223 DISCOVERY REPORTS 



cyrtopias was the only example met with, and the parallel to the peculiar behaviour of 

 Euphausia superba in the Antarctic has already been referred to. It may be noted that 

 where there is evidence of swarming animals in earlier records, the colour is usually 

 described as reddish (Funnel, 1729; Fitz-Roy, 1839; Rayner, 1935). The unusual dis- 

 colorations have some resemblance to the colours that have been described as charac- 

 teristic oi aguaje (Raimondi, 1892 ; Stiglich, 1925). Further reasons for associating them 

 with this phenomenon are given on pp. 229-33, where it is shown to be abnormal. 



A point of interest is that the green colour and the colours of class 2, if produced 

 by plankton, are practically confined to within 30 miles of the coast; whereas the 

 zooplankton seems to be no less abundant at distances of 100 and 200 miles and the 

 phytoplankton reaches its greatest mean volume at 80 miles offshore. It does not seem 

 as though the areas of most intensive organic production can be identified with the 

 coloured water, although the latter coincides with the coolest temperature and the 

 richest phosphate. In searching for another explanation, it should be remembered that 

 the cool inshore temperatures result in a cloud formation which hangs over the littoral 

 as a narrow canopy over most of its length. It seems probable that conditions beneath 

 this cloud are such that the phytoplankton finds its optimum illumination at the very 

 surface close inshore, whereas in the open ocean where the light is more intense, 

 diatoms may sink and scatter at lower levels and the green colour thereby lost. Dr T. J. 

 Hart, with whom I have discussed these questions, has suggested that systrophe on the 

 part of diatoms in the well-illuminated zone might lead to a similar contrast between the 

 blue colour of this water and the green in the zone where cloud allows full expansion 

 of the chromatophores (Marshall and Orr, 1928). The zooplankton, though reacting 

 differently to light, might occupy a higher level in the coastal zone, and in this way fish 

 would also come closer to the surface inshore than offshore, and this might account for 

 the restriction of the birds and seals to the very narrowest zone. This would be brought 

 about equally if the fish favoured littoral rather than oceanic species of the plankton.^ 



BOUNDARIES OF THE PERU COASTAL CURRENT 



Though sharing in the general anticyclonic circulation, the coastal region and the 

 open ocean of the eastern South Pacific have been seen to show profound hydrological 

 differences. In the open ocean the South-East Trade is normally developed, whereas 

 on the Chilean coast the wind weakens and has a meridianal direction. In the open ocean 

 the northerly surface drift has a large westerly component, whereas close inshore the 

 drift is parallel to the coast. These currents, acting in conjunction with the earth's 

 rotation, produce a divergence of surface water from the coast whereby the lower 

 layers are induced to well up in compensation. Contrasted with these inshore vertical 



1 Any modification of coastal colour that might be due to the distribution of zooplankton would be 

 explained if the littoral species were more opaque than the oceanic. It is well known how well adapted to 

 their environment are the pelagic organisms of tropical seas, being either transparent or blue or having 

 silvery scales capable of almost perfect reflection. 



