35» DISCOVERY REPORTS 



wealth of benthic life has been commented upon by the naturalists of nearly all expeditions 

 visiting the Antarctic since those of the ' Challenger' first drew attention to it. Here on 

 the sea floor the animal life is bathed by the cold polar bottom water, so that temperature 

 in itself would hardly appear to be a factor limiting the number of species. May we not 

 tentatively keep in mind the possibility that the abundance of phytoplankton which may 

 develop in any part of the region is a limiting factor, and that only a comparatively small 

 number of species have been able to adapt themselves to the conditions, which may 

 necessitate them making extensive daily excursions from the more congenial layers into 

 the less congenial layers for feeding. Those which have done so occur in enormous 

 numbers. Vertical migration is certainly much more pronounced in this region than in 

 temperate regions, and temperature appears to be an unsatisfactory explanation. All but 

 a small proportion of the large number of species of Copepoda recorded from our region 

 have been taken in the intermediate layer, which whilst being the warmer layer is also 

 well below the phytoplankton zone (see Fig. 54 on p. 122). 



When the benthic fauna is so rich in numbers of species it is a surprising fact that 

 so many of these animals, whose relatives in lower latitudes send up pelagic larvae, 

 should bring forth their young by direct development, often in brood pouches. The 

 almost complete absence of benthic larval forms has also been commented upon by 

 naturalists of many Antarctic expeditions. The small larval forms would scarcely have 

 the power to undertake the extensive vertical migrations which appear to be imposed 

 upon the zooplankton inhabiting the upper layers. 



In the tropics and the sub-tropical waters we find a large assembly of animals such 

 as the floating siphonophores and the heteropods, which live at or near the surface in 

 bright sunlight but are not found in the temperate and polar seas. We may note that 

 only representatives of the Calycophorid siphonophores, which depend upon locomotion 

 rather than flotation for support, extend into the polar regions : e.g. Dimophyes arctica, 

 Diphyes antarctica and others recorded on pp. 10 5- 106. In our survey only two specimens 

 (Pyrostephos vanhoffeni) were taken in the top 150 m. in the Antarctic Zone, and these 

 were taken on different occasions but at night. It is remarkable that a few of the surface 

 tropical pelagic animals have not relatives which have become acclimatized to colder 

 latitudes. We have seen that phosphates are usually absent, or present in only very small 

 quantities, in the tropics (see Table VI ^.so that the phytoplankton production must also 

 be very low, as indeed has been shown by Lohmann ( 1 920) and Marshall (1933). It seems 

 possible that these forms have been able to evolve as daylight surface forms in these 

 regions for just the same reason that in temperate regions zooplankton organisms may 

 come to the surface in bright sunlight towards the end of the summer, when the phos- 

 phate content, and consequently the phytoplankton, is reduced. It is somewhat of a 

 puzzle to know how these tropical surface animals are able to maintain their life with an 

 apparently inadequate phytoplankton. Just as the nutritive salts are taken from the 

 water by the phytoplankton and handed on to the animal plankton in the seasonal 

 changes from spring to summer in the temperate seas, so it would seem there must be 



'P. 77. 



