The biology and functional morphology of Minnivola pyxidatus (Bivalvia: Pectinoidea)

The shallow subtidal scallop Minnivola pyxidutus occurs in the relatively clean oceanic, southern, and south‐eastern waters of Hong Kong and elsewhere in the Western Pacific and is highly polymorphic in terms of shell patterning of the left, upper valve. Locally, because of sea‐bed perturbations, numbers are declining. The species can probably live for ∼ 3 years, with reproduction occurring in winter. M. pyxidatus is monoecious. Shell form and anatomy are described. Normally, the species lives attached by a fragile byssus to sand grains or stones, but can swim, albeit weakly, in response to predation risks. In most respects, Minnivola pyxidatus is a typical scallop, with a bowl‐shaped, right lower valve and a concave, disc‐like, left upper valve. The mantle has pallial eyes and a complex array of tentacles which possess small, ciliary‐based sensory cells. There is also a cleansing tract on the outer surface of the middle mantle fold and a muscular velum formed from the inner fold. The latter is developed into a simple exhalant aperture posteriorly. Both mantle lobes possess extensive posterior pallial glands and an abdominal sense organ on the right lobe only, anterior to the single posterior adductor muscle. Ciliary currents of the mantle cavity are for cleansing and are powerful, particularly on the visceral mass and right mantle lobe but not the left. The ctenidia and their ciliary currents are typical of the Pectinidae except that the ascending lamellae of all demibranchs are greatly reduced and, it is suggested, previse the situation seen in the Propeamussiidae where they are lost and the ventral margins of the inner and outer descending lamellae fuse to create an ‘inside out’ gill. Unusually, the labial palps of Minnivola pyxidatus are fused to the mantle and visceral mass, preventing their collapse in the mantle cavity and thus maintaining particle selection efficiency. The gut is simple. Left and right statocysts are unusually equal and the pericardium is located dorsal to the adductor muscle. Minnivola pyxidatus is adapted to an epibenthic life on subtidal sands and muds. Powerful cleansing currents and an ability to swim have allowed this species to survive in dredged and trawled waters with high levels of settling and resuspended silt.