Atalodoris sparsa

Atalodoris sparsa (Alder & Hancock, 1846)

Atalodoris sparsa 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.

Synonyms

• Doris sparsa Alder & Hancock, 1846
• Onchidoris sparsa (Alder & Hancock, 1846)
• Knoutsodonta sparsa (Alder & Hancock, 1846)

Description
This is a small species of Atalodoris that does not usually exceed 6 mm in length. The body is very flat and with an oval shape. The coloration is generally yellow, with the center of the dorsum slightly reddish because of the internal viscera. The mantle margin has small orange-brown spots that contrast with the somewhat paler color of the rest of the mantle. When the specimens are observed under binocular microscope it becomes apparent that there are thin black dots on the mantle, somewhat more visible on the edge of the mantle in ventral view. Typical dorsum spiculation is visible by transparency: Transverse spicules in the middle, oblique along the sides, and radially arranged around the mantle margin. The dorsum has conical tubercles but with a rounded apex, semitransparent but with an orange-brown tip and a smaller size the closer they are to the mantle margin. The rhinophores are very small, whitish, with few lamellae and some brown spot; there is no rhinophoric sheath but the white gill opening has 2-3 small tubercles like the ones on the dorsum, brown pigmented on the tip. Gill is formed by 5-9 white, unipinnate, small gill leaves, forming an incomplete arc around the anus. The foot is white, the reddish-brown digestive gland is clearly visible in the center of the foot by transparency. The mouth does not have labial palps.

Biology
This small nudibranch, like others of the same genus, is associated with several species of bryozoans they feed on, among which may be mentioned, among others, Cellepora pumicosa, Porella concinna, Celleporina caliciformis (=hassalli) or Schizomavella linearis. Because of its small size, its flattened form, and its peculiar coloration, this is a very cryptic species on the bryozoan colonies on which it is found so it passes almost unnoticed. Little else is known about its biology and reproduction.

Similar species

Idaliadoris neapolitana with similar coloured juveniles (yellowish), but with digitiform tubercles, not conical.

Etymology

• Atalodoris, from  “atalo”, youthful, and “Doris”, name of a marine nymph.
• Sparsa, sparse, referring to the layout of the dorsal tubercles.

Distribution
There are very few reports of this species, mainly found in the British Isles and Ireland, but also in the Iberian Peninsula, where it has been reported in the Cantabrian and Galician coast. First Mediterranean report was from Tossa de Mar (Girona, Spain) (Ballesteros, 1984), later it was also found in L’Escala (Girona, Spain). Also reported in Cabo de Palos (Murcia, Spain).

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
• Encyclopedia of Marine Life of Britain and Ireland
• European Sea Slugs by Morddyn
• Flickr pictures
• Interim Register of Marine and Nonmarine Genera
• NCBI GenBank
• OBIS - Search by Taxon
• Sea Slug Forum
• World Register of Marine Species

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