Particle processing mechanisms in the archaic, peri-hydrothermal vent bivalve Bathypecten vulcani, inferred from cilia and mucocyte distributions on the gill

Abstract

The autobranch bivalve Bathypecten vulcani is a peri-hydrothermal vent 'living fossil'; as such, it is of particular interest both with respect to its trophic strategy and with respect to its particle processing mechanisms. These aspects are investigated through a study of the gill using scanning electron microscopy and mucocyte mapping. B. vulcani possesses a flat, homorhabdic gill, which lacks the morphological specializations associated with endosymbiont presence. Abfrontal and latero-frontal cilia, frontal tract differentiation, organic inter- or intrafilamentar junctions, an ascending lamella on the external demibranchs, and ventral particle grooves are all absent. The lack of latero-frontal cilia in this homorhabdic gill points to direct particle capture on the frontal tracts, a novel mechanism in adult bivalves. The abundant mucus-particle masses on the ventralmost frontal surfaces, which first come in contact with incoming water currents, together with an extremely high density of acid (AM) and acid-dominant (ADM) mucopolysaccharide-secreting mucocytes, suggest that particle capture is initially effected in this zone. The presence of antero-posteriorly oriented mucus cords in the gill arch indicate that transport on the gill is first dorsal, then anteriorward to the buccal region. An increasing postero-anterior gradient in mucocyte numbers and volumes suggests that transfer to the labial palps occurs via an exposed mucus bridge. These findings are the first substantiated representations of primitive autobranch particle processing, prior to the evolution of ventral particle grooves, latero-frontal cilia, or the heterorhabdic condition. It thus serves as a starting point for the understanding of the evolution of particle processing mechanisms in the basal pectinid condition, as well as in contemporary autobranch bivalves.