352 DISCOVERY REPORTS 



While the dominants in Nyctiphanes may differ (i.e. the modal position may shift) from time to 

 time or place to place they do not appear to move out of the stage boundaries. For example, in 

 N. capensis (Fig. 7) it is possible that Fill may at times be expressed as a form having seven terminal 

 spines on the telson instead of five (those with six are abnormalities), but the mode will not be absent 

 altogether. Thus, while a false impression may be registered by dogmatically regarding F III as 

 typified by a form having five terminal spines, this impression will probably be less false than that 

 obtained by disregarding all the modes occurring after F II, and considering all the lateral individuals 

 as belonging to one stage. 



Sheard states that it is highly probable that any individual will pass through only a limited number 

 of the many possible instars in the stages he proposes. This is undoubtedly true, and it seems only 

 logical that the modes represent the limited number of instars through which the majority of 

 individuals passes and that they can therefore be expressed as stages, and the remainder as variants. 



The fact that in Nyctiphanes the modes will shift and that by lumping several series of furcilia 

 taken at different times or places one can hide these modes can, perhaps, be explained without 

 doubting that the dominants may be regarded as valid stages. 



The genus Nyctiphanes is peculiar among the Euphausiacea in that all four known species are 

 shallow-water, neritic forms with limited geographical ranges. N. simplex occurs in Californian waters, 

 N. couchii in the North Atlantic, N. australis in southern Australian and northern New Zealand waters, 

 and N. capensis in South African waters. All these species have a great number of variants in their 

 furcilia series and, therefore, substantiate Frost's (1935) claim that species inhabiting coastal waters 

 regularly exhibit greater larval variation than oceanic species. As Sheard points out, the considerable 

 variation exhibited by Euphansia superba, generally regarded as an oceanic species, may not controvert 

 this claim, since it inhabits the region of pack ice, which simulates neritic conditions in some respects. 

 The marked reduction in the number of instars in oceanic species has led to some speculation that the 

 Euphausiacea represent arthropods in which gradual development is giving way to metamorphosis. 

 On this premise, the genus Nyctiphanes with its great larval variation could be regarded as somewhat 

 more primitive than other genera. It is probable, however, that the large number of variants is not 

 a primitive, but a secondary feature developed during invasion of coastal waters. The genus has several 

 rather specialized features, including an egg-sac in which the first, more vulnerable, larval stages are 

 passed. This, too, is probably a secondary adaptation to the more varied coastal conditions. Thus the 

 genus, in which variation is most marked and the dominant stages are least ' fixed ', is one which is 

 specially adapted to a variety of coastal conditions. The fact that dominant instars do occur in any 

 particular place or time can, perhaps, represent the basis for an argument that these instars are an 

 expression of development under the particular environmental conditions obtaining. In other words, 

 such a genus may have slightly different developmental stages under different environmental 

 conditions. 



I agree with Sheard that much experimental work with live animals is desirable before the question 

 can be examined much further, but feel that under the present circumstances it is less useful to ' lump ' 

 the furcilia stages than to 'split' them. It is possible, unless one is as careful as Sheard has been in 

 giving full codes and counts for all the variants, that much valuable information on development as an 

 expression of environment may be obscured by disregarding dominance. Full data on geographical 

 position, temperature, salinity, oxygen and phosphate content for the stations at which the species 

 reported on herein were taken are available in the Discovery Report, vol. xxvi, pp. 21 1-58. 



