6533b873fe1ef96bd12d5f4c

RESEARCH PRODUCT

The duplivincular ligament of recent Pinna Nobilis L., 1758: further evidence for pterineid ancestry of the Pinnoidea

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description

A correct interpretation of ligament ontogeny and structure is essential for establishing phylogenetic relationships among higher taxa in the bivalve superorder Pteriomorphia. Recent research on pteriomorphian ligaments has focused on understanding ligament morphospace (Thomas et al., 2000; Ubukata, 2003) and evolutionary pathways. In this regard, studies of the transition from larval to post-larval and adult ligaments (Malchus, 2004) have been especially fruitful. Members of the pteriomorphian superfamily Pinnoidea live with their tapered anterior end buried to varying degrees in sediment. The fan shell Pinna may be buried up to one third of its length (Templado, 2004) (Fig. 1), and Atrina is even more completely buried (Hippeau-Jaquotte, 1974). Under these circumstances, shell gaping for respiration and feeding might be expected to allow sediment to enter the mantle cavity. Pinnoidean mantle lobes, unlike those of many other infaunal bivalves, are not extensively fused ventrally (Yonge, 1953). Sediment is largely excluded from entering the ventral shell margins because the pinnoidean ligament has lost its shell-opening and closing function and merely holds the dorsal and, indirectly, the ventral shell margins together. Shell opening is restricted in this genus to the extreme posterior end, where the calcitic prismatic outer shell layer flexes to open the shell when the posterior adductor muscle is relaxed (Yonge, 1953; Carter, 1990, p. 212). In Streptopinna , the shell valves are uniquely fused together dorsoposteriorly by the calcitic outer shell layer (Cox and Hertlein, 1969; Waller, 1990).  Figure 1 —Drawing of a Pinna nobilis individual as is usually found with the tapered anterior third of the shell buried in the sediment, attached to the substratum by byssus threads. Each rectangle delimits the portions of the shell shown in subsequent figures The pinnoidean ligament has traditionally been described as subinternal, opisthodetic with a single couplet …