Abyssal benthic Amphipoda (Crustacea) from the East Iceland Basin 2. Lepechinella and an allied new genus Michael H. Thurston Institute of OceanograpHic Sciences, Wormley, Godalming, Surrey Introduction R.R.S. Discovery Cruise 39 took place in April-May 1971 and was concerned with biological investigations at 60 N 20 W and 53 N 20 W. The main purpose of the cruise was a study of vertical distribution and migration of the zooplankton and nekton. Secondary objectives included an investigation of the benthic fauna, and to this end epibenthic sledge hauls were made at both stations. This paper deals with the dexaminid genus Lepechinella and a closely related genus obtained from five hauls at the northern station. It follows a report on species of the eusirid genus Rhachotropis (Thurston, 1980). Material and methods For station data and methods see previous paper (Thurston, 1 980 p. 44^5). Small circles in some of the illustrations indicate the bases of setae either deliberately omitted for clarity of the underlying structure (particularly coxae and gnathopods) or lost prior to examination. Systematics Over 3500 gammarids representing about 120 species were obtained from the five sledge hauls. Although the sledge had no opening/closing facility, contamination was minimal. Only a very small proportion of the total catch was clearly of midwater origin. More than half of the benthic species distinguished appear to be undescribed. Four species are discussed herein, three belonging to Lepechinella and represented by a total of 340 specimens, and the fourth to a new genus represented by 93 specimens. Together these four species account for 12-2% of the total number of specimens obtained. The bottom in the area investigated consisted of dark grey-brown mud together with small quantities of sand, stones, clinker and pteropod shells. Variation. Barnard (1973) has discussed speciation in Lepechinella, pointing out that many of the interspecific characters currently used in the genus are quantitative rather than quali-tative. Most species are known from few specimens so that the degree of intraspecific variation is unknown. The volume of material in the present collection has made possible an initial insight into this problem, in that a common pattern of sexual dimorphism and allo-metric growth can be seen in the three species of Lepechinella studied. As is true of the vast majority of amphipod species bearing dorsal teeth or carinae, these projections in Lepechinella become relatively more prominent with increasing size. In juveniles of both sexes, all females and juvenile males, dorsal teeth tend to be rather slender and upstanding. As males develop towards maturity, the bases of these teeth become elon-gated along the dorsal midline of the segment, and the teeth themselves become more decumbent. A state is reached in fully developed specimens in which body segments, par-ticularly the posterior peraeonites and the pleonites, become carinate, with each carina ending in a posteriorly directed tooth. Concomitant with these changes is the development of a dorsal tooth on urosomite 3. Terminal males also show marked modifications of the Bull. Br. Mus. nat. Hist. (Zool.) 38 ( 1 ): 69-87 Issued 27 March 1980 69
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