Mechanism of Paddle Cilia Formation in Molluscan Veligers

Paddle cilia fixation artefacts in palliai organs of adultMytilus edulisandPlacopecten magellanicus(Mollusca, Bivalvia)

To ascertain the nature of paddle cilia in adult marine bivalves, three scanning electron microscopic (SEM) fixation protocols were used to fix ciliated organs of the palliai cavity in the blue mussel, Mytilus edulis, and the giant scallop Placopecten magellanicus (Bivalvia: Mytilidae, Pectinidae). SEM examination concentrated on the various regions of the labial palps and gills in M. edulis, with supplementary observations of the osphradium and adjacent regions in P. magellanicus. A regular transect approach was used to examine all surfaces, both for thoroughness and to reveal any eventual patterns in distribution. Cilia with paddle-like expansions were observed on all surfaces when both hypotonic glutaraldehyde–cacodylate buffer and hypotonic Sörensen's buffer were used; no intra- or inter-individual pattern was evident. No expansions were present when isotonic glutaraldehyde–cacodylate was employed. We conclude that paddle cilia may be induced in adult marine bivalves by hypotonic fixation; this has wider implications for the other bivalve species in which such cilia have been reported, in addition to the numerous accounts of such structures in other taxa.