Food & colour in Tritonia hombergi

April 29, 2001
From: Jane Lilley

Dear Bill,
I have just read the past contributions on Tritonia hombergi with interest. I saw quite large numbers of juveniles and one larger individual in the Outer Hebrides of western Scotland, U.K., last autumn. Usually there seemed to be a clear link between the colour of individuals and their diet, but not invariably. I'd appreciate other people's views on what was going on - I'm just guessing!

Very small juveniles, 3 - 8 mm long, were numerous once I 'got my eye in'. They appeared as low bumps on the lobes of the soft coral Alcyonium digitatum, between the polyps; the larger ones showed enough detail to confirm the I.D. They were present on both yellow and white colonies of A. digitatum, and usually matched their host's colour exactly: pure white on white colonies, and the identical shade of yellow on yellow colonies. I assume they were utilising pigment from their food.

But at two sites, I found tiny yellow T. hombergi both on yellow A. digitatum and the adjacent white colonies. In view of the usual perfect colour-match, I guessed that they had recently moved from one host to another. Elsewhere, I saw as 8 mm specimen on a yellow soft coral which had a yellow body but white gills - had it recently moved host and was now gradually changing colour, or was something else going on? (I have seen the same thing in Tritonia nilsodhneri at sites where the host sea fan, Eunicella verrucosa, was present in both pink and white forms.)

I rarely see larger Tritonia hombergi. They are reported to move off the soft corals onto the adjacent substratum, and to become darker and more varied in colour, presumably to be cryptic against the general background. Apparently they live for two years and can grow to 8 inches (20 cm) long, but they are very unobtrusive.

However, I saw a 4 inch long specimen in the Hebrides which was a uniform bright yellow - from a distance I thought it was a sponge. So they don't all change colour. It was feeding actively on a small yellow colony of Alcyonium digitatum, and descriptions state or imply that large individuals still feed exclusively on this species. But is this invariably the case, or is it possible that most of them change colour because they have adopted a more varied diet? (Tritonia nilsodhneri seem to change colour to some extent when they are off their usual prey - I can give details if anyone is interested.)

Can anyone make any useful comments?
Jane Lilley

PDL@mm-brig.mottmac.com

Lilley, J., 2001 (Apr 29) Food & colour in Tritonia hombergi. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/4251

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