SAGITTA GAZELLAE 273 



I shall describe the life history of one particular brood, referred to as the March/April brood, which 

 is observed during the winter months in the surface 100 m. Then having established the pattern of its 

 life history I shall proceed to discuss that of other broods. 



Since the fully mature specimens are found only in deep water it can be assumed that the eggs are 

 laid also in deep water, and that they, or the newly hatched young, rise to the surface. Certainly the 

 youngest individuals taken in our nets occur in the surface layers. Judging by the growth rate indi- 

 cated in Fig. 26 it seems likely that eggs are laid in about late March or early April. The youngest 

 individuals are found between 250 m. and the surface in early May. Although at this stage they are 

 mainly in the 10-15 mm - g rou P> there are still quite a large number in the 5-10 mm. group; individuals 

 at a length less than 5 mm. are rare at all times. It seems likely that hatching takes place in the top 

 250 m. as no very small individuals have been found below this depth (see Table 16), though specimens 



Table 16. Monthly distribution of 0-10 mm. size group, from seventy-one vertical stations 



of other forms (S. maxima, S. planctonis and Eukrohnia hamata) are common in the deeper nets, at 

 lengths from 2 to 5 mm. I have been unable to find the eggs, but judging from their size in the mature 

 ovaries they must be very small when laid, and I do not know how to distinguish them from eggs of 

 other species. Kramp (1939) states that some of the eggs of Sagitta maxima, a species which also 

 breeds in deep water, rise to the surface to hatch. The ovaries of the living mature S. gazellae are pale 

 yellow in colour, and this may be due to the presence of fat or oil which might help the eggs to rise. 



From May to late July the March/ April brood is concentrated in the 100-50 m. layer. In late July 

 and August they sink to the 250-100 m. layer (Fig. 22), and the upper layers are almost completely 

 empty of this species (the haul at Station 2869 in August is quite exceptional (see p. 263) though very 

 fortunate, as without it there would be no data for August in Fig. 26). 



During the winter months the animals grow at a rate of 5 mm. a month, and by September are 

 mainly in the 25-30 mm. group. The growth rate increases in the spring, and by the end of October 

 specimens have reached the 35-40 mm. group, but the individuality of the brood is lost by mixture 

 with other broods which have ascended from deeper water. The March/April brood remains in the 

 surface until February or March, and attains stage II maturity and full size. The breeding migration 

 (p. 270) now commences, and the animals sink, passing from stages II to III at about 750-500 m., down 

 to the 1000-750 m. layer, where they are usually at stage III, and on to the 1 500-1000 m. layer where 

 they are mainly at stage IV. Loss of the posterior teeth occurs between stages II and III at about 

 750 m., and the rest of the head armature also decreases. At the advanced stages of maturity the 

 hooks are often damaged and broken, but the main reduction in hook numbers is effected by normal 

 shedding from the ventral end of the hook rows. After leaving the surface the animals do not appear 

 to increase in length; their food is presumably used almost entirely by the rapidly developing ovaries, 



