Cilia with a distal membrane expansion enclosing a coiled end of the axoneme (paddle cilia or discocilia)

Cilia with a distal expansion of the membrane enclosing a curved or coiled end of the axoneme (paddle cilia or discocilia) have been widely reported in a variety of marine invertebrates. Various functions have been ascribed to paddle cilia, from acting as paddles to chemoreceptors (see Short and Tamm. 1991). Nevertheless, several authors have argued against the genuine nature of these organelles (Ehlers and Ehlers, 1978; Bone et al., 1982; Nielsen, 1987). Short and Tamm (1991) recently showed that paddle cilia in veliger larvae of bivalve molluscs are not genuine structures, but artifacts of hypo-osmotic solutions used by previous workers (Campos and Mann, 1988). Short and Tamm (1991) argued that most, if not all, examples of paddle cilia and discocilia are artifacts of non-physiological conditions. They proposed that coiling of the axoneme within the paddle is not due to tension by the surrounding membrane, but to a helical conformational change of the doublet microtubules induced by a Ca or proton flux across an osmotically stressed membrane. We tested this model for paddle cilia formation in order to gain further knowledge of the structural and mechanical properties of ciliary axonemes and membranes. We found that axonemal coiling is due to tension exerted on an elastic axoneme by the enclosing membrane. Furthermore, the direction of coiling is specific, indicating an intrinsic asymmetry of stiffness of the axoneme within the ciliary beat plane. These findings illustrate important mechanical properties of ciliary axonemes and membranes that will be useful for understanding the mechanism and control of ciliary movement. Some of these findings have been published elsewhere in preliminary form (Deiner and Tamm, 1991; Deiner et al., 1992).