192 DISCOVERY REPORTS 



suitable for its spawning. The stock of krill at South Georgia must therefore be carried thither by the 



current in the Antarctic surface layer '.^ 



It may be that temperature plays a major part in determining the distribution of the spawning krill. 

 I quote from Moore (1958): 



...there are likely to be marginal zones around the area inhabited by a particular species in which temperatures are 

 suitable for survival but not for breeding. Such areas depend for repopulation on a supply of immigrant larvae or adults.'^ 

 An animal with a short life span may not have time to be carried far from its breeding area before it dies and may, 

 therefore, not attain a region where temperature would limit survival. Euphausiids, with their relatively long life 

 span, however, have the opportunity of being carried long distances by the currents, and Einarsson (1945) has shown 

 that they may have extensive fringes of non-breeding population. 



As Einarsson himself remarks, with marine animals generally, the spawning area is more restricted 

 than the whole geographical range, the distribution of the freshwater eel {Anguilla anguilla L.) pro- 

 viding a classic example.^ He notes, too, that whereas with fishes, in so far as oceanic dispersal is 

 concerned, the larvae alone are influenced by the currents, euphausians ' are subjected to the influence 

 of the currents throughout life ', deliberate wanderings towards certain spawning areas such as are 

 undertaken by some fishes being ' out of the question because of the feeble swimming power of the 

 animals. Consequently ', he continues, ' the individuals which are carried out of the reproduction area 

 will seldom contribute to the maintenance of the stock '. 



Paulsen (1909), referring to the wide temperature range of Meganyctiphanes norvegica, and other 

 boreal euphausians, observes that they are not ' strict in their claims on the surroundings ', adding that 

 if it proves correct that they spawn in warm and not in cold water, ' diflFerent claims on the surroundings 

 may be made for propagation than for the maintenance of life '. 



In his survey of the zoogeography of the Atlantic Hydromedusae, Kramp (1959) writes on similar 

 lines : ' How far away from their place of origin the neritic medusae may be carried along with the 

 currents depends on the velocity and direction of the currents and the duration of life of the 



medusae We must also bear in mind that the geographical distribution of a medusa does not 



necessarily coincide with its native habitat, because it may have been carried to an area where it 

 may continue its swimming existence and take nourishment for its own maintenance, but is unable to 

 propagate'. 



Or to quote Tucker (19596) on his new eel hypothesis, 'Parallel cases of expatriated populations 

 failing to breed are the British Octopus vulgaris, the Norwegian Palinurus elephas and the Lagos 

 Branchiostoma nigeriense, all of which are maintained by immigrations of larvae bred elsewhere and so 

 represent similar cases of wasted reproductive potential '. Or as Gunter (1957) has put it in his remarks 

 on temperature and the distribution of marine life as a whole, ' Species restricted by reproductive 

 stenothermy may live in other areas but must be recruited every year from a spawning center '. 



In his recent account of the zoogeography of the pelagic polychaetes of the South Atlantic Tebble 

 (i960) finds that juvenile Tomopteris carpenteri (specimens less than 12 mm. long) are plentiful 

 round South Georgia but apparently absent from the deep oceanic water near the South Sandwich 

 Islands and along the meridian of Greenwich. He suggests that T. carpenteri may breed round 



1 See, too, the passage from Risting quoted on p. 43. 



^ The italics are mine. 



* According to the new hypothesis of Tucker (19590) it would seem that the Leptocephali that regularly cross the Atlantic 

 from the Sargasso Sea, and eventually as elvers ascend the European rivers, are now to be regarded as of North American 

 origin, and that the supposed return of the European females to spawn in the Sargasso area does not in fact take place. Even 

 so the European eel still provides us with a classic example of discrete spawning coupled with wide geographical range. The 

 distribution of larval and adult Swordfish {Xiphias gladius) in the North Atlantic (Taning, 1955) points to a similar 

 phenomenon. 



