Halgerda willeyi Eliot, 1903

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 : 845

Halgerda willeyi Eliot, 1903

(®gures 7, 11b)

Halgerda willeyi Eliot, 1903c: 372 , pl. 32, ®gure 5; Yonow, 1990: 293, pl. 13.

? Chromodori s pulchella (?) (sic.) Bergh, 1873a: ®gure only: pl. 25, ®gure 11.

Material. Chag96/77a, b: 29 mm (20 3 15 mm preserved, slightly curled; radula preparation), 12 mm (113 10 mm preserved, slightly curled); outer reef of Middle Brother, Great Chagos Bank; 1 March 1996; at 24±25 m depth.

Description. Body translucent white with series of raised yellow-orange ridges on the dorsum (®gure 11b). These ridges were symmetrical, enclosing a double series of six depressions; these depressions each contained a longitudinal black streak, ¯anked in the larger specimen by thin yellow streaks. The intersections of the ridges were marked by tubercles. Many black lines were present around the edge of the mantle, mostly radiating outwards to meet the mantle edge at right angles. Rhinophores long: lower two-®fths of club translucent and central ®fth yellow anteriorly; distal two-®fths of club dark brown; posterior dark brown line on translucent stalk. Rhinophore sheaths short, orange. Gills white, with dark brown lines externally.

The preserved specimens have lost all traces of orange pigment but the black remains on the mantle, hyponotum, foot and rhinophores; the rhinophores are extended in both specimens. The radula was dissected out of the larger specimen, and there were no markings on the membranes; often species of Halgerda have a dark covering or pigmentation on the bulb. The formula is 44±453 35.0.35. All the teeth are hook-shaped: the central # 14 on each side are as small and crowded as in H. iota Yonow, 1994 . There are no bristles on the marginals, which are very reduced hooks (®gure 7).

Geographic distribution. Indo-West Paci®c.

Remarks. Halgerda willeyi is usually very large, up to 90 mm (Yonow, 1990), and has white rhinophores and gills with black speckles distally and spots proximally. Large specimens diOEer in having yellow ridges surrounding depressions which contain mainly longitudinal black and yellow markings, and yellow lines and yellow tubercles around the margin in addition to the perpendicular black lines. The radula of the

100 m m