Study of the importance of the soft abdomen of the hermit crab Pagurus bernhardus in minimising the mechanical effects of osmotic uptake of water

It has been previously suggested by the author (Davenport, 1972) that the abdomen of the hermit crab Pagurus bernhardus (L.) acted as a swelling reservoir, protecting the structures of the cephalothorax from the mechanical effects of osmotic inflow of water. To confirm this, evidence of fluid transfer from the cephalothorax to the abdomen is required. Experiments to demonstrate this transfer were performed, and it was found that, although the abdomen is more permeable to water than the cephalothorax, much of the abdominal swelling in dilute media is caused not by abdominal water uptake, but by transfer of excess fluid from the cephalothorax to the abdomen; 55.4% of the potential weight increase of the cephalothorax, caused by exposure to 30% sea water (100%=34 S) for 60 min, is transferred, presumably as haemolymph, to the abdomen.