EUPHAUSIA SUPERBA AND E. TRIACANTHA 427 



fine filter feeding of this voracious and extremely successful herbivore as distinct from the coarser, more 

 actively predatory, feeding of other copepods such as Candacia, Temora, Euchaeta and most bathy- 

 pelagic forms, which, having much stronger and less setose mouth parts, triturate their food either 

 before or during feeding. It has been suggested too (Millar, i960) that the large size attained by certain 

 Antarctic ascidians springs indirectly from the rich phytoplankton diet of the plankton animals on 

 w^hich they so largely feed. 



Dell (1952), referring to the immense quantities of phytoplankton in the Antarctic and the super- 

 abundance of nutrients, phosphates and nitrates, that goes with it, also calls attention to the benefits 

 that accrue from an algal diet, noting that the rich food supply available for the phytoplankton in 

 these southern waters will ' determine the relative abundance of Euphausia which will in turn influence 

 the numbers of the whales and their distribution '. 



Fisher and Goldie (1959) refer to the particularly high vitamin A reserves present in the Meganyc- 

 tiphanes norvegica population of Loch Fyne and to the relatively great size attained by this species there, 

 noting that these phenomena spring principally from the presence of /?-carotene in the dinoflagellates, 

 diatoms, algae and fern sporangia that contribute to its diet in this western Scottish loch. They find 

 that this diet produces larger specimens of M. norvegica in Loch Fyne than any they have seen from 

 the North Sea, the North Atlantic and the Mediterranean, and that these large animals are much richer 

 in vitamin A than those they have analysed from these localities. The rich, superlatively rich, phyto- 

 plankton diet of E. superba, and the /^'-carotene that goes with it, might contribute much it seems to the 

 relatively enormous size to which it grows. 



It may be of some significance too that the krill seem to be completely immune to infection by the 

 EUobiopsid Protozoan parasite Amallocystis fagei Boschma, which Boschma (1949) records on 

 Euphausia vallentini, E. frigida, E. recurva, E. lucens, E. hemigibba and Thysanoessa gregaria, and 

 which I have repeatedly noticed is particularly liable to attack E. frigida, an Antarctic species co-existent 

 with the krill in the plankton. It attaches itself to the dorsal surface of the carapace of its host in the 

 region of the genital gland, ' protoplasmatic excrescences ' from the organ of fixation penetrating into 

 the ovary. According to Einarsson (1945), who describes what Boschma thinks is the same species on 

 Thysanoessa inermis, it seems probable that it ' castrates the animals ' it attacks. 



When one contemplates the hosts of its predators and the vast weight of whale flesh, seal flesh, 

 fish, penguins and other birds that is built up on its wholesale destruction, one is left wondering 

 as to how in its seemingly inexhaustible myriads it continues to survive, and above all at its astonishing 

 capacity year after year for making its staggering losses good. This capacity, by itself, must be one of 

 the overriding factors contributing to its phenomenal biological success. 



Perhaps the winter ice-sheet, extending as it does over such a vast area of the circumpolar sea, 

 and under which such an immense part of the larval population spends virtually six months of its 

 surface existence, contributes a lot to the ability of this species to maintain its numbers at their 

 existing fantastic level. For above all the winter ice must act as an ecological umbrella under which 

 the overwhelming mass of the krill in its most vulnerable phase is secure from the ravages of its 

 largest predators, enjoying a long period of relative immunity denied to other euphausians that 

 spend their entire existence in the open sea. 



Above all it seems clear that in the course of evolution this overflowing community has adapted 

 itself superbly both to tolerate and to take the fullest possible advantage of the external metabolites of 

 its neighbours. I quote from Lucas (1947): 



That microplanktonic organisms do produce substances externally, either during life or in death, which assist some 

 and hinder others of their fellows in their growth, is now confirmed. One point, however, can scarcely be emphasized 

 too strongly: since planktonic organisms do produce such substances, then only those forms which are unharmed 



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