. Fig. 2. Limacina bulimoides. (a) Shell of 'mature female' at stage 6. Shell heightapprox. 1-9 mm. (b) Operculum, on slightly enlarged scale. ECOLOGY—DEPTH DISTRIBUTION The samples from WS 996 and WS 997 were from an area poor in phytoplankton according to Dr Hart (1953).1 There was some evidence of recent heavy grazing, though the abundant green faeces present were probably attributable to euphausiids. Whatever may have been the economics of this population of Limacina, its presence in such numbers must have made greedy inroads on the phyto- plankton of the vicinity. Although presumably they
. Fig. 2. Limacina bulimoides. (a) Shell of 'mature female' at stage 6. Shell heightapprox. 1-9 mm. (b) Operculum, on slightly enlarged scale. ECOLOGY—DEPTH DISTRIBUTION The samples from WS 996 and WS 997 were from an area poor in phytoplankton according to Dr Hart (1953).1 There was some evidence of recent heavy grazing, though the abundant green faeces present were probably attributable to euphausiids. Whatever may have been the economics of this population of Limacina, its presence in such numbers must have made greedy inroads on the phyto- plankton of the vicinity. Although presumably they had fed rather recently, the stomachs of almost all the numerous specimens sectioned were empty of recognizable contents. In one specimen, coccolithophores were present, which may represent an important part of the diet of this population, and which would, at c. 40/x, be likely to be missed by the N 70 V. plankton net. Of predators probably feeding on Limacina, we may mention the abundance of Pneumodermopsis paucidens accom- panying Limacina at a depth of 50-100 m. at WS 997, and a few large Atlanta at the same station and at WS 996. Partial exclusion of Limacina at 50-100 m. at WS 997 (night station) is evidently not to be explained by the action of predators or by existing hydrological features. An explanation has been ventured below after considering the pattern of depth migration of Limacina bulimoides. Depth distribution. Stubbings (1937) regards the known facts of the diurnal depth migrations of thecosomatous pteropods as consistent with the idea of an optimum light intensity for each particular species. 'At dusk the animal follows the optimum towards the surface and after dark the stimulus to 1 Also in personal conversation.
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