T 



THE PELAGIC MOLLUSCA OF THE 

 BENGUELA CURRENT 



PART I. FIRST SURVEY, R.R.S. 'WILLIAM SCORESBY', 



MARCH 1950 



WITH AN ACCOUNT OF THE REPRODUCTIVE SYSTEM AND 



SEXUAL SUCCESSION OF LIMACINA BULIMOIDES 



By J. E. Morton, Ph.D. 



(Text-figs. 1-17) 



INTRODUCTION 



he Mollusca collected by the R.R.S. 'William Scoresby' during the first of her two surveys of 

 the Benguela Current off the coast of South West Africa (March 1950— Stations WS 964-1002) 

 form a small collection, representing in all eleven species of pelagic gastropods, one larval gastropod, 

 two lamellibranch post-larvae (one species in considerable numbers), and some cephalopod larvae. 

 Of the gastropods the ' pteropods ' are by far the largest group and they yielded two species of numeri- 

 cal importance, the gymnosome Pneumodermopsis paucidens, and— in special abundance at certain 

 points— the thecosome Limacina bulimoides. The latter was taken at five offshore stations lying to the 

 west of the continental shelf, and at two stations was the dominant zooplankton species. The material 

 secured gives valuable information on the depth distribution, reproductive activity and sexual succes- 

 sion of this pteropod. Further, the fixation of the Limacina in 5 % formalin was in many cases good 

 enough to enable detailed morphological work to be carried out on this species, and the second section 

 of this report has been devoted to an account of reproduction in L. bulimoides, in which the genital 

 system has been described in detail and the sexual phenomena of this mollusc discussed. 



Lamellibranch larvae occurred close inshore at Walfisch Bay, at the inshore station WS 989, and at 

 the mouth of the Orange River. The cephalopod larvae, which were four in number, have kindly been 

 examined by Dr W. J. Rees of the British Museum (Natural History) and I am grateful to him for 

 allowing me to publish his comments on them. In addition to material from the South West African 

 coast, one haul of Ianthina taken with a surface net in north equatorial waters off the north coast of the 

 Gulf of Guinea is conveniently referred to here; while one of the cephalopods (from WS 1091) 

 belonging to second survey material is included for comparative purposes in this report. 



For full hydrological data, reference should be made to the detailed Station List already issued for 

 the 1950 cruise of the 'William Scoresby' {Discovery Reports, xxvi, 211-58). Preliminary accounts of 

 the hydrological results of the expedition and of the distribution of phytoplankton in the South West 

 African area have already been published (Currie, 1953 ; Hart, 1953). A table of positions and dates 

 together with surface temperatures and salinities from the stations which yielded mollusca during the 

 first survey is given on p. 166. 



ACKNOWLEDGEMENTS 

 I am indebted to Dr N. A. Mackintosh, C.B.E., of the National Institute of Oceanography far per- 

 mission to report upon this material. I must also record my appreciation of the kindness of Dr T. J. 

 Hart, at whose suggestion the work was placed in my hands, and who was personally responsible for 

 the heavy task of sorting and classifying the molluscan material from the general bulk samples. 



