Atalodoris pusilla

Atalodoris pusilla (Alder & Hancock, 1845)

Atalodoris pusilla @ U.K. by Bernard Picton

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.
Atalodoris albonigra (Pruvot-Fol, 1951) was described in the Mediterranean Sea, but Furfaro et al. (2022) consider is is confused with Atalodoris pusilla because its original description is totally insufficient to differentiate them. A. albonigra was redescribed by Schmekel based on specimens from the Gulf of Naples by using the same general approach (Schmekel & Portmann, 1982), but some inconsistencies in the description support the possible synonymy with A. pusilla. It was presumedly found again by Perrone (1986) but his description corresponded to A.pictoni, as did other findings from the Ligurian Sea (Betti et al., 2017). Later A. albonigra it was found in Catalunya, Spain (OPK, 2012), and these specimens really correspond to A. pusilla, both in the molecular analysis and in the external morphology. In specimens so small (2.5 to 7.5 mm) the pigmentation of the mantle may appear with a prevalence of brown/black, but with dark spots on a creamy white background in those specimens three times larger. Furthermore, the coloration and structure of gills and rhinophores match between the two taxa. Based on these considerations and on the fact that the samples described by Pruvot-Fol (1951), Schmekel & Portmann (1982) and Perrone (1986) are not available, Furfaro et al (2022) consider A. albonigra as the junior synonym of A. pusilla, according to ICZN rules.

Synonyms

• Doris pusilla Alder & Hancock, 1845
• Onchidoris pusilla (Alder & Hancock, 1845)
• Knoutsodonta pusilla (Alder & Hancock, 1845)
• Onchidoris albonigra (Pruvot-Fol, 1951) junior synonym
• Knoutsodonta albonigra (Pruvot-Fol, 1951) junior synonym
• Atalodoris albonigra (Pruvot-Fol, 1951) junior synonym

Description
This species of doridacean can reach a size of 9-10 mm in length. The body is oval shaped and very flattened, nearly circular when the animal is at rest. The body color is dark brown because of numerous densely packed colour patches covering it. Near the edge of the mantle there are usually whitish stains. All the dorsal mantle is covered with small conical brown tubercles, although frequently their base is whitish; tubercles located near the margin of the mantle are smaller. As in other species of Knoutsodonta, the mantle is very espiculous with elongated transversally laid spicules in the center, and diagonally laid near the edges, but this spiculation is not apparent due to the pigmentation of the body. The rhinophores are white, short, with up to nine lamellae and do not have a high rhinophoric sheath; the rhinophoric orifice has two tubercles in an anterior-internal position and in lateral-external position. The gill consists of up to nine small simply pinnate leaves arranged in a circle on the back of the dorsum, almost completely surrounding the anus; The gill crown is also tuberculate, with two tubers in anterior position and another one closing the gill circle. The gill leaves colour is white, like rhinophores, but Mediterranean specimens are pigmented dark brown with scores, like the mantle, although the spine is coloured cream. The cephalic region under the mantle does not differentiate oral palps; the cephalic veil border is usually pigmented brown. The foot is whitish with tiny brown spots; internal viscera can be glimpsed through the center of the foot as a reddish-brown stain.

Biology
The specimens of this species tend to live both in the intertidal and subtidal stratum, often under stones and on the bryozoan colonies from which they feed. There have been cited, as food or substrate,  the following species of bryozoans: Callopora spp., Escharella immersa, Microporella ciliata, Escharoides coccineus, Porella concinna and Schizomavella linearis (McDonald & Nybakken, 1999). The spawn consists of a semitransparent narrow cord with white eggs, wound in a tight flat spiral with up to 9 turns; sometimes there are two egg spirals surrounded by a unique exterior cord. The spawns have been observed in the month of May.

Etymology

• Atalodoris, from  “atalo”, youthful, and “Doris”, name of a marine nymph.
• Pusilla, from Latin “pusillus”, petty, trifling, insignificant, very small.

Distribution
There are few known references of this species. It has been observed in the Norwegian coast, in the British Isles, and in the Atlantic Iberian Peninsula shores (Galicia, Asturias). In the Iberian Coast it has also been cited in the Strait of Gibraltar, and in Catalunya (Furfaro et al., 2022) as A. albonigra.

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

• AsturNatura
• Biodiversity Heritage Library
• Encyclopedia of Marine Life of Britain and Ireland
• Flickr pictures
• Interim Register of Marine and Nonmarine Genera
• NCBI GenBank
• OBIS - Search by Taxon
• World Register of Marine Species

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