ZOOPLANKTON 273 



50-0 m. haul at station WS 989. Evidently spawning took place within 25 miles of the coast, in 

 depths of about 50-150 m. A few post-larvae were taken in the deeper net hauls, but all were con- 

 fined to the waters on the continental shelf. The heaviest catches occurred where the mean temperature 

 of the upper 50 m. of the water column lay between 13 and 14-5° C. The importance of locating a 

 spawning ground of this, the South African pilchard, is referred to on p. 270. 



The eggs and young stages of other fish were also taken, notably those of the stockfish (Merluccius 

 capensis), and of anchovies {Engraulis sp). 



Distribution of the zooplankton 

 From the foregoing notes it is evident that there are many characteristic features in the zooplankton 

 distribution, several of which might be expected from the known behaviour of the organisms. The 

 ostracods, for example, are in this region an essentially mid-water group, lying off the edge of the 

 continental shelf and showing a fairly well-defined distribution of species with depth, Conchoecia 

 elegans being the dominant species and occupying the 500 250 m. layer. In the shallower waters, 

 the role of the ostracods appears to be taken over by the Cladocera, which locally reached great 

 abundance inshore on the Walvis Bay line. 



The mysids were, on the whole, very poorly represented numerically, and only the one species, 

 Gastrosaccas sanctus, which is normally particularly gregarious, attained any considerable numbers. 

 This concentration occurred in the rather diluted water off the Orange river mouth. On the more 

 northerly lines very few specimens were taken, and it is interesting to compare this with the abundance 

 of cumacea at these more northerly stations. 



The dominant cumacean, Iphinoe fagei, is closely related to other bottom living forms, and shows 

 no particular adaptations to a planktonic existence. The large numbers occurring planktonically 

 inshore on the Sylvia Hill line may be explained as being a nuptial swarm. But when one considers 

 other phenomena — such as the presence of larval tubicolous polychaets with their tubes partly 

 developed, the larval lamellibranchs with shells developed, and the post-larval Ophiuroids, all in the 

 plankton — then one is more inclined to accept the alternative explanation, that the sea-bed of the 

 anaerobic zone did not present a desirable environment and forced into the plankton many organisms 

 which would otherwise have been present on the sea-bed at these stations. 



The presence of the euphausiid Nyctiphanes capensis on the continental shelf is quite characteristic 

 of its habit, and the other euphausiid, Euphansia Incens, only reached large numbers in the vicinity of 

 the Orange river mouth, again in the shallow coastal waters. The close similarity in succession of 

 species between the euphausiid fauna of this region, and that found off California, described by Boden, 

 seems significant in view of the hydrological similarities between the two regions. 



At the offshore end of the Orange river line the large numbers of the pteropod Limacina bulimoides, 

 evidently grazing heavily on the phytoplankton (p. 229) and probably being preyed upon in turn by 

 the gymnosomatous pteropod Pneumodermopsis pancidens, is the only major occurrence of the pelagic 

 molluscs in the collection. 



The distribution of the Larvaceae is strongly indicative of the presence of two separate populations, 

 one in the coastal waters to the north, and the other in the oceanic waters to the south. 



The association between the young amphipods (Vibilia sp.) and the salps, both of which occurred 

 in numbers at the offshore end of the Sylvia Hill line, is no doubt an example of the well-known 

 commensalism between these animals. 



The correlation between the numbers of cumacea and the numbers of eggs and young stages of the 

 pilchard is probably purely coincidental, but may be of some value since the cumaceans are fairly 

 large and easily seen, and could perhaps be used as an 'indicator' for the pilchard eggs. 



