Scyllaea pelagica Linnaeus, 1758

Yonow, Nathalie, Anderson, R. Charles & Buttress, Susan G., 2002, Opisthobranch molluscs from the Chagos Archipelago, Central Indian Ocean, Journal of Natural History 36 (7), pp. 831-882 : 871

Scyllaea pelagica Linnaeus, 1758

(®gure 18b)

Scyllaea pelagica Linnaeus : Thompson and Brown, 1981: 441, ®gures 3, 4.

Material. Chag96/55: four specimens approximately 18 mm; on ¯oating Sargassum weed, inside Peros Banhos Atoll; 22 February 1996. Chag96/56: 23 mm; collection details as above. No photographs.

Description. The preserved specimens, although ¯attened, appear identical to the photograph in Wells and Bryce (1993: 173; ®gure 223): the body is translucent and the organs within are opaque white; the brown radula and jaws are visible. The orange pigment spots remain on the preserved material. The two pairs of cerata with the gills on the mesial surface are squared in shape, irregularly lobed on their distal edges, and the dorsal ¯ap posterior to the cerata is large and bears gills, ending bluntly at the short metapodium. In the smallest specimen (the only specimen without orange spots), the rows of teeth in the radula are clearly visible at 3 80 magni®cation. When removed, examination provided a formula of 17 (1 3)3 24±25.1.25 ±24 (®gure 18b). There are no colour notes or photographs, but preserved body shape is identical to those illustrated by Baba (1949), Marcus and Marcus (1963) and Thompson and Brown (1981).

Geographic distribution. Scyllaea pelagica is considered semi-planktonic, associated with the ¯oating oceanic alga Sargassum . It has been recorded under numerous names from all areas of the world, including Antarctica, southern Britain and the western Atlantic (see Thompson and Brown, 1981).

Remarks. Numerous species and varieties of Scyllaea have been described, all of which are found in Sargassum and analogous ¯oating habitats. Odhner (1936) has synonymized the majority into ®ve species, distinguished by colour, shape and internal characteristics such as the radula, stomach plates and liver masses. Following Eliot’s (1906b) example, Odhner’s (1936) primary distinctions between the species were in radula ribbon width, narrow (16±17 lateral teeth) or broad (201 lateral teeth). This specimen, with 24±25 laterals, clearly falls into the second group, which contains three species distinguished partly by colour and size. Scyllaea pelagica is the most common and the largest, characterized by blue patches or small spots and squared cerata. The other two species with a broad radula are S. marmorata Alder and Hancock, 1864 and S. viridis Alder and Hancock, 1864 . The former, redescribed