On the Nature of Paddle Cilia and Discocilia

Cilia with paddle-shaped or disc-shaped tips enclosing a curved end of the axoneme (paddle cilia or discocilia) have been described in a variety of marine invertebrates. Although numerous studies, in which fixed specimens were used, claimed that paddle cilia and discocilia are genuine structures of unknown function, several studies, in which fresh living material was used, reported that modified cilia are artifacts. We have re-investigated a recent SEM report that paddle cilia are genuine organelles in veliger larvae of marine bivalves (Campos and Mann, 1988). Using high-speed video and electronic flash DIC microscopy, we find no paddle cilia in living larvae of Spisula solidissima and Lyrodus pedicellatus. Hypotonic seawater, however, induces formation of paddle cilia and vesiculations of the ciliary membrane in these veligers, as does the hypotonic SEM fixative used by Campos and Mann (1988). Fixatives that are isosmotic with seawater, on the other hand, do not induce paddle cilia. We conclude that paddle cilia are artifacts, and we propose a unifying mechanism to explain their production in various animals under different conditions.