, growth and survival rate.