I07 



THE DEVELOPMENT AND LIFE-HISTORY OF KRILL 



rapidly until they begin to feed independently. Sars (1898) thought that this occurred in the meta- 

 nauplius, when the mouth opens to the exterior ; but Taube has shown that at this stage, in Nyctiphanes 

 norvegicus, there is as yet no connexion between the mouth and the mid-gut, and Macdonald (1927) 

 states that, in Meganyctiphanes norvegica, "although an open mouth is present in the metanauplius it 

 was not found to feed ". The internal yolk supply suffices until the gut is fully established. This occurs 

 after the calyptopis stage is reached, when the mouth and proctodaeum become connected with the 

 mid-gut. The cells of the mid-gut still contain a certain amount of yolk, but Sars (1898) writes that 

 the larva now begins to feed actively, " chiefly upon small Diatomeae, the remains of which could be 

 distinguished by microscopical examination of the contents of the intestine". In E. superba, "the 

 more typically oceanic species of diatoms are evidently digested rapidly: recognizable fragments are 

 rather rare even in the crop (Hart, 1934).. • .Two forms that appeared constantly in the stomachs of 

 adult specimens and remained clearly recognizable were Fragillaria antarctica^ and Thallassiosira 

 antarcttca". 



Table i has been compiled from the data in Eraser's paper and gives the average lengths of the 

 different larval stages. It will be noticed that at the ist calyptopis stage, when the larva begins to feed, 

 its length is at once almost doubled, after which growth proceeds again more regularly throughout 

 the summer. "By the time the euphausiid reaches the 6th furcilia stage, the major developmental 

 changes have been effected and in appearance it is characteristically a euphausian." 



Table i . Average lengths of larval stages 



By plotting half-monthly average lengths of the larvae for the period of one year, Fraser found 

 that from November to March (the period of spawning) growth was slow, but that it increased steadily 

 from March to June, was retarded during the mid-winter months and began to increase again at the 

 end of August, by which time the first adolescents had made their appearance. Evidently, growth 

 from the egg of the adolescent occupies an average period of about nine months, although under 

 optimum conditions it can proceed more rapidly. 



Eraser's work on larval krill shows clearly that spawning in E. superba is not restricted to one short 

 period, but is spread over most of the southern summer, with the result that new broods of larvae are 

 continually being hatched out, and the stock is constantly replenished. Taube (1915) and Ruud (1936) 

 found that in northern waters, Nyctiphanes and closely allied euphausians had a similarly extended 

 spawning season. Consequently, eggs, larval forms, adolescents in every stage of development, and 

 adult individuals can be, and frequently are, found to exist side by side, and the euphausian population 

 presents a very heterogeneous appearance. 



The larvae of E. superba after one year of growth have attained by the following November an 

 average length of 13 mm. Their subsequent development from adolescence to maturity forms the 

 subject of this paper. 



' Revised by Hendey (1937) and now called Fragillariupsis antarctica. 



