Idaliadoris neapolitana

Idaliadoris neapolitana (delle Chiaje, 1841)

Idaliadoris neapolitana @ Costa Brava (Spain) by Enric Madrenas

Taxonomic note: The family Onchidorididae Gray, 1827 includes 5 genera: Acanthodoris Gray 1827; Adalaria Bergh, 1878; Atalodoris Iredale & Donoghue; 1923, Onchidoris Blainville, 1816; and Onchimira Martynov, Korshunova, N. Samayan & K. Samayan, 2009; and includes a total of 56 currently accepted species (WoRMS, 2022). Except for the genus Acanthodoris, which includes 15 accepted species and three dubious ones (one nomen dubium and two taxa inquirenda) (WoRMS, 2022; Hallas et al., 2016), and the recently described monospecific Onchimira genus, represented by Onchimira cavifera Martynov et al. (2009), the rest of the genera show a troubled systematic history. In fact, Adalaria, Atalodoris, and Onchidoris (including respectively, 7, 17, and 16 currently accepted species) have undergone numerous revisions over the last few years, producing dissimilar systematic assessments, according to the different opinions of various authors (Hallas & Gosliner, 2015; Martynov et al., 2009). The validity of the genus Adalaria is nowadays disputed; Hallas & Gosliner (2015) treated it as a synonym of Onchidoris whereas Martynov et al. (2009) considered it as a valid genus. Furthermore, Hallas & Gosliner (2015) established the genus Knoutsodonta to include all Onchidorididae without a rachidian tooth, but this diagnostic character was previously stated as the synapomorphy of the genus Atalodoris, originally described by Iredale and Donoghue (1923), and for this reason, was recently re-established by Ortea (2021) and is currently considered as valid. Furfaro et al.(2022) introduced a new genus Idaliadoris Furfaro & Trainito, 2022 (type species Idaliadoris neapolitana) grouping 8 species with finger-like more or less swollen dorsal papillae and with a common egg spawn shape. According to Furfaro et al (2022), the shape of the egg masses was revealed to be a useful diagnostic character allowing one to distinguish between genera in the Onchidorididae systematics.

Synonyms

• Idalia neapolitana Delle Chiaje, 1841
• Onchidoris neapolitana (delle Chiaje, 1841)
• Knoutsodonta neapolitana (delle Chiaje, 1841)
• Atalodoris neapolitana (Delle Chiaje, 1841)

Description
This species can reach a size of 10 mm. The body is very flat and coloured red-orange with many brown spots in the center of the back. Some individuals may have a yellowish hue. The most characteristic feature of this species are the numerous digitiform papillae on the back, of the same colour as the dorsum, smaller in the center and longer on the margins, which may extend past the edges of the mantle. These papillae provide a “villous” aspect to the specimens. The rhinophores are semitransparent with an orange hue, can be fully retracted and have about 6-7 lamellae; the rhinophoric orifice is smooth. The gill consists of 12 unipinnate gill leaves arranged in an incomplete oval open in the back. The gill leaves are semitransparent orange. The interbranchial area holds the anus and it also has 6-7 elongated papillae. The mantle is very broad and, on its ventral side, radial spicules of the margin of the body can be seen by transparency. The foot is red-orange, like the oral veil. The foot features a dark brown spot corresponding to the digestive gland of the animal.

Biology
This species, like others of the same genus is often found under stones, on encrusting bryozoans. It is often located on Schizobrachiella sanguinea, of the same red color as the nudibranch, so it is possibly eating the pigment that gives its color while it mimics on the bryozoan. It is easy to tell when predation of I.neapolitana occurs, since predated areas of the colony appear white. This nudibranch can also be found on red algae like Sphaerococcus coronopifolius. The spawn is a transparent cord laid forming an spiral of 2 to 5 turns with large (225 micron) orange eggs, aligned in two rows along the cord. The diameter of the spawn is about 6 mm.

Etymology

• Idaliadoris. The type species of the genus, Idaliadoris neapolitana, was described with the original combination Idalia neapolitana, but the generic epithet was not usable as it was previously assigned to another genus, Idalia (Leuckart, 1828). The union between the genus used in the original description with “doris” maintains a connection with the taxonomic history and the belonging to the suborder Doridina..
• Neapolitana, related to Naples (Italy) where there is an important oceanographic station.

Distribution
This is a mediterranean species that, in the Iberian Peninsula, it has only been cited in the Strait of Gibraltar and in Catalonia (Cadaqués, Tossa de Mar, Colera). Outside the Iberian coasts it has been observed in the French Mediterranean coast (Banyuls, Villefranche), the Gulf of Naples and the Adriatic Sea. There are two recent reports from the French Brittany coast, in the Atlantic.

More pictures

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
• BiologiaMarina.org
• CIB - Club Immersio Biologia
• Flickr pictures
• GROC
• Interim Register of Marine and Nonmarine Genera
• MedSlugs (Med)
• NCBI GenBank
• OBIS - Search by Taxon
• Sea Slug Forum
• World Register of Marine Species

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