Thuridilla gracilis
(Risbec, 1928)

Order: SACOGLOSSA
Superfamily: ELYSIOIDEA
Family: Elysiidae

DISTRIBUTION

Western Pacific

PHOTO

UPPER RIGHT, LOWER LEFT: North point of Île Nou, Nouméa, New Caledonia, intertidal: 10 October 1988, AM C156617.
LOWER RIGHT: Pointe de Pandop, Koumac, New Caledonia, 20°35.2'S, 164°16.6'E, 2 m, hard substrate, 18 October 1993, 3 specimens (13, 20 & 24mm long alive), AM C200509.
PHOTOS: Bill Rudman.

Very characteristic colour pattern, with a dark, almost black, background, fine white longitudinal lines and the foot, head and rhinophores edged with bright orange. In some specimens the parapodia can also be bordered with bright orange and in some specimens elongate white or bright blue spots are present along the outside of the parapodia just down from the edge.Thuridilla bayeri is clearly a synonym and so is T. ratna.

Described by Risbec as having 8 rows of white streaks down the parapodia, which are bordered with orange. He also describes a whitish T-shaped mark on the head between the rhinophores. Gosliner (1995) discusses the synonymy of T. bayeri and T. ratna but considers that the original description of Risbec's Elysia gracilis is insufficient to allow an accurate identification. My photographs from New Caledonia clearly fit Risbec's description and painting, which I have reproduced below. Gosliner considers that the 10 denticles on the radular teeth in Risbec's description shows that E. gracilis is distinct from T. bayeri with 10-15 denticles. Considering the optical equipment available to Risbec to view these tiny teeth, I don't think this difference can be considered significant. I have retained the Thuridilla bayeri page as a cross-reference, but I feel that it is a synonym of T. gracilis.

Reference:
• Risbec, J. (1928) Contribution a l'etude des nudibranches Neo-Caledoniens. Faune de Colonies Francaises: Paris. 1-328.
• Gosliner, T.M. (1995) The genus Thuridilla (Opisthobranchia: Elysiidae) from the tropical Indo-Pacific, with a revision of ther phylogeny and systematics of the Elysiidae. Proceedings of the California Academy of Sciences, 49(1): 1-54.

Authorship details
Rudman, W.B., 2000 (August 12) Thuridilla gracilis (Risbec, 1928). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/thurgrac

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