A MECHANISM OF ADAPTATION TO HYPERGRAVITY IN THE STATOCYST OF APLYSIA-CALIFORNICA

The gravity-sensing organ of Aplysia californica consists of bilateral ly paired statocysts containing statoconia, which are granules compose d of calcium carbonate crystals in an organic matrix. In early embryon ic development, Aplysia contain a single granule called a statolith, a nd as the animal matures, statoconia production takes place. The objec tive of this study was to determine the effect of hypergravity on stat oconia production and homeostasis and explore a possible physiologic m echanism for regulating this process. Embryonic Aplysia were exposed t o normogravity or 3 x g or 5.7 x g and each day samples were analyzed for changes in statocyst, statolith, and body dimensions until they ha tched. In addition, early metamorphosed Aplysia (developmental stages 7-10) were exposed to hypergravity (2 x g) for 3 weeks, and statoconia number and statocyst and statoconia volumes were determined. We also determined the effects of hypergravity on statoconia production and ho meostasis in statocysts isolated from developmental stage 10 Aplysia. Since prior studies demonstrated that urease was important in the regu lation of statocyst pH and statoconia formation, we also evaluated the effect of hypergravity on urease activity. The results show that hype rgravity decreased statolith and body diameter in embryonic Aplysia in a magnitude-dependent fashion. In early metamorphosed Aplysia, hyperg ravity decreased statoconia number and volume. Similarly, there was an inhibition of statoconia production and a decrease in statoconia volu me in isolated statocysts exposed to hypergravity in culture. Urease a ctivity in statocysts decreased after exposure to hypergravity and was correlated with the decrease in statoconia production observed. In sh ort, there was a decrease in statoconia production with exposure to hy pergravity both in vivo and in vitro and a decrease in urease activity . It is concluded that exposure to hypergravity downregulates urease a ctivity, resulting in a significant decrease in the formation of stato conia.