Form and functional morphology of Raetellops pulchella (Bivalvia: Mactridae): an example of convergent evolution with anomalodesmatans

. The bivalve Raetellops pulchella is a highly specialized, deposit‐feeding member of the Mactridae. Studies of its form and function provide an example of how the bivalve body plan can be modified to facilitate the exploitation of mud as a food resource, and help in understanding how this lifestyle has evolved. Adaptations to this lifestyle include an overall reduction in ctenidial size and loss of the descending lamellae of both outer demibranchs. This reduction is associated with the enlargement of the labial palps to process inhaled sediment. In the mantle cavity, a waste canal below the posterior mantle flaps facilitates pseudofeces removal. The midgut is long and capacious, presumably to cope with the large amounts of ingested organic material. In addition, individuals of R. pulchella have unusually thin, brittle, and rostrate shells, with narrow siphonal gapes. They possess a shell buttress in each valve extending from the hinge plate to above the posterior adductor muscle. This buttress functions to prevent the brittle shell valves from fracturing when adduction occurs. A buttress is also seen in some representatives of the Anomalodesmata; in particular, the situation in R. pulchella is most like that seen in individuals of the similarly deposit‐feeding species Offadesma angasi (Anomalodesmata: Periplomatidae). I interpret the similar shell form of these deposit‐feeding clams as an example of convergent evolution.