Cuthona gymnota? from Cork, Ireland

Dear Jamie,
There are a number of aeolids found in your waters which look like these animals. Unfortunately the way they are curled around the hydroid stalk - and each other - hide most of the characters we could use to identify them easily. So my identification of these as Cuthona gymnota is a little tentative.

Cuthona gymnota feeds on Tubularia, which is what your animals are on, and it does not have any white pigmentation on the body or head tentacles, which as far as I can is the case in your animals. What would be positive proof is if the anterior corners of the foot were rounded, but I can't see them. Another point in favour of Cuthona gymnota is the shape of the egg ribbon, which Picton & Morrow (1994) say "consists of curved lozenge shaped packets of eggs", which fits the egg mass in your photo.

There are three species of Flabellina which are also very similarly coloured: Two of them, Flabellina gracilis and F. pellucida usually eat hydroids of the genus Eudendrium, which suggest your animals are not those species. F. verrucosa feeds on Tubularia, but it has a white median line on the body. The other species of Flabellina  have white markings on the head tentatcles (rhinophores, oral tentacles) and the body, none of which are visible in your photos.

Hopefully, a local expert can confirm or correct my identification

Best wishes,
Bill Rudman