THE DISTRIBUTION OF 

 SAGITTA GAZELLAE RITTER-ZAHONY 



By P. M. David 

 (Plate XIII, text-figs. 1-27) 



INTRODUCTION 



ON E of the main objects of the plankton studies carried out by the Discovery Committee and the 

 National Institute of Oceanography is to disclose the principles which underlie the distribution of 

 oceanic organisms. Among the different species of pelagic zooplankton there are very wide contrasts 

 in horizontal and vertical ranges, and in the relations between distribution, life cycle, and environ- 

 ment. Yet there are few oceanic species of which even the limits of distribution have been fully 

 delineated, and fewer still whose life cycle has been adequately related to their distribution. To under- 

 stand the basic factors it is clearly desirable to know how much the range of habitat and the life cycle 

 vary among planktonic organisms, and to consider whether any types of distribution are associated 

 with certain methods of reproduction or feeding habits, or with the phylogenetic position of the species. 

 One method of approaching these problems is to work out as fully as possible the distribution of a 

 number of species at all stages of their life history, so that those of different habits and from different 

 taxonomic groups can be compared; and the present paper is intended as a contribution to this end. 



The Discovery Committee's field of work was mainly in Antarctic and Subantarctic waters, and 

 these regions were very widely sampled by the ' Discovery II ' in five voyages before the war. During 

 a sixth voyage, under the National Institute of Oceanography in 1950-51, much material was collected 

 in the winter months, which had been insufficiently represented in the pre-war plankton samples. The 

 collections are now well suited to studies of distribution since they cover the whole range of a number 

 of southern species at all times of year. The closing nets provide good information on vertical distribu- 

 tion, and detailed hydrological data are available from most stations. 



At present, a most exhaustive study of the life history and distribution of Euphaiisia superba by 

 Mr J. W. S. Marr is nearing completion, and it is hoped that a study of a carnivorous species such as 

 Sagitta gazellae will be of special interest when compared with a herbivorous species such as Euphaiisia 

 superba which has wholly different habits. 



Sagitta gazellae was chosen as an example of a carnivorous species for various reasons ; experience 

 had shown that it is widely distributed ; it is easily recognized ; it occurs in large, but not inconveniently 

 large, numbers; the animal itself is of considerable size, and is thus easily picked out of samples; it is 

 transparent and the state of its gonads can be determined without dissection. In addition to these 

 points, very considerable data on this species had been amassed by Mr Marr during two pre-war 

 commissions of ' Discovery II '. 



Although in its distribution S. gazellae is restricted to the Southern Ocean, data from other areas 



concerning other species collected during the course of this work suggest that its habits are not unique 



and that S. lyra, a more cosmopolitan species, has, at least in the subtropics of the southern hemisphere, 



a very similar life history. It may well be that other species will be found to conform to a similar 



pattern. 



ACKNOWLEDGEMENTS 



I am particularly grateful to Mr Marr for advice and encouragement, and for permission to use the 

 very large collection of data which he had assembled during the second and fourth commissions of 

 R.S.S. 'Discovery II'. 



