Light- and electron-microscopic analysis of the statocyst of the American crayfishOrconectes limosus (Crustacea, Decapoda)

Abstract

The statocyst ofOrconectes limosus contains static hairs arranged in four groups. All the hairs are the same in basic structure; they differ only in length and diameter and in their positions with respect to the other hairs in the group and to the statolith. In terms of functional morphology, each static hair is part of a unit consisting of an acellular lever string, three receptor cells, a scolopale cell, sheath cells, and enveloping cells. The lever string comprises two components in a characteristic longitudinal arrangement. The structure of the receptor cells resembles that of the arthropod chemo- and mechanoreceptors studied previously. The cilium and the postciliary section lie within two receptor cavities, formed by the scolopale cell and the sheath cells; the two cavities communicate with one another. The receptor cells are fixed in position by various structures. Proximally they form desmosomes with the scolopale cell, medially they are joined by filaments to the inner wall of cavity 1, and distally they are retained by a constriction between the two cavities. Two possible stimulus-mediating mechanisms are discussed: pressure changes in the receptor cavities and shearing of the base of the cilia with respect to the preciliary region. The lever string is part of the cuticle and hence is shed during molting. Nevertheless, the statocyst remains functional during this process because new structural units are formed below the old cuticle prior to ecdysis.