Sheppard Charles - Advances in Marine Biology

Anomalodesmata; Bentholyonsiidae; Carnivorous lifestyle; Ctenidia; Evolution; Prey; Phylogeny; Septum

Considered the second most diverse class of the Mollusca, it is currently estimated that there are between 8000 and 20,000 species of Bivalvia Linnaeus, 1758 contained in 106 families (. Of the, as currently considered, 106 families there are, at present, 1257 known species (13.7%) belonging to 12 (11.3%) families occurring in freshwater habitats. The remainder (86.3%) is marine and here they occupy all latitudes and virtually all habitats from the intertidal to the abyss.

The bivalves first evolved in the geologically brief period of the Cambrian explosion, which took place between 540 and 520 million years ago ().

Subsequent to the Ordovician, the bivalves have radiated to occupy most aquatic habitats and on hard and soft substrata the numbers of individuals can be enormous. For example, on a large beach in South Wales in the United Kingdom, reported that on a North Yorkshire rocky coast in the United Kingdom, the numbers of newly settled plantigrade individuals among an established population of Mytilus edulis Linnaeus, 1758 varied between 100 and 1000 per 5 cm2. Much of this abundance and success relates to the filter feeding and gregarious behaviour of many bivalves and a generalised reproductive strategy of lecithotrophy in which the external fertilisation of huge numbers of gametes and the development of planktonic larvae can produce an equally large number of settled juveniles.

The great majority of bivalves are restricted to fresh waters and intertidal and shallow subtidal habitats and ).

In a major review of the living bivalves of the Eastern Pacific Ocean all the way from the Bering Strait to Cape Horn and extending out into waters around the Galapagos Islands, recording 168 bivalves from these cold waters and of these but 2% (~ 3) were septibranchs.

The waters around the Canary Islands support a large number of septibranchs: Verticordiidae (6), Poromyidae (1) and Cuspidariidae (15) ().

The septibranchs thus have a global distribution on and, especially, in the deep sea floor raising questions of as yet unstudied cosmopolitanism and genetic diversity at abyssal and, even, hadal depths. And, the septibranchs, plus other deep water but non-predatory anomalodesmatans, thus constitute some 22% of the deep water bivalve fauna with the predators making up approximately 20% of them. It is these that are the principal subject of this review.