The importance of mangrove species association to the population biology of the sesarmine crabs Parasesarma affinis and Perisesarma bidens

The population biology of two sesarmine crabs, Perisesarma bidens and Parasesarma affinis, was studied over a two-year period in two mangrove forests, one dominated by Kandelia candel and the other by Avicennia marina. Despite the fact that the mangrove forests were contiguous, shared the same water supply and were less than 20 cm different in tidal position, significant differences in population biology of both crab species were recorded. Individuals of both species associated with the topographically higher Kandelia candel forest were more abundant, had a higher tissue organic content, higher gonosomatic and hepatosomatic indices than their counterparts in the Avicennia marina forest. Female individuals of both species were also larger in size in the Kandelia forest. These differences result in higher secondary production of the crabs at the Kandelia forest. By contrast, association with different mangrove species did not result in significant differences in the sex ratio, growth and reproductive periodicity, and size-specific reproductive output of the crabs. The results suggest significant influence by the associated mangrove species on the population biology of the crabs, through processes such as modification of the physical habitat fabric and/or provision of trophic resources of different quality and quantity by the mangroves. This influence further reiterates the bi-directional nature of mangrove-crab interactions and the need to consider these intricate interactions in future studies on tropical mangrove structure and function.