The biology and functional morphology of Nucula pusilla (Bivalvia: Protobranchia: Nuculidae) from Western Australia, Australia: primitive or miniature simplicity?

WA Museum Records and Supplements | Updated 7 years ago

ABSTRACT – The southern Australian nuculid Nucula pusilla is < 3 mm in shell length, making it one of the smallest known protobranchs and one of the smallest bivalves. It lives in clean, well-aerated, coarse, offshore marine sands in the southern half of the Australian continent. The bivalve lies in the sand with the pointed anterior end of the shell directed upwards at an angle of ~45° to the sediment surface and inhales water into the mantle cavity from this direction. The more rounded posterior shell margin sits within the sediment. The posterior exhalant current, created by the posterior, largely respiratory ctenidia assist in the removal, along with ciliary currents on the mantle surface and visceral mass, of unwanted particles. Nucula pusilla possesses a pair of large, closed statocysts with numerous statoconia, situated above the pedal ganglia and these are likely to be responsible for the bivalve’s orientation. There is also a minute Stempel’s organ located on the outer face of the anterior adductor muscle. Feeding is largely by means of palp proboscides that collect subsurface material and transport it along anterior ciliated grooves where it reaches and is sorted by the labial palps where some particles are accepted and others are rejected. The intestine is coiled complexly in the visceral mass but the style sac is much simplifi ed in comparison with N. sulcata.

The mantle margin of Nucula pusilla is simplified, with the periostracal groove, in contrast to N. sulcata, extending almost to the pallial line. Other features of N. pusilla anatomy are described and although this species is clearly closely similar in morphological terms to other widely accepted ‘primitive’ nuculids, it appears that miniaturisation has resulted in some anatomical simplifi cation. As with all species, form and function result ultimately from ancestry and the evolutionary path arising from natural selection. Aspects of the morphology of N. pusilla, such as the mantle margin, the simply structured style sac, the uniquely protobranch Stempel’s organ, and the mantle cavity brooding of lecithotrophic larvae, are indicative of a ‘primitive’, perhaps ancestral, nuculid condition. Other aspects of the overall body plan, such as the loss of the anterior pedal retractor muscles, suggest simplifi cation resulting from miniaturisation. It is suggested that through paedomorphosis, miniaturisation of a nuculid ancestor could be the means by which the ‘higher’ lamellibranch bivalves have so successfully adopted a ctenidial and suspension feeding lifestyle and been released from the constraints that deposit feeding imposes. These bivalves have subsequently diverged into a complex array of species inhabiting virtually every aquatic habitat on Earth.