The Shell Gravel Deposits, and the Infauna of the Eddystone Grounds

Abstract

1. The limitations of the Conical Dredge as used in obtaining a sample for quantitative estimation of the fauna of a ground are discussed, and are found to be related to(a)The great distance over which the instrument must be hauled, with consequent inability to detect patchiness of fauna.(b)The inability of the dredge to capture members of the epifauna.2. The nature of the shell gravel, its position relative to the Eddystone reef, and the various factors conditioning the degree of coarseness of the deposit and the segregation of its elements are considered. The main points of note are,(a) There are three well-defined areas, within which the gravels have their own particular characteristics.(1) TheInner Shell Gravelarea and theMiddle Gravelarea, composed of material of inorganic and organic origin, the former being gneissic in character, and the latter being formed of the remains of animals which normally live on this particular type of deposit.(2) TheOuter Shell Gravelarea, a mixture of matter of local and of outside origin. The area is more extensive on the western than on the eastern side of the reef.(b) Segregation of particles according to size occurs, and is illustrated by the movement of the valves of a number of species of molluscs. Wave action and the action of gravity are considered to be the chief factors in inducing movement of particles, which movement occurs in directions radiating from the reef outwards.3. The epifauna of the rocky bottom is referred to briefly.4. The infauna of the shell gravel has been examined quantitatively and qualitatively, and the range of the various species over the ground noted.5. The nature of the shell-gravel community and its relation to some of the important environmental factors, are discussed.6. Mortality among the lamellibranchs, and the depredations of the gastropodNatica alderiare considered; figures relating to the numbers of lamellibranchs killed byN. alderiare given.