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The bivalve Limopsis cristata pursues a semi-endobenthic life in abyssal soft sediments. It attaches to particles by up to three byssal threads and filter feeds by inhaling water from posterior and anterior directions. Because of partial burial, however, only the latter is functionally significant. Complex ciliary currents in the mantle cavity concerned with the rejection of unwanted particles keep most material out of the simplified intestine. It is generally considered that the ligament is the constraining force in arcoid evolution. This may be true in part, but the lack of pallial fusions and the retention of anterior and posterior inhalant flows are more powerful limits to radiation in the Arcoida. In the deep sea, the Limopsidae has radiated into many micro-niches through micro-morphological adaptations. Loss of the arcoid 'heel' has resulted in the union of the separate rejectory currents of the visceral mass and foot, creating a single discharge point in more advanced bivalves. This greatly simplifies the rejectory roles of the visceral mass and foot and is thus of functional and selective advantage. [ABSTRACT FROM AUTHOR]

Nuculid bivalves of the Cape Melville Formation (Early Miocene, King George Island) are reviewed. Ten bivalve taxa are listed from the formation in the families Nuculidae (two species), Sareptidae, Malletiidae, Limopsidae (two species), Limidae, Pectinidae, Hiatellidae, and Periplomatidae. The Nuculidae consist of two species of Leionucula Quenstedt, 1930. One of these, L. melvilleana n. sp., is described and the other consists of the two species named previously by Anelli et al. (2006), which are demonstrated to be synonymous and are assigned to the species Leionucula frigida (Anelli, Rocha-Campos, Santos, Perinotto & Quaglio 2006). This assemblage, dominated by protobranchs (89% of specimens), is a typical fauna of offshore soft substrates, with a few specimens transported from hard substrates nearby. The diversity of Nuculidae has decreased in the Antarctic region through the Cenozoic. [ABSTRACT FROM AUTHOR]

The diversity and adaptive radiations of modern Arcoidea, here considered to contain the families Arcidae, Noetiidae, Cucullaeidae, and Glycymerididae, are reviewed. Most fall into either epibyssate or endobyssate life habits with only the Glycymerididae living as free burrowers. The phenetic characters of the families within the Arcoida are reviewed and the families are shown to be supported by very few synapomorphic characters. Homoplasy is shown to be widespread and is illustrated in a series of discussions on the ligament, epibyssate–endobyssate radiations, and possible parallelism within genera, and in a review of arcoid anatomical characters. Previously published molecular data are reviewed and these support the inclusion of the Glycymerididae in the Arcoidea. They also indicate, however, that polyphyly is probably widespread at the subfamily level. © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society, 2006, 148, 237–251. [ABSTRACT FROM AUTHOR]

Early ontogenetic shells of Limopsis angusta , L. aurita , L. cristata , L. friedbergi and L. minuta are described in detail and their morphogenetic traits discussed in a phylogenetic context. Prodissoconch length is found to range from 170–370 µm discounting the so-called prodissoconch 2 stage of authors. Reports of such a stage refer to the early postlarval ‘interdissoconch’. Prodissoconch sizes are indicative of lecithotrophy but not of brooding. Limopsid-like prodissoconchs are rather common among pteriomorphs and at present of little use for phylogenetic interpretations, therefore. The early postlarval hinge is characterized by two tooth generations: an early one, (G1), representing the postlarval continuation of provincular teeth, and a late one, (G2), representing the independent adult dentition. This is a plesiomorphic trait for Pteriomorphia. The postlarval ligament is continuous with the larval resilium; it is neither co-functional with nor is it substituted by the typical duplivincular ligament as in other arcoids. Occasionally developed ridges, gutters or multiplication of ligament sublayers remain restricted to the primordial resilifer. It is concluded that the limopsid adult ligament represents a fusion of adult sublayer repetition within a retained larval resilium, a phenomenon best described as a heterochronic process. Comparisons with homologous characters of other arcoids support the view that Limopsidae evolved from parallelodontid arcoids and that they gave rise to Philobryidae but not to Glycymerididae. [ABSTRACT FROM AUTHOR]