Assemblage structure, distribution and habitat type of the grapsoid crabs (Brachyura: Grapsoidea) of the coastal forested swamps of northern Borneo

Abstract Several intertidal and supratidal systems were investigated in the Brunei Bay to assess the diversity and assemblage structure of grapsoid crabs (Brachyura: Grapsoidea) in different salinity and substrate conditions, type of vegetation, and distance from water bodies. The structure of the assemblages differed remarkably across and within sites, and several taxa were not previously recorded from this region. Diversity decreased towards more terrestrial conditions, with systems closer to the sea supporting more diversified assemblages. High and low intertidal areas also hosted different assemblages in terms of taxonomic composition. Most of these species can be considered habitat specialists, and only occurred in a small number of the investigated areas. A small number of habitat generalists were found in several areas, exhibiting eurytopic ecological traits. A time-based sampling method was used to estimate population densities. A few habitat specialists were recorded in high densities, suggesting adaptations to specific environmental conditions in which these species can bloom. These results highlight how the diversity of these assemblages needs to be considered in management plans, to prevent local extinctions and loss of diversity in the area.