Gill-cleaning mechanisms of the amphibious freshwater crab Geothelphusa dehaani (Decapoda, Brachyura, Potamidae)

Abstract

Geothelphusa dehaani (White, 1847) is an amphibious crab inhabiting rivers and streams in Japan. Its gill complement consists of nine phyllobranchiate gills, which are cleaned by the passive action of setiferous epipods on all maxillipeds. Epipodal setae are of two types; anchor setae distally have two rows of stout outgrowths with opposing recurved ends (anchor-like), and brush setae have dense needle-like setules distally surrounding the setal shaft. Both setae project from the epipod margins mostly onto gill surfaces, with a few setae inserting between the lamellae. The functional morphology of epipodal grooming in brachyurans is inferred to be a better design solution than in other decapod taxa. Subjecting crabs to dehydration resulted in severe fouling and breakage of epipodal setae, thus indicating the importance of branchial water to gill grooming. Form-function analysis suggested that the use of setiferous epipods as a common gill-cleaning mechanism in the Brachyura was secondarily derived as a result of evolution to a crab-like body form, which caused the tight enclosure of the gills in and the exclusion of the limb bases from the branchial chambers. The epipodal setae, especially anchor setae, of G. dehaani were morphologically compared with those of several brachyuran taxa, including other Japanese potamoid species.