The adaptations of Frenamya ceylanica (Bivalvia: Anomalodesmata: Pandoracea) to life on the surface of soft muds

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A representative of the Anomalodesmata — Frenamya ceylanica (Pandoracea:Pandoridae) — from Hong Kong is described. Details of the ligament confirm that Frenamya, hitherto considered but a subgenus of Pandora, is a valid genus and is re-established. Frenamya is either slightly buried or lives on the surface of soft sublittoral muds. In various ways, including a deeply recessed mantle margin, powerful cleansing currents in the mantle cavity, copious mucus production and long proximal oral grooves, Frenamya is adapted to high amounts of sediment in suspension and the consequent removal of large quantities of pseudofaeces from the mantle cavity. Frenamya shows convergent similarities with the discoidal Placuna placenta (Anomiacea:Placunidae) to life on such substrates, i.e. extreme lateral compression and a form that enables both to ‘float’ on the mud surface. Frenamya is not discoidal but a greatly expanded posterior shell face, gives a broad scimitar shape, that fulfils the same function. In both, a sunken, dorso-ventrally elongate, primary ligament and long ‘secondary’ hinge teeth or crura, serve to prevent shear and align the shell valves. A dorsal ‘secondary ligament’ of fused periostracum assists in valve alignment and prevents sediment gaining access to the dorsal region of the shell. Frenamya occupies a habitat essentially similar to that of Placuna but has a narrower range because of the restrictions imposed by simultaneous hermaphroditism and, probably, as in Pandora, a short larval life. The reproductively simpler Placuna is more widely dominant on such substrates in the tropics.