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The biology and functional morphology of Claudiconcha japonica (BivaMa: Veneracea)

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Abstract

Claudiconcha japonica Dunker 1882 is a member of the Petricolidae in which family there is a clear trend towards a tunnelling mode of life culminating in the evolution of species such as Petricola pholadiformis–a borer of soft rocks and shales–with appropriate anatomical and morphological adaptations. Such convergent adaptations recall those seen in another unrelated group of rock-borers–the Pholadidae.

The tunnelling habit in modern petricolids undoubtedly arose from a nestling mode of life which is commonly encountered in other members of the Veneracea. Some, possibly more primitive, petricolids only have the ability to enlarge the crevice they inhabit.

C. japonica is different, however, in that for the first part of its life it is a motile (though usually found byssally attached), equivaive bivalve. With site selection, however, the margin of the right valve undergoes considerable enlargement to curl around and partially enclose the unmodified left valve. This development, associated with the selection of a crevice for permanent habitation, has no effect upon the disposition of the organs of the mantle and body cavities as in other inequivalve bivalves and they remain typically bilaterally equal.

It is impossible for C. japonica to tunnel since the secondary development of the right valve fills the chosen site and in so doing comparatively immobilizes the left valve and closes the pedal gape. Thus C. japonica and its close relatives must be considered to be on a separate line of evolution from other petricolids and it is doubtful that the present sub-generic status of Claudiconcha is sufficient to highlight this difference. It seems more appropriate to genericize this name thereby amplifying this trend.

In other morphological respects, which are described, C. japonica is clearly a member of the Petricolidae and functional aspects of these adaptations suit the animal to a life in relatively clear waters but where nevertheless the niche tends to accumulate detritus which must be kept out of or removed from the mantle cavity.

The secondary enlargement of the right valve, associated with the selection of a suitable site for permanent habitation also appears to be associated with the onset of sexual maturity –a feature again reminiscent of the tunnelling Pholadidae.

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