Particle processing mechanisms of the eulamellibranch bivalves Spisula solidissima and Mya arenaria

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 solidissima 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. solidissima, AMPS is secreted onto the gill filament frontal surface from cells remotely located on the lateral faces of the filament. Although S. solidissima also presents mucocytes on its frontal surface, its lateral mucocytes are much more developed. In M. arenaria, mucus-particle masses destined for ingestion were mechanically fluidized by the labial palps. The presence of mucocytes containing 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. solidissima. Ingestion 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.