Molting of Mud crab (Scylla serrata) Cultured in Cage(prepared by Edmundo Enderez – Sept 21, 2009)

1. A trial on the culture of syclla serrata (without chelipeds) with carapace width ranging from 4.0-4.9 cm to 9.0-9.9 cm with peak of 6.0-6.9cm was undertaken to determine period of molting and growth. Culture period covered Aug 25 – Sept 12, 2009.

2. It was expected that the results will be similar to what had been observed and experienced by observers or researchers as presented in the literature.

a. It has been observed that majority of mud crabs undergo molting during the period of quarter moon phases; during high tides; and mostly at night time as shown in the following Table.

a. The research paper titled “A preliminary study on the response of mangrove mudcrab (Scylla serrata) to different feed types under drive-in cage culture system” published by the Journal of Ecology and Natural Environment Vol. 1(1), pp. 007-014, April, 2009 showed data on extent of molting. It has been observed that majority of mud crabs undergo molting during the period of quarter moon phases; during high tides; and mostly at night time as shown in the following Table.

b. In Myanmar, the practice is that fisherfolk harvest mud crabs from the wild over 4-5 days during the full moon period. After capture, the mud crabs are put in individual containers then after 4-5 days the stock will molt. The stocks have size range of 65-100 grammers or 7.0-7.5 to 8.5cm carapace width. This particular cases goes to show that the period of molting of mud crab coincide with the occurrence of 3rd quarter moon phase which occurs after full moon.

c. In Thailand, mud crabs are abundant in mangrove swamps. Based on studies, mud crab catching is related to tidal cycle: fishing season or highest catch occurred during spring tides that coincide with the new moon and full moon. Since newly-molted mud crabs will take cover in their shelters or burrow in the mud to allow their shells to harden in 3-4 days, hence only hard-shell mud crabs are expected to be caught.

d. In the paper of Alice Fe D. Lavina (Courtship and Mating Behaviour of Scylla serrata in Captivity), she cited Arriola’s paper (1940) stating that an autotomized appendage would resume its normal form and shape after 4 succesive molts. But her finding in her experiment was that a regenerate would assume the normal form and shape upon one molting activity; in cases where a body part was just regenerating and another injury occurs on its pair molting is delayed; and it was observed that the other regenerate's growth was delayed and the newly injured part would start regenerating.

3. Based on what has been presented in the reference literature, a trial on the culture of mud crab was conducted in a fishing community of Barangay Tamulaya, Polillo, Quezon. This was conducted as preparatory to the engagement of a fisherfolk association in the small-scale production of mud crab in pen (a form of aquasilviculture project). The activities conducted are presented in the following pictures.

4. The picture below shows the stocks without chelipeds before culture period. Stocking rate was 29 individuals per sq.m. Feeding rate was 10% of the total biomass and feeds consisted of fish and shells. After the culture period of 18 days or on Sept 12 almost all of the stocks (27 out of 29 pcs) had molted but most of the pincers were relatively small and 2 with full pincers; about 3 pcs of mud crab had soft-shells a condition of newly-molted mud crab.

5. The Graph below shows the change in carapace width frequency distribution of the stocks on August 25 (time of stocking) and September 12 (time of harvest) or after 18 days of culture. Survival was 100%.

6. During the culture period, the phases of the moon were as follows: August 20 (new moon); August 28 (1st quarter moon); Sept 5 (full moon); Sept 12 (3rd quarter).

7. Based on the results of the trial and the reference literature, the following can be speculated:

a. The fishing season of mud crab or when there is plenty of catch occures during the period of spring tides (few days before and after new and full moon phases). This means that molting around the new and full moon is absent or negligible and that molting has its peak period around the 1st quarter and 3rd quarter moon phases.

b. Molting could had occurred 4-5 days after stocking or August 28-29 which coincided with the 1st quarter moon phase or after the new moon phase. Then on Sept 2-3 (closed to Sept 5 with full moon phase) which was 3-4 days after molting the shell underwent hardening and the mud crabs started to look for food in the open wild.

c. Molting had also occurred on or a few days before Sept 12 (3rd quarter moon phase) which was the time of harvest. On Sept 12 harvest, 3 out of the 29 individuals had soft-shells.

8. Molting occurs when there is the formation of a brown membrane sticking to the underneath side of the carapace which is a precursor to the new shell and that the remains of the mud crab that molted contains all the parts except for the organs like heart and the digestive system as shown in the pictures below.

9. Molting occurs when the mud crab is fat or robust or meaty and when there is supply of calcium from the feeds like shells (golden kohol, bagongon, and balukaret) as shown below.

10. Since the cage is installed in natural body of water along the edge of natural canal (wherein water depth is 1 foot during the lowest tide) within the mangrove area which is subjected to semi-diurnal tidal fluctuation (2 high tides and 2 low tides), water exchange occurs every 6 hours. This situation prevents the accumulation of organic sediments and wastes coming from the stocks. This condition makes water inside the cage of high water quality (dissolved oxygen, pH, and temperature). The Illustration below shows the source of dissolved oxygen in the water and different reactions on dissolved oxygen with ammonia and hydrogen sulfide formed from organic wastes and removal of acidic water and soil through water flow or exchange. High water pH of about 8 in the project site facilitates the molting process.