Thuridilla gracilis (Risbec, 1928)

Yonow, Nathalie, 2012, Opisthobranchs from the western Indian Ocean, with descriptions of two new species and ten new records (Mollusca, Gastropoda), ZooKeys 197, pp. 1-130 : 16-17

Thuridilla gracilis (Risbec, 1928) Fig. 5Plates 1819

Elysia gracilis Risbec, 1928: 278, fig. 93, pl. 10 fig. 5 (New Caledonia).

Thuridilla bayeri Er. Marcus, 1965: 270, figs. 5, 6; Yonow et al. 2002: 835, fig. 2c (Chagos) (syn. n.).

Thuridilla ratna Er. Marcus, 1965: 270, figs. 7, 8; Jensen 1992: 270, figs. 14B, 16A-C, 17A, 18B (syn. n.).

Thuridilla cf. bayeri. - Yonow 1994a: 104, figs. 5a, 6a (Maldives).

Thuridilla gracilis . - Apte 2009: 168, fig. 1v (Laccadive Islands).

Material.

Seychelles: 15 × 5 mm (PK-P, "stripy black/mauve rhinophores and body, scattered blue spots, orange line edging parapodia"), Lilôt, NW Mahé, 7 m depth on underside of rock, 25 March 1992, leg. P Kemp; 25 × 5 mm (PK-W, "striations along parapodia but with white edge with orange line, vivid blue spots, rhinophores white with longitudinal stripes and brown orange at tips, orange under mouth"), Lilôt, NW Mahé, 20 m depth under rock, 09 April 1992, leg. P Kemp; photo of one individual, Lilôt, NW Mahé, 1988-1989, P Kemp. - Maldives: three pres. specimens 6 mm, 7 mm, and 12 mm. (MDV/AB/96/10), Miyaru Kandhu, Felidhoo Atoll, 05 May 1996, leg. RC Anderson & SG Buttress; 15 mm (MDV/AB/96/12; badly preserved, no radula found), Fulidhoo Channel, Felidhoo Atoll, 10 m on algae, 07 May 1996, leg. RC Anderson & SG Buttress; photos of two individuals, 1986-1994, J Hinterkircher.

Description.

Several colour forms exist in the Indo-West Pacific under the names of bayeri Er. Marcus (1975) and ratna Er. Marcus (1975). Ernst Marcus (1975) named these two species in the same publication based on specimens from the Marshall and Palau Islands: his species were differentiated by the presence or absence of blue spots and the shapes of the radular teeth. As noted in the review by Gosliner (1995), the numerous photographs now available show an overlap in colour pattern; morphologically he thought they had very little to distinguish between them and synonymised ratna with bayeri. The pericardia of bayeri appear to be simpler than those of ratna, but Risbec does not describe or illustrate it for gracilis. The pericardia of these three specimens are illustrated for information, and are clearly variable (Fig. 5). The sizes of the teeth are also variable, with the range of the teeth of bayeri being slightly larger than that of ratna (measured from Jensen 1992 and Gosliner 1995). The form with blue spots is the more common in the Indian Ocean, although the form with orange margins is the only one recorded in Mayotte (http://seaslugs.free.fr/nudibranche/a_intro.htm). All the specimens listed above from the Maldives and the Seychelles had bright blue spots in life (Plate 18), and the Seychelles animals additionally had orange margins (Plate 19).

Remarks.

Thuridilla gracilis (Risbec) was mentioned in a discussion of a new species as being impossible to re-identify ( Gosliner 1995), but later, Gosliner (2001) appears to have reconsidered, comparing splendens (Baba) with gracilis, both species having dark brown to black bodies with cream longitudinal lines and reflective blue pigment spots which may be absent in gracilis, and subsequently retaining gracilis as a valid species ( Gosliner et al. 2008). Risbec’s drawing and description of a 20 mm specimen from New Caledonia is clearly within the cline between bayeri and ratna in having dark and light green lines on the body and orange tips to the rhinophores, parapodial margins, and oral tentacles. Photographs of many individuals from the type locality, New Caledonia, demonstrate that the specimens occurring at the type locality are well within the extremes shown by bayeri and ratna ( Hervé 2010 and pers. comm.). Risbec’s species is here determined as both recognisable and valid, and the name Thuridilla gracilis should take precedence.