Tidal periodicity of molting in giant mud crab, Scylla serrata (forskål, 1775)

Molting is the most crucial phase in the life cycle of mangrove crabs, leading to mortality in various culture systems. As a result of this exigency, this study was conducted to offer a cue in molting using the tidal cycle as one of the visible events in a natural context. Monitoring was carried out at every 1-hour interval (24 hours) day and night for 57 days to check for molting while taking into account key environmental elements such as tidal cycles, water current speed, and flow rate. The results showed that 70.6% of molting happened during high tide, with 93.8% of it occurring at night and 6.3% during the day. By contrast, only 29.4% occurred during low tide, showing a significant difference (T-test = 0.011, p < 0.05) between mean molts. Hence, the highest molting rate (88.2%) was observed between tidal episodes between the neap and middle cycles, with the middle tide (55.9%) occurring at 12.4±0.9 m/s, neap tide (32.4%) occurring at 16.8±2.0 m/s, and spring tide (11.8%) occurring at 9.5±1.2 m/s. Molting occurs at high tide because crabs' rheotactic behavioral responses to incoming new water help the crab's body fill with air and water throughout the molting process. Mangrove crabs employ their tidal periodicity of molting as a defense strategy against potential predators and mortality, and it can be used as a molting indicator in many culture systems.