230 DISCOVERY REPORTS 



Summing up he notes that most of the underwater Unear polarisation springs from the scattering 

 of refracted sun's rays by particles in the water, a phenomenon affecting much of the photic zone. 



The pattern of polarization therefore is dependent on the sun's position in the sky and hence changes markedly with 

 time of day. Since a number of arthropods are known to be highly sensitive to linearly polarized light, it is suggested 

 that the behavior of certain pelagic forms may be determined in part by this environmental parameter. The most 

 likely significance of this factor in nature would be in directing local diurnal movements of populations and in pro- 

 viding a sort of celestial compass that could be used in orienting horizontal migrations. 



Although linearly polarised light is the principal component of natural underwater illumination, 

 elliptically polarised light is also present as a definite but less prominent component (Ivanov and 

 Waterman, 1958). It will be interesting to determine, as these authors remark, whether this too is of 

 any biological significance. 



It is obvious, however, that much experimental work is needed, together with further observation 

 on the swarms in their natural state, before we can even begin to understand what keeps them together 

 and behave as they do. To quote Tinbergen (1953) on the behaviour of congregating animals: 



Once the animals have come together, we see co-operation of numerous kinds. The simplest kind of co-operation 



is 'doing the same thing' as others. When one Herring Gull flees, the others flee as well This principle of 



'sympathetic induction', as McDougall has called it, can be seen at work in many social animals, Man included. 

 We yawn when we see another yawn, we get scared when we see signs of intense fear in another man. It has nothing 

 to do with imitation ; the reacting individuals do not learn to perform certain movements by watching others perform 

 them, but they are brought into the same mood, and react by making their own innate movements. 



Watching the flight manoeuvres of a flock of Starlings or waders reveals another type of co-operating. Such 

 animals not only fly when the others fly, but they direct their flight to that of the others. It is highly fascinating 

 to see how thousands of Starhngs, flying round above their roost on a winter evening, turn as if at command ; left, 

 right, up, and down. Their co-operation seems so perfect that one forgets the individuals and automatically thinks 

 of them as one cloud, as one huge 'super-individual'. 



The swarming krill, with their superlative co-operation or capacity for doing the same thing, may 

 be Ukened to Tinbergen's starlings, for their mass behaviour, too, seems to be that of a super-individual. 



Beebe (1932) gives a superb description of the schooling of the aj-inch long Bermudan fish 

 Atherina, a passage recalling in almost every line Hardy's classic account of the incredible capacity 

 for concerted action, the incessant fluxing or amoeboid movement, the mass awareness, sense and 

 intent, exhibited by the swarming krill. 



A few feet away from me was a concentrated school of small fish. When they were drawn out into a long ribbon 

 I counted fifty, and a conservative estimate of the whole was fourteen hundred — and this was an extremely small 

 school, as schools of Little Arrows go. Slowly the school swung first in one, and then in the opposite direction, it 

 changed from a circle to a dumb-bell, to beads on a string, to a crescent, and now and then I halved it momentarily, 

 when it rejoined and became some unnamed figure. It had length, breadth, and depth, intentions, achievement, 

 attempt, refusal, impetus — but one never thought of individual units. As I watched, it became more and more a 

 sentient individual, with particular desires and activities; in the little harbour beneath me were snappers, and 

 squirrel-fish, cow-fish and bream and — it. It kept halfway from the bottom, and its shadow on the bottom was the 

 verisimilitude of a slow creeping worm, which flowed over pebbles, reached fingers out sideways, drew back and 

 then slipped out of sight, as the sun went under a cloud. 



I could do strange things with this Atherina organism — like boring three holes at once through it. I waited until it ! 

 became a broad oval and then tossed three tiny pebbles simuhaneously. As they sank, there opened beneath each of them a 

 round hole, and soon they all dropped through three separate wells and the trio of apertures closed up again. There was no 

 especial fright, simply a very reasonable withdrawing from the unusual phenomenon of pebbles dropping from the sky. 



The reaction of the krill it will be recalled to Gunther's dropped lead on line was precisely the same. 

 In its beginnings then, the inherent capacity of this species for maintaining itself in swarms is a , 

 purely larval phenomenon. It can be traced, however, from the time when the larvae first begin to 

 become adolescent (Fig. 38), through the whole of adolescence and up to the adult state, the measure- 



