Cuthona yamasui? from Nth Sulawesi

Dear Roberto,
Yes this is C. yamasui, but it is certainly out of place sitting on a live coral colony, probably a species of Porites. I suspect this animal has been placed on this coral head by another diver who perhaps brought it back to show his friends, and then he/she carefully 'put it back' so it wouldn't come to harm. Unfortunately unless you put animals back in the right place you are usually condemning them to death.

If you look at the way all the cerata are contracted, as are the rhinophores and oral tentacles, this is not a happy animal. I would say it is being stung or is in contact with noxious chemicals excreted by the coral colony. Because of that it won't move. That's why I suspect it can't have crawled onto the colony itself.

It reminds me of work I did on the coral-feeding species of Phestilla. When I was trying to see just how choosy the Porites-feeding Phestilla lugubris was, I noticed that some colonies of what was thought to be the common local species of Porites were slightly different in colour and colony shape than the rest, and I never found Phestilla on them in the field. So I brought about 50 small colonies - some 'normal' some not - back into the lab and one test I did was to place a specimen of Phestilla on each colony and see what they did. Usually after a few seconds they would relax, extend their foot and after a bit of crawling were obviously quite comfortable. However any Phestilla placed on the 'different' colonies immediately reacted by contracting all their cerata and head tentacles and curling their foot down and in, trying to avoid the sole touching the coral. They would stay in this distressed condition until they rolled off the colony or I removed them. I could never get coral taxonomists then to accept that there might be two species involved, but I am sure there were. Anyway the point is that although the nematocysts in some corals are not apparently as powerful as those in hydroids and anemones, there is certainly something in some Porites which is painful to aeolids. That's why I think your animal is sitting there in pain, unable to move anywhere.
Best wishes,
Bill Rudman