Studies on the physiology of a shrimp, Metapenaeus sp. (Crustacea: Decapoda: Penaeidae). V. Calcium metabolism

Abstract

The gills, functioning as simple membranes in exchange with the external medium, account for 90 % of total ⁴⁵Ca uptake and 70 % of total output. Exchange also occurred across abdominal cuticle and via the anus (net outwards exchange 13 % and 7 %, respectively, of total). The long midgut appeared to be a site of calcium excretion. Antenna1 gland output was only 0.5% of the total. Blood ⁴⁵Ca uptake gave an exponential curve, resolved by graphical analysis into three components: (1) a "fast" component due to completely reversible exchange across the gills; (2) a "medium" component due to loss via the abdomen; and (3) a "slow" component due to deposition in the cuticle. Calcium was lost rapidly in calcium-low artificial seawater, and most of the loss was made good within 24 hr. Calcium above normal levels did not result in a significant increase in body calcium. ⁴⁵Ca was freely exchangeable between soft tissues but complete exchange did not occur with the external medium, largely due to "bound" calcium in the cuticle.

Calcium in the exocuticle was revealed only by histochemical tests, but visible calcium deposits occurred at the exocuticle-endocuticle interface. The latter begin at scattered calcification centres, grow concentrically, and may become confluent; deposition continuing through the intermoult period. This type of calcification appears to occur in a number of Malacostraca. Calcium ot the exocuticle seems to be "bound", but that of the interface is labile and apparently serves as a calcium (or alkaline) reserve, though this calcium does not appear to be resorbed prior to ecdysis.