On the functional morphology of the family Verticordiidae (Bivalvia) with descriptions of new species from the abyssal Atlantic

Abstract

This study forms part of a larger study on the ecology and functional morphology of deep sea bivalves of the Atlantic. Eighteen living species of the family Verticordiidae are described, of which eight are new species. The descriptions include shell morphology and a detailed analysis of anatomy, particular attention being paid to the mantle, gills and alimentary canal. The Verticordiidae are only taken in deep water and form a small but persistant fraction of the bivalve fauna. They have a morphology that is intermediate between species of the order Anomalodesmacea and the septibranch species of the Poromyidae and Cuspidariidae, and this study shows that the latter two families have been derived from the Anomalodesmacea. The structure of the hinge is very similar to that of species of the latter order as, too, is the gill which, although reduced, is typically eulamellibranch in form. Progressive reduction in the size and the number of lamellae of the gill is evident. With reduction there is an increase in the extent of the gill membranes that are attached by tissue fusion to the mantle, the body and the septum between inhalent and exhalent apertures, i.e. an intermediate stage between the eulamellibranch and septibranch conditions. In contrast, most of the eighteen species have a stomach identical to that of the septibranch families. However, one or two species show limited ciliated surfaces intermediate in extent between the anomalodesmacean, as exemplified byLyonsia, and the septibranch condition. In addition, the Verticordiidae show features peculiar to themselves. The mantle edge is glandular. The glands take the form of discrete multicellular radial structures. The region of inhalent and exhalent tentacles is richly supplied with papillae that secrete an adhesive fluid. The secretion has two functions; to assist the animal to maintain a position at the surface of the very soft abyssal sediment, and to capture food organisms such as copepods. The sticky tentacles are spread out over the surface of the sediment. On capture the food is ‘licked’ off the tentacles by an inhalent valve and falls, or is carried, to a posteriorly facing buccal funnel formed by the expanded lips. The palps are extremely reduced in size. Oesophagus and stomach are muscular and the stomach is lined with a scleroprotein and is a crushing organ. It is interesting to note that the rectal region of the hind gut is expanded, probably to assist in further digestion of food remains. This is a further example of hind gut modification in abyssal bivalves, where it is essential that food should be utilized to the full. Other unusual features are the large size of the kidney, often extending along each side of the body as far forward as the mouth. Associated with the kidney is a lacunar system of unknown function. The Verticordiidae are hermaphrodites, producing small numbers of large eggs. Prodissoconch measurements indicate that they have at most a short pelagic life. However, individual species are widespread, many being found on both sides of the Atlantic from arctic to equatorial latitudes.