Studies on the pedal gland and pedal epithelium of the protobranchiate bivalvia

The morphology and secretory functions of the pedal glands of eight species of protobranch bivalves were examined using light microscopy. The morphology of the gland varies among species. It is entirely absent in Acila castrensis, Yoldia hyperborea, and Y. limatula. Nuculana taphria has a reduced, and apparently nonfunctional vestige of a gland. Functional glands are found in Solemya velum, Ennucula tenuis, Portlandia arctica, and P. lenticula. Glands are most likely used to reduce friction between the bivalve\u27s foot and the substratum into which they burrow. The resulting pedal mucous gland data were incorporated into a cladistic analysis of the bivalve Subclass Protobranchia. While there is a pressing need for identification of additional characters for incorporation into the analysis, it appears that the protobranch pedal gland is a potentially useful taxonomic character, and merits further investigation. The ultrastructure of the epithelium from three regions of the foot of the awning clam Solemya velum was also investigated. Microvillous cells were found on the sole and shaft epithelium. Microvillous cells have been implicated in absorption of dissolved organic matter as well as free amino acids in a variety of mollusc taxa. The presence of such an epithelium is of particular interest for S. velum because it has a reduced, but functional gut and symbiotic sulfur-oxidizing bacteria in its ctenidia. Absorption of substances through the pedal epithelium could be an important source of nutrition for this bivalve. The epithelium of the papillae that fringe the sole is pseudostratified and comprised entirely of secretory goblet cells containing numerous electron-dense granules