Storage and translocation of integumentary calcium during the moult cycle of the terrestrial isopod Oniscus asellus (L.)

Abstract

The calcium content of pieces of integument of standardized size from various regions of the body was measured at all 15 stages of the intermoult cycle. Calcium resorbed from the posterior integument during premoult is stored mainly in the sternal calcium deposits of the anterior region, which contain 20% of total body calcium in late premoult. Earlier arguments that the deposits are not a store of calcium are refuted. These deposits are similar to gastroliths in both structure and function. After posterior ecdysis, calcium in the deposits is resorbed rapidly and employed in calcification of the new posterior exocuticle. Calcium is then resorbed from the general anterior integument within 24 h and may accumulate transiently in the haemolymph. During this period resorption of integumentary calcium and calcification proceed simultaneously in the anterior and posterior regions of the animal. Ecdysis of the anterior region then occurs, and calcium from the haemolymph is employed in calcification of the new anterior exocuticle. Forty-eight percent of intermoult calcium content is conserved. However, both exuviae are eaten and it is argued that further calcium is conserved by resorption from the exuviae within the gut. It is suggested that this calcium, together with dietary calcium, is employed in calcification of the endocuticle after ecdysis. It is proposed that comparable events occur in all crustaceans that conserve calcium, but are rendered especially dramatic in isopods, owing to the unusual biphasic pattern of cuticle secretion.