On the Occurrence of Discias Atlanticus Gurney, 1939 in the Western Indian Ocean (Decapoda, Caridea)

Abstract

The caridean family Disciadidae contains only a single genus with four species. All species are of tropical distribution, with two species, D. atlanticus Gurney, 1939, and D. serratirostris Lebour, 1948, from the Atlantic Ocean, D. exul Kemp, 1920, from the Indian Ocean and Western Pacific Ocean and D. serrifer Rathbun, 1902, from the Eastern Pacific Ocean. This note reports the discovery of D. atlanticus in the western Indian Ocean, off northern Kenya. Three specimens, two apparently adult and one late larval stage were obtained from the catch of an Isaacs-Kidd midwater trawl, operated from the R. V. Manihine (Cruise 341) on 2nd June 1972 at a position 2?44'S 4l?00'E in water of 110 fm depth. D. atlanticus was first reported from Bermuda by Gurney (1939) but has since been reported from Guadeloupe by Monod (1939) and also from the Cape Verde Islands and Gabon by Holthuis (1951). All previous records have therefore been in the tropical Atlantic Ocean and the occurrence of this species in the Western Indian Ocean represents a considerable extension in range, which is particularly surprising in view of the very small number of shallow water caridean shrimps that occur in common to the Indo-West Pacific and Atlantic faunal regions. The two specimens, both male, have a post-orbital carapace length of 2.7 mm and are essentially similar. One of them has been compared with a specimen of D. atlanticus from Bermuda by Dr. F. A. Chace, and no significant differences have been noted. This specimen was presented to the Smithsonian Institu tion by R. Gurney and is apparently the male illustrated in Gurney's report. The British Museum (Natural History) has no material of D. atlanticus and none is in the collections of the Marine Biological Association of the U.K. It therefore seems to represent the only part of Gurney's original material to be still extant. The Mombasa specimens have also been compared with the Atlantide specimens reported upon by Holthuis (1951), which were kindly made available by Dr. Tor ben Wolff. The present specimens show a closer resemblance to the Bermudan material with regard to the form of the rostrum, which is relatively broad, rather than to the Atlantide specimens, which have a comparatively narrow rostrum. In the Mombasa specimens the lateral margins of the rostrum distinctly overlap the eyestalks, but do not do so in the Atlantide specimens. The lateral margins of