ABSTRACT: The pseudolamellibranch is the only main bivalve particle processing system whose mechanisms of pseudofeces rejection on the mantle have not been elucidated. To this end, the entire mantle surface and underlying tissue characteristics were investigated in the oysters Crassostrea virginica and C. gigas, using scanning electron microscopy and histology, as well as observations of particle transport on half-shell preparations. Pseudofeces transport is effected by short simple cilia atop specialized radial ridges, which extend from the gill-mantle junction near the adductor muscle to the mantle edges. The histology of the ridges is characterized by a densely-ciliated pseudostratified epithelium and numerous acid mucopolysaccharide (AMPS)-containing mucocytes, and differs markedly from that of the inter-ridge regions which present a cuboidal epithelium containing few mucocytes and isolated tufts of long simple cilia. In contrast to the pseudofeces transport (infrabranchial) region of the mantle, the suprabranchial mantle region is uniformly and densely ciliated, and devoid of ridges. The sparse ciliation of the inter-ridge region and the counter-current beat of the ridge cilia (presumably isolated to varying degrees from the pallial flow by the mucus-particle rafts involved in mucociliary transport) indicate that the maintenance of pallial flow is accomplished almost exclusively by the gill and the suprabranchial mantle ciliation in these 2 species. Pallial pseudofeces rejection mechanisms to date are summarized for the 4 main bivalve gill systems, which may be grouped into 3 categories: (1) pallial transport with vertical elevation accomplished via composite cilia (homorhabdic filibranchs and eulamellibranchs), (2) pallial transport with vertical elevation accomplished via mantle ridges (pseudolamellibranchs), and (3) voidance via valve clapping (heterorhabdic filibranchs).

KEY WORDS: Oyster · Feeding · Mantle · Rejection · Pseudofeces

Published in MEPS Vol. 190 (1999) on December 14
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1999