Responses of Two Semiterrestrial Isopods, Ligia exotica and Ligia taiwanensis (Crustacea) to Osmotic Stress

When immersed in fresh water, Ligia taiwanensis is a poorer osmoregulator than Ligia exotica, as judged by a lower LD 50 at 96 hr and by the osmolalities of haemolymph. Animals appear to osmoregulate more efficiently in air. On immersion, both species displayed hyper- and hypo-osmoregulatory ability. Both species subjected less osmotic selection pressure during their inland colonization. The results suggest that a route of terrestrial colonization not involving transitional freshwater stresses had been taken by L. exotica and L. taiwanensis.