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.

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Two species of fiddler crab, Uca tetragonon (Herbst, 1790) and Uca vocans (Linnaeus, 1758), which belong to the subgenus Gelasimus, dwell on rocky shores and muddy-sandy tidal flats, respectively, in Phuket Is., Thailand. We investigated their feeding ecology in relation to the morphology of their feeding organs: minor food-handling chelipeds and maxillipeds. U. tetragonon fed chiefly on rocks covered by filamentous green algae. U. vocans fed on the emerged sand and in shallow water along the shoreline and in pools. While feeding, both crabs made sand pellets beneath their mouthparts and discarded them, indicating that they divided the matter scooped up with their minor chelipeds into edible and inedible fractions by using the maxillipeds in the water passing through their buccal cavity. The morphology of maxillipeds hardly differed between the two species, which means that both species are flotation-feeders. The morphology of their minor chelipeds, however, differed: the tips of the dactyl and pollex were flat in U. tetragonon and pointed in U. vocans. When the minor cheliped was closed, U. tetragonon had a hemispherical space in the distal one-fourth of the gape, which was closed by the framing keratin layers and a few setae of the dactyl and pollex. On the other hand, U. vocans had an ellipsoidal space in the distal half of the gape. We consider these morphological characters to be adaptations to the different feeding substrates for retaining more food-laden sediment. We discuss the role of the setae on the minor chelipeds on the basis of the morphological differences between populations of U. tetragonon in Phuket Is. and East Africa where the crab inhabits muddy-sandy tidal flats.