Thuridilla ratna (Er. Marcus, 1965)

Description

A small elysiid sacoglossan up to about 15–20 mm in length (Gosliner, 1995). Body dark brown to black with longitudinal cream to yellow lines on the head and parapodia. The free edge of each parapodium is conspicuously bordered by a bright red-orange line, contrasting strongly with the dark body, and the typical T. ratna colour form lacks (or has greatly reduced) the row of submarginal blue ovoid spots characteristic of Thuridilla bayeri. Rhinophores often bear an orange band. Sole of foot red or orange.

Distribution

Western Pacific. Records under the name T. ratna include the Marshall Islands (type), Palau, Guam, Pohnpei, Taiwan, Philippines, Malaysia, Indonesia and Papua New Guinea (Marcus, 1965; Carlson & Hoff, 1978; Gosliner, 1995). Type locality: Eniwetok, Marshall Islands.

Etymology

The specific epithet 'ratna' is a Sanskrit-derived noun used as a name in apposition; 'ratna' means 'jewel' in Sanskrit, referring to the gem-like coloration of the living animal.

Remarks

Described by Ev. Marcus 1965 as Elysia ratna; transferred to Thuridilla by Jensen 1992. Gosliner 1995 reviewed the genus and treated T. ratna as a colour morph of T. bayeri, but MolluscaBase / WoRMS currently accept it as a separate species. The two taxa often co-occur and are most reliably separated by the presence (in T. bayeri) or absence (in T. ratna) of the submarginal row of blue ovoid spots on the parapodium.

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References

• Thuridilla ratna, Gosliner, T. M. (1995). The genus Thuridilla (Opisthobranchia: Elysiidae) from the tropical Indo-Pacific, with a revision of the phylogeny and systematics of the Elysiidae. Proceedings of the California Academy of Sciences (4th series) 49(1): 1-54.
• Thuridilla ratna, Martín‐Hervás M.D.R., Carmona L., Malaquias M.A.E., Krug P.J., Gosliner T.M. & Cervera J.L. (2021). A molecular phylogeny of Thuridilla Bergh, 1872 sea slugs (Gastropoda, Sacoglossa) reveals a case of flamboyant and cryptic radiation in the marine realm. Cladistics. 37(6): 647-676. https://doi.org/10.1111/cla.12465