DEPTH DISTRIBUTION 273 



number of specimens were found in the 50-0 m. sample, indicating a swarm of a part of the population 

 near to the surface. This station was worked at night, while the neighbouring Stations WS 976 and 978, 

 where this phenomenon is not found, were worked during the day. This then would indicate that a 

 nocturnal migration to the surface might be taking place. Similarly at Station WS 986 and 996 in 

 daytime the majority were at 250-500 m., while at WS 997 at night, though the catch was small, 

 there is evidence of a movement towards the surface. Fowler (1909) does in fact show such a migration 

 for this species. 



C. nasotuberculata (Fig. 5) and C. curta (Fig. 4) also show a depth distribution centred about 

 500-250 m. or rather less, though, in the latter case, the numbers are only sufficient at Stations 

 WS 976 and 996 to indicate this. The rise in numbers of C. nasotuberculata above 250 m. at Station 

 WS 977 may indicate a vertical migration, but in neither species is there a clear indication of such a 

 migration as in the case of C. elegans. 



The numbers of C. alata were much smaller but as will be seen from the Appendix (pp. 276-8), 

 the majority of those specimens captured were in the 500-250 and 750-500 m. samples. This indicates 

 a distribution around a level of about 500 m., or rather deeper than in the case of the three species 

 already considered. 



C. teretivalvata occurred in moderately large numbers (Fig. 5). Here the indications are that this 

 species is distributed around a depth of 250-100 m., or at a much less depth than the other species 

 considered. A large number of adults of the species occurred in the 750-500 m. sample at Station 

 WS 976, but on the basis of the data available no explanation can be offered for this. There are some 

 indications of nocturnal migration to the surface at Station WS 977. 



Some of the species such as C. symmetrica occur in large numbers at depths of 1000 m. or more; 

 hence the small numbers of these species in the samples may be due to the absence of samples from 

 depths of more than 1000 m. It seems that, at the time when the samples were taken, the majority 

 of the species found in moderate numbers in the samples had a greatest density of population at 

 depths between 250 and 750 m. 



SUMMARY 



The Ostracod material from samples collected by the ' William Scoresby ' during a first survey of the 

 Benguela current (March 1950) has been examined. Twenty-three species of the genus Conchoecia 

 have been identified and numbers of adult males and females as well as juveniles determined for each 

 species. Single juveniles of Halocypris brevirostris and a Cypridina sp. were also present. 



Data regarding depth distribution obtained by means of closing-nets, together with morphological 

 examination, has made it possible to conclude that the Ostracods described previously under the name 

 Conchoecia rotundata Muller are probably three closely allied species. Two of these species present in 

 the Benguela current have been re-named. 



An almost complete absence of specimens has been noticed in samples taken over the Continental 

 Shelf. This is in contrast to the presence of quite a number of specimens of several species in samples 

 taken at similar depths at deeper water stations. 



A few species were present in sufficient numbers to obtain indications of their depth distribution. 



There is evidence of a nocturnal migration towards the surface in C. elegans Sars. 



C. echinata Muller and C. obtusata Sars var. antarctica Muller have been found south and north of 

 their previously known areas of distribution respectively. 



