Function of Molluscan Statocysts

The gravity sensors of most molluscs are spherical organs called statocysts. The wall of the sphere contains mechanosensory cells whose sensory cilia project into the lumen of the cyst. The lumen is filled with fluid and dense “stones”, the statoconia or statoliths, which sink under the influence of gravity to load and stimulate receptor cells which are at the bottom. The composition of the statoconia is known in only a few species. Data presented here suggest that they are aragonite in Aplysia. The statoconia of Aplysia californica are shown to be calcified about a lamellar arrangement of membranes. Similar lamellar membrane arrangements are seen within the receptor cells, and their possible role in the formation of the statoconia is discussed. Scanning electron micrographs of unfixed statoconia reveal plate-like crystallization on their surface. Elemental analysis shows a relatively high strontium content. This is of interest in light of the recent report (Bidwell et al., 1986) that strontium is required in the culture medium of several laboratory-reared molluscs before statoconia develop.