172 DISCOVERY REPORTS 



shell is tall and turreted and quite transparent, sometimes exhibiting faint longitudinal striae inter- 

 rupted by growth lines on the last whorl. The aperture is drawn out in front into a beak or rostrum, 

 rounded or more or less pointed. In the Benguela material this is 

 especially well marked in the younger stages, while tending to be 

 obscured in the mature females. A distinctive character is the 

 frequent marking of the suture line with light chestnut brown, 

 which may also be present at the columellar edge of the aperture 

 and around the vestigial umbilicus. A figure of a shell (mature 

 female — 1-8 mm.) and of an operculum is given here (Fig. 2). 



Recorded distribution. According to Tesch this species is very 

 generally distributed in the tropical Atlantic, being nearly always 

 present in large numbers. Its centre of abundance is probably in 

 the Sargasso Sea area ; it reaches the eastern United States seaboard 

 and extends, according to Meisenheimer's chart, into the Mediter- 

 ranean where, however, it is less abundant. Sparser records are 

 from the Indian Ocean and the tropical Pacific. The Atlantic area 

 of Limacina bulimoides appears to lie roughly between 40 N and 

 40 S. Comparing its distribution with that of L. inflata, Tesch 

 found its greatest abundance per fishing unit in the upper 100 m. to 

 be 243 against 280 of inflata in the Sargasso Sea. In the Benguela 

 hauls of the first survey, this predominance is reversed and inflata 

 is never present in large numbers. Off South West Africa buli- 

 moides was never taken inshore and was confined in March 

 to the five stations WS 986, WS 996, WS 997, WS 998 and 

 WS 999, near the edge of the continental shelf. It is clearly an oceanic species avoiding the 

 colder up-welled waters towards the coast. 



Fig. 2. Limacina bulimoides. (a) Shell 

 of 'mature female' at stage 6. Shell 

 height approx. 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. 



