THE LARVAL STAGES 119 



Although we do not know the rate of development of the larval stages of the southern euphausians 

 it is known from northern waters (Lebour, 1924) that laboratory reared specimens of Nyctiphanes 

 couchii (Bell) take roughly 8 or 9 days to grow from the Nauplius to the First Calyptopis stage, while 

 the plankton investigations of Ruud (i 927) and Einarsson (i 945) seem to indicate that the corresponding 

 development in Thysanoessa inermis (Kroyer) and Thysanopoda acutifrons (Holt and Tattersall) takes 

 about the same time. In laboratory reared specimens of Meganyctiphaties norvegica the same 

 developmental phase, Nauplius to First Calyptopis, has also been found to take about 9 days 

 (Mauchline, 1959). While it would be unwise to assume forthrightly a similar rate of development 

 in Eiiphausia superba there seems to be some justification for supposing that it might not be of 

 a widely dissimilar order. Fraser (1936, p. 113), referring to this species, says, 'Conclusions based 

 on the presence of eggs in the plankton involve the assumption that the time between the laying of the 

 eggs and the development into Naupliar and Calyptopis forms is very brief. The occurrence of all 

 stages of development up to the clearly distinguishable First Nauplius in eggs in one catch, and the 

 fewness of First and Second Nauplii in any of the catches, suggest that this is possibly the case '. While 

 the existence of the Second Nauplius, however, is manifestly less ephemeral (p. 98) than Fraser 

 imagined, the life-span of the First Nauplius on the other hand might well it seems be very brief, 

 for in other euphausians where the development is known (Lebour, 1926) this stage is said to slough 

 its skin immediately on emerging from the egg and become a Second Nauplius. 



If the development from First Nauplius to First Calyptopis does in fact take 8 days in E. superba 

 and the eggs as we may well suppose are hatching somewhere between 2000 and 3000 m., say at 

 2500 (see p. 100 and Tables 13 and 14), then since the Metanauplius (p. 97) does not moult to become 

 the First Calyptopis until it reaches the 1000-750 m. layer, it would follow that the ascending larva 

 would be taking 8 days to travel a distance of at least 1500 m., and presumably another 5 to reach the 

 surface. Such a rate of ascent, approximately 2-2 mm. per second, would mean that in its upward 

 passage the larva, assuming it to have left the cold bottom water at say 2000 m. and to have entered 

 the Antarctic suface layer at say 200 m., would, always provided the rate of climb were sustained, spend 

 from 9 to ID days in the warm deep current moving towards the south (but see p. 123). Even for such 

 relatively minute and feeble organisms as the Nauplii, Metanauplii and First Calyptopes of this 

 species the speed of ascent, approximately 26 ft. in an hour, even if it could only be maintained for 

 short bursts, is at least within the bounds of possibility. It seems to come within reasonable range 

 for instance of the known swimming powers of the Calanoid, C alarms finmarchicus, which as Hardy and 

 Bainbridge (195 16) have demonstrated experimentally, is capable of climbing at approximately 

 double this speed, a higher rate no doubt associated with its greater size and more powerful swimming 

 appendages. At any rate the speed could not be enormously less nor could it be enormously greater. 

 For if it were exceedingly slow and the rate of development were correspondingly slow it is con- 

 ceivable that all the larvae might perish from starvation long before they could attain the Calyptopis 

 form when (Taube, 1915; Gauld, 1959) first they begin to feed. On the other hand if it were 

 exceedingly fast, if for instance they took only 12 hr. or a day to reach the Antarctic surface layer, 

 unless we assume a rapidity of development in larval euphausians unknown so far as I am aware in 

 nature, we should expect to find the oceanic Nauplii and Metanauplii much nearer the surface than 

 they have ever been found to be. It is possible, however, that the ascending larva, without unduly 

 over-taxing the non-feeding period of its Naupliar and Metanaupliar existence, might travel con- 

 siderably more slowly and spend a correspondingly longer time in the warm deep layer than we have 

 supposed it to do. For as we have seen it does not have to traverse the whole extent of this layer 

 before reaching the feeding stage but only about half, the First Calyptopis emerging from the non- 

 feeding Metanauplius between 1000 and 750 m. where presumably it could obtain some sustenance 



