Thordisa filix

Thordisa filix Pruvot-Fol, 1951

Thordisa filix by Manuel Ballesteros

Description
This species can reach 40 mm in length, but usually the cited specimens measure between 15 and 25 m in length. The color of the animal is a more or less homogeneous yellow with 1-2 dark areas in the mantle due to internal viscera, however, some specimens are coloured in lighter shades. The dorsum has numerous conical or slightly digitiform (finger like) tubercles and other filiform (thread like) tubercles sparser and coloured brown at the tip, forming a line of 5-6 tubercles in the center of the dorsum. The dorsum is very spiculous, as are the tubercles, which have spicules on their base in a starry arrangement. The rhinophores are yellow but have a thin brown pigmentation in the lamellae, the apex is whitish; the rhinophoric sheath is rather high and its upper edge is tuberculate. The gill consists of four tripinnate gill leaves that remain high and growing from a single basal protuberance; they are thin and semitransparent but with a light brown raquis. Branchial sheath is slightly raised and its upper edge is also tuberculate. In the center of the gill leaves lies the anal papilla, which is quite high, brownish and with 6-8 lobulations with rounded tips. The foot is relatively narrow, coloured uniform yellow and is furrowed and cleft on the front side. The mouth is located in a highly developed oral protuberance and has two whitish elongated labial palps. The genital orifice is very apparent and it is positioned on the underside of the mantle, to the right of the body.

Biology
This is a species living at a certain depth on the continental shelf, so it is difficult to observe by scuba diving. Very few details of its biology are known. Most of the known specimens have been collected by commercial trawling on soft or maerl bottoms at depths of 40-150 m. Schmekel cites that the spawn is an spiral ribbon with yellow eggs of about 120 microns. It’s supposed to feed on sponges, like other species of the same genus.

Etymology

• Thordisa. Woman name of Old Norse origin, cited in the Sagas. A fair woman to look on, high-minded, and rather hard of heart.
• Filix. Latin word meaning fern, bracken, probably related to the shape of the gills.

Distribution
This is an uncommon species that has been recorded mainly in the western Mediterranean basin as in Banyuls-sur-mer, France (Géry Parent, 1990; 1992); Gulf of Naples, Italy (Schmekel & Portmann, 1982); Cabo de Palos, Murcia, Spain (Templado et al., 1988); Blanes, Barcelona, Spain (Cervera et al., 1988); Mallorca (Dominguez et al., 2013); San Isidoro, Nardò, Italy (Perrone, 1998) and Bay of Piran, Slovenia (Lipej and Mavrič [in] Zenetos et al., 2015) but there are also records in the Eastern Mediterranean basin from Greece (Koukouras, 2000); from Yassiada, Istanbul, Turkey (Yokeş, 2001) and from Haifa Bay, Israel (Barash & Danin, 1992). The only reports from outside the Mediterranean Sea correspond to specimens found along the coasts of Arrábida, Portugal (Calado et al., 1999) and Sagres, Algarve, Portugal (García-Gómez et al., 1989). In the Catalan coast it is a relatively common species in commercial trawling fishing grounds off Blanes, Spain.

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Bibliography

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Bibliography based on the works by Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 and Gary McDonald, 2009. Bibliographia Nudibranchia, with later updates from other resources.

Further reading

• Biodiversity Heritage Library
• Flickr pictures
• Interim Register of Marine and Nonmarine Genera
• ISRAMAR
• NCBI GenBank
• OBIS - Search by Taxon
• Sea Slug Forum
• World Register of Marine Species

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