MORPHOLOGY AND ULTRASTRUCTURE OF THE GILLS OF TERRESTRIAL CRABS (CRUSTACEA, GECARCINIDAE AND GRAPSIDAE) - ADAPTATIONS FOR AIR-BREATHING

The terrestrial crabs Geograpsus grayi, Geograpsus crinipes, Cardisoma hirtipes and Gecarcoidea natalis have a reduced number of gills and show a reduced planar gill surface (SA) compared to aquatic species. Gill lamellae are stiffened and thickened (increasing blood/gas (BG) diffusion distances) and nodules maintain wide spacing between lamellae. Haemolymph is directed through the gill lamellae by rows of pillar cells and in the afferent region an intralamellar septum splits the haemolymph into two parallel networks. Gaps in the lines of pillar cells allow movement of haemolymph between adjacent channels. The afferent vessel distributes haemolymph to the lamella via a number of direct channels including the marginal canal and in large gills with the aid of a long, forked sinus which supplies the ventral and central regions of the lamellae. The marginal canal functions in both distribution and collection of haemolymph; the role varies with species. Potential flow-control sites were identified at the junctions between afferent and efferent areas and where the efferent channels enter the efferent branchial vessel. Each gill receives a branch from the sternal artery which supplies all the lamellae. Transport epithelia is the principal cell type in the gills of all species examined though its location varies between species, either being confined to certain gills or specific parts of the lamellae. The gill lamellae of air-breathing crabs are clearly modified to breathe air (stiffening and presence of nodules), though the overall contribution of the gills to gas exchange has been reduced (smaller SA and longer BG diffusion distances). The role of the gills in air-breathing crabs thus appears to have switched from one of an efficient aquatic gas-exchanger (thin with large surface area) and transport tissue. to one that is predominantly set up for ion-regulation.