NUCULANIDAE

La Perna, R., 2004, The identity of Yoldia micrometrica Seguenza, 1877 and three new deep-sea protobranchs from the Mediterranean (Bivalvia), Journal of Natural History 38, pp. 1045-1057 : 1046

Family NUCULANIDAE H. and A. Adams, 1858

Genus Yoldiella Verrill and Bush, 1897 Type-species Yoldia lucida Lovén, 1846 by original designation.

The systematic arrangement of protobranchs is problematic and not well settled yet (e.g. Allen and Hannah, 1986; Maxwell, 1988; Ockelmann and Warén, 1998). According to the most conservative usage (e.g. Warén, 1989; CLEMAM web-site), the genus Yoldiella includes species with elliptic valves, a short, blunt to sharp rostrum whose position ranges from postero-dorsal to almost postero-ventral, thick to delicate hinge with chevron-shaped teeth and amphidetic inner ligament. In this sense, Yoldiella is a provisional ‘pigeon-hole’, where many species are allocated in a kind of waiting list, rather than a natural group. Within this confused ‘genus’, some clusters of morphologically similar species can be recognized, with the morphological distance among these clusters suggesting distinct systematic ranks. The systematics of these protobranchs might be more clearly settled when, or if, it will be possible to limit these clusters and highlight distinct lineages. Possibly the genus Yoldiella s. l. might prove to be polyphyletic. It is worth noting the convergence in shape and hinge features of the type-species Y. lucida (Lovén, 1846) and of many allied species, to the juveniles of the family Malletiidae . If confirmed, these shared features might point to an origin of Yoldiella s. s. from malletiids through a progenetic retention of the juvenile inner ligament. Other species currently referred to Yoldiella show nuculanid relations ( Ockelmann and Warén, 1998). The unnatural composition of ‘ Yoldiella ’ was remarked upon by Schileyko (1985), who reported differences in shell and soft parts between some species close to Y. lucida , and others referred to Portlandia Mörch, 1857 . A similar subdivision was followed by Allen et al. (1995), but the systematic arrangement of this wide group remains unclear, as well as its systematic position (e.g. Schileyko, 1985; Allen and Hannah, 1986; Maxwell, 1988; Allen et al., 1995; Ockelmann and Warén, 1998).

Due to this state of confusion, in the present work the genus Yoldiella is applied in a broad sense, but excluding those species which can find a better placement in Ledella Verrill and Bush, 1897 (see below). Also, the family Nuculanidae is used in its broadest sense, following Ockelmann and Warén (1998).