HEMOLYMPH OSMOREGULATION AND THE FATE OF SODIUM AND CHLORIDE DURING DEHYDRATION IN TERRESTRIAL ISOPODS

Osmoregulation of the haemolymph during dehydration was investigated i n a selection of temperate oniscidean isopods. Inulin tracer studies s how that the haemolymph contributes approximately 69% of water losses in Porcellio scaber, significantly more than predicted from the volume of this compartment (42% of total water), Haemolymph osmolality incre ases linearly as a function of haemolymph dehydration but at a signifi cantly lower rate than predicted from the change in haemolymph fluid v olume, Similar results for Oniscus asellus show that both species disp lay efficient osmoregulation until lethal dehydration, Osmoregulation is associated with significant hyporegulation of haemolymph sodium and chloride, These findings indicate that: (1) cell water is conserved a t the expense of the haemolymph; and (2) haemolymph dehydration is ass ociated with the removal of Na+ and Cl- contributing to net osmoregula tion, During dehydration, accumulations of both Na+ and Cl- are seen i n the hindgut, with significant accumulations of electrolytes also see n in the luminal fluid of the hepatopancreas, Low fluid volumes in the foregut and hindgut suggest macromolecular association as the most pl ausible mechanism of ion sequestration, Evidence refutes ion excretion and haemocyte sequestration as osmoregulatory mechanisms, Sequestrati on of Na+ as urate salts, as shown for Periplaneta and generally assum ed for other insects, is insignificant in isopods. (C) 1997 Elsevier S cience Ltd.