DISTRIBUTION OF ANTARCTIC MACROPLANKTON 93 



occurrence in small numbers, none is recorded between 2200 and 0159 except for a 

 single batch of twenty. On the other hand there is a clear maximum about midday. 

 Few were caught at St. 461, but there is no sign of such a reversed migration. Diurnal 

 variation is not of course necessarily proof of a corresponding vertical migration. It is 

 conceivable, for instance, that a species like this might become localized at night into 

 small shoals which are missed by the net. 



Vibilia antarctica (Fig. 18). The material is not really adequate and the numbers are 

 patchy. However, there is a most distinct variation. The fact that the peak comes rather 

 before midnight is due to only two or three samples which were abnormally large. 



Ensirus antarcticus (Fig. 19). A very doubtful species. Among those occurring south 

 of 6o° S there is no evidence of any diurnal variation. Those occurring north of 6o° S 

 suggest a sharp variation, but there are so few of them that the apparent maximum 

 around midnight may be quite accidental. 



Cyllopus sp. (Fig. 18). There is not very much to go on, but there seems to be a marked 

 variation. As in Vibilia the peak comes before midnight, but with the available material 

 we cannot be sure whether this is accidental or whether it represents the actual state 



of affairs. 



Antarctomysis sp. (Fig. 18). There is a pronounced diurnal variation, though the genus 

 has been taken practically as often during the day as at night. This is perhaps connected 

 in some way with the neritic distribution of this genus. 



Enphausia superba (Fig. 19). This is a difficult species to deal with owing to its extreme 

 patchiness and tendency to form shoals. Many of the big shoals have been seen at the 

 surface during both the night and the day, but the deeper shoals and the more scattered 

 individuals might undertake vertical migrations. For the estimation of the average per 

 haul at different hours, samples containing over 1000 E. superba have been disregarded. 

 This should eliminate the disturbing influence of heavy shoal catches and give some idea 

 of the general behaviour of the species. The resulting curve suggests only a minor degree 

 of diurnal variation. At St. 461 the majority seemed to remain near the surface, while 

 those living at greater depths gave some signs of moving up and down. At Sts. 618-25 

 there was quite a marked diurnal variation. The explanation would seem to be that while 

 a section of the population of this species undergoes some vertical migration, the greater 

 part remains at the same level, especially perhaps when forming shoals. 



Euphausia frigida (Fig. 18). The variation is strongly defined, but fair-sized catches 

 appear now and then during the day and the period of abundance at night is a little 

 more extended than in such organisms as Pareuchaeta and Metridia. St. 461 suggests 

 a vigorous migration, and the diurnal variations are clearly shown at Sts. 618-25. 



Euphausia crystallorophias. This is a neritic species of which few examples have been 

 taken, and none north of 6o° S. The few that occur south of 6o° S give some suggestion 

 of diurnal variation, and since most of the other species of Euphausia undertake vertical 

 migrations, it is probable that E. crystallorophias does so too. 



Euphausia triacantha (Fig. 18). Here the variation is almost as sharply defined as in 

 Pleuromamma. Examples occurred in only three out of 127 hauls between 0600 and 1759. 



