To investigate particle transport mechanisms on the bivalve pseudolamellibranch gill, the mucocyte secretion types and distribution were determined for this organ in the eastern oyster Crassostrea virginica, and related to previous endoscopic data. Three adult oysters were collected from Shediac Bay (New Brunswick, Canada) in July 1994, immediately fixed, then dissected and processed for histology and whole mount rnucocyte mapplng uslng a modification of the penodic acidSchiff-alcian blue protocol. One type of mucocyte contained acid-secretion rnucopolysaccharides (AMPS), while the other type contained neutral rnucopolysaccharldes (NhIPS). A clear grad~ent in nlucocyte density was observed from the plical crest to the trough; for all but the anteriorn~ost 15 plicae the proportions of each mucocyte type remained constant; the 15 anteriormost plicae presented an increased proportion of AMPS. These proportions would produce a relahvely viscous acid-dominant mucus after mixing on the c~liated gill surfacr. The principal filament troughs contained relatively few Inucocytes, aligned on the median ndge This arrangement could account for the observed range of partlcle velocities in these filaments. The results of the present study conform to a pattern of specialization of nlucus types and functions on the diverse types of bivalve gill, depending on particle trajectory, transporting surface architecture, and dominant current flow.
To date, the marine eulamellibranch gill type is the only one of the 4 principal bivalve gill types whose mechanisms of particle processing have not been thoroughly Investigated using the contemporary techniques of endoscopy and determination of mucocyte distribution. To this end, particle handling on the pallial organs (gills, palps, lips, mantle) of Spisula S~~l d~s s i~a and Mya arenaria was examined using endoscopy and histology, as well as half-shell preparations. In both species, all particles intercepted by the gill were transported ventrally to the gill particle groove and then anteriorly to the labial palps. Rejected particles (i.e. pseudofaeces) were shunted to the palp ventral margin, and thence posteriorly to the palp tip and ultimately to the mantle. Pseudofaeces were transported along a narrow, distinct pathway on the ventral margin of the mantle to the inhalent siphon. It is postulated that such a pathway is characteristic of bivalves possessing a single gill particle groove and siphons. The transport medium for particles on the gill was acid mucopolysaccharides (AMPS). This conforms to an emerging pattern for counter-current particle transport on exposed surfaces Examination of mucocyte distribution and residual AMPS suggests that in M. arenaria, and perhaps also S. soljdissirna, AMPS is secreted onto the gill filament frontal surface from cells remotely located on the lateral faces of the filament. Although S, solidissm~a also presents mucocytes on its frontal surface, its lateral mucocytes are much more developed. In M arenana, mucus-particle masses dest~ned for Ingestion were mechanlcally fluidized by the labial palps. The presence of mucocytes contaming neutral-mucopolysaccharide In the gill particle groove suggests that there may also be a biochemical component to fluidization. It was not possible to observe palp treatment of particles destined for ingestion in S. solidissirna. Ingestlon volume control was effected in both species at 2 levels: closure of the gill particle groove, and closure of the lower lip of the mouth. Although few differences in pseudofaeces pathways were observed between specimens examined endoscopically and in half-shell preparations, the latter were not suitable for study of particle processing for ingestion.
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