The structure of some cephalopod statoliths

Abstract

The statoliths of Sepia officinalis, Octopus vulgaris, Alloteuthis subulata and Taonius megalops have a smooth outline, but an irregular shape. They have projections and indentations. The statoliths from a pair of statocysts are usually quite similar in size and shape, and the general pattern is probably maintained throughout the size range of the species. Statoliths from large animals are marginally larger than those from smaller ones. The statolith usually occupies only a small part of the cavity of the statocyst, and it is situated in the anterior part of the statocyst. They are joined to the macula by hairs extending from it. These hairs are very delicate and easily broken during preparation of the specimens. The hairs are much longer and narrower than the receptor cilia of the macula. The receptor cilia are enclosed within holes in the tangled hairlike anchoring fibrils. The statolith is made up of crystalline subunits, the statoconia. The crystals vary in size, they are usually elongated, hexagonal with pointed ends. The statolith consists of a closely packed mass of these crystals, sometimes they are irregularly arranged, where in others they are stacked with their long axes parallel. In Sepia officinalis and Taonius megalops, the crystals are arranged in regular shaped packets and these packets of crystals are stacked together. These larger subunits are not always arranged in a regular way, and their major axes can be organised in several different ways. The size and outline of these large subunits do vary in different parts of the statolith. The external surface of the statolith is macroscopically smooth. Over some parts there is a surface layer covering the rod-like crystals that make up the major bulk of the stone. In other regions, the surface is rough at a microscopic level, the roughness is produced by the exposed ends of the filamentous crystals. The crystals are composed of calcium carbonate in the form of aragonite.