LABIAL PALPS OF THE BLUE MUSSEL MYTILUS-EDULIS (BIVALVIA, MYTILIDAE)

As a basis for understanding the functions of labial palps in the blue mussel Mytilus edulis, the structure and histology of palps were stud ied using light and scanning electron microscopy. Mussels used in the present study were collected in August 1993 and April 1994. The palp r idged surface is characterized by the presence of a smooth but densely ciliated dorsal fold, upon which rests the corresponding demibranch v entral region. The underside of the dorsal fold and the palp ridges fu se to form vestigial ciliated tracts. The dorsal fold is capable of co ntraction, allowing it to cover variable amounts of the ridged surface . Two different types of mucocyte are present on the palp ridged surfa ce: subepithelial, glandular, acid-dominant secretion mucocytes and ep ithelial mucocytes characterized neutral secretions. In histological s ection, these mucocytes appear to be concentrated on anatomical featur es known to intervene in particle handling. The anatomical and histolo gical features of the smooth surface are typical of bivalve labial pal ps, except that the dense ciliation of the dorsal fold begins in the d orsal region of the smooth surface, indicating the possible origin of this feature. Previous studies on M. edulis point to the palps as the probable site for both ingestion volume control and particle selection ; the anatomical basis of the present study should facilitate further research on these aspects.