240 DISCOVERY REPORTS 



centrations of brittle stars recently photographed in Cook Strait, New Zealand, it would seem to be 



'an aspect of the normal behaviour of a naturally gregarious group of animals'.^ 



Johansen (1925) also calls attention to the probability that swarming or schooling in marine animals, 

 particularly in fish, is a long-term phenomenon, having its origin far back among the larval forms. 

 He notes, for instance, that many fishes, even while still young, move about in shoals, adding that the 

 highly irregular yield of cod and herring larvae from his serial (consecutive) net hauls in the Kattegat 

 distinctly suggested that the phenomenon was ' manifested already ' in the larval phase. 



As Cassie (1957) has recently said, 'One of the simplest assumptions which can be made as to the 

 pattern formed by organisms in space is that they are randomly distributed. In practice randomness 

 is fairly uncommon, the usual pattern being an "aggregated" one in which organisms tend to cluster 

 in certain regions. . . . Sometimes the reason for aggregation is obvious. Shade loving plants cluster 

 in shady niches. Young animals like to stay near their parents. On the other hand there is no obvious 

 reason why fully grown plankton should not be randomly distributed, since there seem to be no 

 specially favourable niches in the ocean '. Proneness to lifelong swarming such as seems to be mani- 

 fested in the Antarctic krill, however, would provide a simple explanation, even in such a relatively 

 changeless environment as the open sea. 



Colebrook (i960 a) calls attention to the highly non-random horizontal and vertical distribution of 

 the zooplankton in Windermere, describing (19606) a swarm of Daphnia hyalina at the surface there 

 as 'fairly distinct, about i m. in diameter and confined to the top 10 cm. of the water'. He notes that 

 the vast majority (82%) of the individuals of this swarm were in the same developmental phase and 

 suggests that such a gathering might be produced and maintained partly by water turbulence and 

 partly, if not indeed entirely, by ' social activity of some kind implying purposeful movements by the 

 animals '. 



IMPACT OF THE SWARMS ON THE OCEAN PASTURES 



With such a vast herbivorous population so discretely disposed, and the enormous capacity for grazing 

 concentrated in a single swarm, it is not surprising that in nature we should find, side by side, places 

 where the phytoplankton is rich and others where it is scanty or poor. In general, one would expect, 

 the gaps or 'voids ' separating the swarms, so long as they remained untenanted, would be richer, indeed 

 immensely richer, than the water occupied by the congregating krill, an average-sized large swarm 

 (p. 148), say 60 by 40 yards in surface area and i yard thick, containing, at 40,000 to the cubic yard,^ 

 96 million euphausians, the whole comprising a compact grazing unit of enormous local destructive 

 power. One could well in fact imagine it making a clean sweep of the diatoms among which it swam, 

 then moving on, for the swarms it seems (p. 156) do move bodily about, to wreak equally effective 

 havoc among the richer pastures of the voids. As Bigelow (1926) has said of Calanus finmarchicus, a 

 pelagic swarmer ranking high in ecological importance with the Antarctic krill, it probably 'makes 

 greater inroads on the planktonic plants on which it preys than do all other copepods combined, and 

 conceivably it may practically exterminate them locally and temporarily'.^ No doubt salps too wreak 

 similar destruction, Bigelow (1909) recording that they sometimes occur in the Gulf Stream ' in hordes, 

 and on such occasions strain the water bare '. The swarming salps of the Antarctic could equally well 

 I believe bring about the local extermination of the plants on which they graze (Foxton, 1961). 



^ Cf. the marked gregarious habit in Ophiothrix quinquemaculata recently demonstrated by Czihak (1959) in the Adriatic 

 Sea, or Thorson (1957), who remarks that brittle stars occur in such dense swarms that 'every centimeter of surface must be 

 swept by their arms'. 



- Approximately (see p. 151) one to the cubic inch. 



' Cf. Bainbridge (1953, 1957) who has said, 'grazing and migration into a large uniform mass of phytoplankton could result 

 in the eating out of lines of plants'. 



