THE POPULATION STRUCTURE OF THE SYMBIOTIC CRAB, <i>TRAPEZIA</i> <i>SEPTATA</i> (DECAPODA, TRAPEZIIDAE) AT THE EARLY STAGES OF CORAL COLONY DEVELOPMENT

Abstract

Symbiotic invertebrates play crucial roles in coral reef ecosystems due to their extremely high abundance and biomass, and by maintaining coral fitness and protecting them from predation. Among these invertebrates, symbiotic crabs of the family Trapeziidae are especially important for branching corals of the family Pocilloporidae. In this research, we studied the spatial arrangement and parameters of the populations of the symbiotic crab, Trapezia septata at the early stage of Pocillopora verrucosa colony formation. The study was conducted based on symbionts sampled from small coral colonies exposed for three months at a coral nursery in the Nha Trang Bay, Vietnam. As much as 59.5% of the study colonies are shown to be inhabited by the T. septata crab. The percentage of inhabited colonies grows proportionately to their volume. The male to female ratio is 1.4, being skewed to males. The number of males and females is increased with the volume of inhabited colonies, as is the average size of individuals. Individual colonies are commonly inhabited by a single crab or heterosexual pairs. Sexual dimorphism was found, females being larger than males. A positive relationship between the sizes of males and females in pairs was revealed, as in 73% pairs females were larger than males. The size of the clutch correlated positively to the size of the female. We hypothesize that the predominance of males in the population, as well as the individuals located among the hosts one by one, is the result of intersexual competition