The biology and functional morphology of Clavagella australis (Bivalvia: Anomalodesmata)

Abstract

The biology and functional morphology of the unusual boring anomalodesmatan bivalve Clavagella (Dacosta) australis (Clavagellacea) is described. The animal occupies a borehole in calcareous (including coral) rocks to which it cements itself by the left shell valve. Radial mantle glands developed in the middle folds of the siphonal tip produce the glue but also stick sand grains to the periostracum to camouflage the siphons. The right valve is free. The shell is somewhat inequilateral in form, the adductors large and slightly heteromyarian. The pallial retractor muscles are also large so that the ventral mantle is very thick. Because mantle fusion involves inner, middle and inner surfaces of the outer folds (type C), the periostracum is continuous so that shell and mantle are almost completely, except for the siphonal orifices and the pedal gape, enclosed in periostracum. The simple sunken primary ligament comprising inner ligament layer only and located between chondrophores is flanked anteriorly and posteriorly by fused periostracum, assisting, in the absence of teeth, in hinge alignment.Boring is probably a chemical process, a secretion from the inner surface of the anterior mantle being discharged onto the burrow wall from the pedal gape. The calcium carbonate eroded in this way is possibly used to partially line the burrow. The distal siphonal tube, however, is secreted from siphonal glands discharging from the tip of the siphons.In most other anatomical respects Clavagella is typical of the Anomalodesmata, the ctenidia and labial palps for example being of characteristic form. The statocysts, however, are unusual in that the statolith comprises a large number of small, diamond‐shaped crystals that stimulate statocyst cilia. C. australis appears to be dioecious, whereas most other anomalodesmatans are simultaneous hermaphrodites. Clavagella may be linked to a pholadomyacean ancestor and most closely resembles members of the infaunal Latemulidae. It represents an evolutionary line that has adopted first a nestling then a boring mode of life. It seems probable that the more specialized infaunally buried clavagellids with both valves incorporated into the adventitious tube are derived from a nestling/boring ancestor.