Statocyst structure in representatives of each, as currently recognized, family of the bivalve sub-class Anomalodesmata has been investigated.
Three types are recognized—A, B and C. Type A is possessed only by Pholadomya Candida, a representative of the ancient Pholadomyacea. Type C is found only in the Cuspidariacea. Type B, possessed by all other anomalodesmatans is divided, on the basis of statolith structure, into three sub-types—B1, B2 and B3.
Type A statocyst is extraordinarily complex with a capsule penetrated by nerve endings and a multicellular statolith. Such a sophisticated structure is adjudged essential for a bivalve that is postulated to lie angled on its back in the sediment and to collect subsurface deposits via the pedal gape, using the foot as a suction piston.
Type B statocyst comprises a large, multicellular capsule of ciliated cells as the sensory receptors. Variations in structure relate to whether or not there is a single, large, statolith (B1), a large statolith and other statoconia (B2), or numerous crystal-like statoconia, one of which may be enlarged into a statolith (B3).
Type C statocyst comprises a small capsule of but four or five cells in transverse section, each of which is swollen and lined by microvilli. A large, oval statolith is not freely mobile within the lumen and probably confers little fine orientation discrimination.
It is concluded that statocyst structure has some phylogenetic significance, widely separating the Pholadomyidae from other anomalodesmatans and indicating that the Cuspidariacea may have a separate origin from the other deep-water ‘septibranch’ predators. Type B3 statocyst (crystal-like statoconia) appears typical of the Clavagellacea, but is also possessed by Myadora striata.
All other anomalodesmatans possess either Type B1 or B2 statocysts, and with so much variation between and within constituent genera, families and superfamilies, little adaptive significance is attached to form. All other bivalves investigated to date have a statocyst structure of Type B, this probably representing ‘primitive’ conditions. Earlier conclusions from studies of other bivalves that type B2 or B3 is more primitive than B1 cannot be substantiated, because more comprehensive studies of these phylogenies may reveal a wider diversity of form than is at present realized.
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