Age of the Mangrove Crab Scylla serrata at Colonization by Stalked Barnacles of the Genus Octolasmis

Cyprid larvae of the lepadomorph Octolasmis colonize the gill chambers of the edible mangrove crab, Scylla serrata (Forskaal, 1755), sometimes in debilitating numbers. We set out to determine when, in the life cycle of the host, barnacle infestation begins. A total of 856 mangrove crabs, ranging in size from 10.9 to 132.3 mm carapace width (instars 5 to 18), were collected from natural populations in Phuket, Thailand, and examined for these barnacles. Almost a third harbored one or more barnacles. The smallest crab to host a barnacle was 34.3 mm (instar 10); 233 smaller crabs, representing instars 5-9, had none. Infestations by more than one barnacle were uncommon among crabs of less than 70 mm carapace width (instar 13). The percentage of crabs hosting barnacles increased as the crabs approached sexual maturity, and the magnitude of infestation on individual crabs increased with their size. The distribution of octolasmids on the gills of immature crabs differed from that on mature crabs. In the former, all barnacles were on the inside of the gill surfaces and none were on the outside, whereas in the latter, 11% were on the outside of the gills. The numbers of barnacles on the inside and the outside of the gills is a function of the number of barnacles in the gill chamber. The major inhalant aperture size, and gill chamber size were eliminated as possible factors limiting infestation. Instars 10 and 11 may be suboptimal for infestation by octolasmids because the intermolt time between instars does not allow sufficient time for production of barnacle nauplii. Current data do not permit us to distinguish the relative influences of microhabitat use, host hormonal changes, and behavioral changes on infestation.