First record of Discodoris rosi from the UK

Dear Mark,

This is Discodoris rosi. It has never been recorded north of the French Coast before, so this is quite exciting. It is a fairly large and conspicuous nudibranch so I suspect that we are seeing a true range extension here. Sally Sharrock, who was leading your trip, sent me another picture of the same species taken by Anna Cawthray, so there seems to be a population there, not just a chance individual.

The Scilly Isles are probably one of the places we can expect species crossing the English Channel to be seen first, if global warming continues to affect the North Atlantic by warming it. It was first described from Galicia, NW Spain as recently as 1979, indeed I saw it there myself in 1978. At the time I identified the sponge it was feeding on as Microciona strepsitoxa, a red encrusting sponge on which it is reasonably well camouflaged. I suspect that is the sponge in Anna's picture. Since then it has been reported from a few scattered sites throughout the Mediterranean Sea and here on the Forum as far north as Belle-Ile in Southern Brittany. The sponge seems to have a more northerly distribution in Britain and Ireland, so perhaps that made it possible for the nudibranch larvae to get started once they had somehow crossed the English Channel.

Best wishes,
Bernard Picton