Size and location relationships of stalked barnacles of the genus Octolasmis on the mangrove crab Scylla serrata

Cyprid larvae of the lepadomorph Octolasmis colonize the gill chambers of t he edible mangrove crab Scylla seratta (Forskal, 1755). In a natural popula tion of 856 mangrove crabs from southern Thailand, 260 individuals were inf ested by 3,670 Octolasmis cor and 1,758 O. angulata, including 1,014 subadu lts, 168 cyprids, and 38 peduncles of the two species. This population of g ill chamber symbionts was examined to investigate the relationship between barnacle size and barnacle spatial distributions. The habitat of the branch ial chamber was partitioned for study into gills one through eight, the ins ide (hypobranchial) and outside (hyperbranchial) gill surfaces, and the pro ximal, medial, and distal regions of each gill. The collective data from 26 0 crabs were pooled for an analysis that showed a nonrandom relationship be tween the size of octolasmids and thier location within the gill chamber. O n the inside gill surfaces O. angulata attained its largest average size on gills 3, 7, and 8, whereas on the outside surface the barnacles were large st on gills 4 and 5. Octolasmis cor attained its greatest average size on t he inside surface of gill number 6. Comparisons of barnacles from the three gill regions also revealed some significant differences in average barnacl e size. Positive correlations among barnacle size, barnacle number, and bar nacle density were present. Moreover, there was a significant correlation b etween the total numbers of barnacles and the average size of O. cor on the inside surfaces of gill numbers 1 to 8, whereas it was not significant for O. angulata. There was also a significant correlation between barnacle den sities and barnacle size in O. cor but not O angulata. Positive correlation s were also observed among higher numbers of barnacles, larger barnacles, a nd higher numbers of advanced reproductive stages. Areas with higher densit ies also were areas of higher average fecundity.