Stylocheilus polyomma

Stylocheilus polyomma (Mörch, 1863)

Stylocheilus polyomma @ El Hierro 17-10-2012 by Aketza Herrero

Taxonomic note: There was a long time discussion about the identity of Stylocheilus longicauda (Quoy & Gaimard, 1825) and Stylocheilus striatus (Quoy & Gaimard, 1832). Unfortunately these authors noted that S. striatus was probably the same as S. longicauda, beginning a confusion which has persisted until the present. Today most researchers consider that there are two species of Stylocheilus: S.striatus, with dark longitudinal lines, a mottled colour pattern and compound papillae, and S.longicauda with a uniform yellow or green colour, simple papillae, without lines and an extremely long slender ‘tail’. Despite these species were clearly differentiated, Yonow (2012) reverted back to the confusion by ignoring evidence presented by Rudman (1999) that they are distinct species.

According to Bazzicaluppo et al. (2020) Stylocheilus striatus is a cryptic species complex formed by three allopatric different species: the name S. striatus is retained for an Indo-Pacific species, while the names S. polyomma and S. rickettsi are resurrected for species inhabiting the Western Atlantic and the Eastern Pacific respectively. These authors also state that “Whereas Bursatella has colonised the Mediterranean, most likely from tropical Atlantic populations (Bazzicalupo et al. 2018), Stylocheilus has not. Even though benthic and pelagic species of Stylocheilus are present in both the Red Sea (Yonow, 2008) and the Canary Islands (Ortea et al., 2008), and favourable conditions have allowed the dispersal of several other species of sea slugs across the Suez Canal (Crocetta et al. 2013; Zenetos et al. 2017) and the Gibraltar Strait (Valdés et al. 2013), something must be preventing species of Stylocheilus from making such a move”.

In our opinion is is mostly because of a water temperature and / or sality issue. For these reasons, as the only Stylocheilus species present in the Atlantic Ocean would be S.polyomma (its native distribution range is from Florida to Brazil), and until genetic analysis are performed on the Canary Islands specimens, the most probable ID for the Canary Islands is, effectively, S.polyomma.

Synonyms

• Aplysia striata Quoy & Gaimard, 1832 (original)
• Notarchus polyomma Mörch, 1863
• Stylocheilus lineolatus Gould, 1852

Description
Stylocheilus polyomma is a medium sized sea slug, reaching up to 65mm, it has a translucent body with patches of greens, browns and white, and often with many branched papillae. This species is distinguished by fine longitudinal brown-to-black lines usually interrupted by ‘eyespots’ with bright blue centres; it has a swelling present in the middle of body, where the parapodial lobes protect a large gill and the anal siphon. The rhinophores and oral tentacles are long and slender. It may have few or numerous villi, depending on the population.

Biology
It can be locally common in algal beds and shallow water, especially during the breeding season, but this does not happen every year. Releases a purple dye when disturbed.

Etymology

• Stylocheilus. From Latin “stylus”, column, pillar + from Greek χεῖλος, “cheilos”, lip, edge.
• Polyomma. From Greek “Poly”, multiple and “ὄμμα”, eye in reference to the multiple bright-blue eyespots scattered along the body.

Distribution
Although Stylocheilus striatus is actually a group of cryptic species with an apparently circumglobal subtropical distribution, a recent work by Bazzicalupo et al. (2020) confirmed that there are three distinct species of Stylocheilus striatus and that they are allopatric; the name S. striatus is retained for an Indo-Pacific species, while the names S. polyomma and S. rickettsi are resurrected for species inhabiting the Western Atlantic (type location is Virgin Islands) and the Eastern Pacific respectively. Eastern Atlantic populations are believed to be Stylocheilus polyomma until molecular analysis are performed on specimens from this area to confirm this adscription.

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
• Flickr pictures
• Interim Register of Marine and Nonmarine Genera
• NCBI GenBank
• OBIS - Search by Taxon
• Sea Slugs Guadeloupe
• SeaSlug Forum
• World Register of Marine Species

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