Food and colour in Tritonia nilsodhneri

May 10, 2001
From: Jane Lilley

Dear Bill
Here I my observations on food, colour and behaviour in Tritonia nilsodhneri to match those I sent earlier on Tritonia hombergi

This species feeds on the sea fan Eunicella verrucosa, which is usually pink, and the Tritonia matches the colour of its host. However, at some sites in south-west Britain, a minority of the Eunicella are white. Tritonia nilsodhneri appears to feed just as readily on the white form as the pink, and again matches the colour of its host. So white fans have white nudibranchs, and pink ones, pink, though the pink fans vary a bit in colour, and the Tritonia is not always a good match - it can be perfect, but it is often too salmony or too beige a shade.

During a series of dives in August last year, when Tritonia nilsodhneri was breeding - mating pairs and egg threads all over the sea fans - several people saw them crawling rapidly across the algae and rocks, often well away from any sea fan. Between us, we saw enough individuals to be fairly sure that they had not been accidentally dislodged, but were deliberately migrating, presumably in search of mates. Specimens of Eunicella with only one Tritonia are common, so for mating to be possible, migration would be needed. At one site, adjacent to a small Eunicella which had a single nudibranch on it, I saw another individual on an algal frond. It was stretching forwards as far as it could, with the front half of its body unsupported as it attempted to reach the sea fan, and was making a determined effort to reach its stem at the point where a small individual of Tritonia nilsodhneri was coiled round it. There seemed no doubt that it knew that the other nudibranch was there, and that it, rather than just the Eunicella, was its goal. Had it perhaps followed a 'scent' in the water to find it?

We also saw several white individuals on pink fans, which I assume had recently moved from one host to another. There were also a significant number of 'pink' Tritonia on pink fans which had unusually dark bodies, although the gills were pink (in Tritonia hombergi, individuals which have recently moved from one colour of Alcyonium digitatum to another appear to change their body colour first, and then the gills). In some cases the whole body was darker, in others it seemed to be just the back. The individuals seen 'migrating' were also often darker-bodied. I wondered whether they were feeding on whatever was available while they were off the Eunicella, and were either making use of a darker pigment from their food, or gradually breaking down their pink pigment and not being able to replace it. If so, the darker-coloured individuals seen on sea fans would have recently migrated, just as the white nudibranchs on pink sea fans must have. I wonder whether individuals which appear to be the wrong shade of pink have also recently moved from one host fan to another of a slightly different colour. If so, such movement must be quite common.

I'd appreciate it if anyone can comment usefully on my speculations, or add further observations.

Jane Lilley

Lilley, J., 2001 (May 10) Food and colour in Tritonia nilsodhneri. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Thanks Jane,
Bill Rudman

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