How to distinguish Trinchesia with blue and yellow cerata

Those who dive in winter in the waters of the Mediterranean and Eastern Atlantic Ocean have the opportunity to observe certain white nudibranchs that have cerata with a blue subapical band and a yellow apical band. It was initially thought that they were different phases of the same species, Trinchesia caerulea, quite common in the Western Mediterranean and near Atlantic, but since the paper published by Korshunova et al. (2019) it was possible to prove by morphological and molecular techniques that they were different “cryptic” species.

Below is a list of points you can look at to differentiate your findings.

Trinchesia morrowae

• White line running along the dorsum.
• White line running along both sides of the body, below the cerata.
• Smooth rhinophores and oral tentacles coloured whitish with orange tips.
• Cerata with yellow tip, followed by an electric blue ring and a yellow / gold dots ring. Brown digestive gland, deppending on the food, visible by transparency.
• Maximum size: 10 mm.

Trinchesia morrowae by Enric Madrenas

Trinchesia cuanensis

• Absence of white line running along the back.
• Absence of white line along the sides of the body.
• Rhinophores and oral tentacles smooth, without coloration or with a very pale yellow coloration.
• Orange spots may be present on the head and body.
• Cerata with yellow / orange tips, followed by an electric blue ring and a more or less dense yellow dot ring. Black digestive gland, deppending on the food, visible by transparency.
• Maximum size: 15 mm.

Trinchesia cuanensis by Enric Madrenas

Trinchesia caerulea

• Absence of white line running along the dorsum.
• Absence of white line on the sides of the body.
• Smooth rhinophores and oral tentacles with pale yellow tinted tips, which may be inconspicuous.
• Cerata with yellow tips, followed by a more or less intense blue ring, which may be absent.
• Cerata without a third ring below the blue ring.
• Maximum size: 25 mm.

Trinchesia caerulea by Enric Madrenas

References

• Cella, K., L. Carmona, I. Ekimova, A. Chichvarkhin, D. Schepetov, and T. M. Gosliner. 2016. A Radical Solution: The Phylogeny of the Nudibranch Family Fionidae. PLoS ONE 12/2016; 11(12):e0167800
• Korshunova, T., A. V. Martynov, and B. E. Picton. 2017. Ontogeny as an important part of integrative taxonomy in tergipedid aeolidaceans (Gastropoda: Nudibranchai) with a description of a new genus and species from the Barents Sea. Zootaxa 4324 (1): 1-22.
• Korshunova, T., B. Picton, G. Furfaro, P. Mariottini, M. Pontes, J. Prkić, K. Fletcher, K. Malmberg, K. Lundin, and A. Martynov. 2019. Multilevel fine-scale diversity challenges the ‘cryptic species’ concept. Scientific Reports. 9: 6732.

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