Nudibranchs

[The Kapenta Kid]

Anyone know anything about these critters?
I have found a hitchhiker which I have tentatively identified as a nudibranch. It is about half an inch long, bright red, slug-like with two dark colored 'horns' sticking up from its head and lots of little red papillae protruding from its body. I have not been able to get a good pic yet.
I found it on the underside of a rock when I was cleaning the tank. It looked dark red and squishy like a small closed anenome of the type that you find in rockpools in the North Sea. But when I replaced the rock the other way round it started to move with a slug-like motion and eventually moved to an undersurface again.
I googled around a bit and found the slug site but there appear to be about 10 zillion species and didn't know where to even begin.
Any pointers to narrow the search will be helpful.
Of course, it might be some other critter completely.

Gotcha.

After I had typed the above I took another look at the tank and found the critter crawling across the sand. It is clearly a nudibranch, and you can see why they are called sea hares from the 'ears', but the species??
It is much more red in real life than my photo. I was in such a hurry to get some shots that the color balance is wrong.

 

[johnanddawn]

looks like a nudi - most are obligate feeders so likely it'll be doomed before you ID it. BTW many nudis feed only on a specific species of cnidarian such as the monti eating nudi and the zoathid eating nudi - this isn't either of them - but still very specialized in what they eat

 

[The Kapenta Kid]

Yep, I've now learned that it is a dorid nudibranch--which still leaves a pretty big field :mrgreen:
Apparently they graze on certain sessile inverts like sponges and bryozoans. I've got a few sponge fragments growing in the tank but not enough to provide many square meals for a dorid I guess.
Bryozoans are undoubtedly there by the zillion so if that's what it eats it should be OK.
I know that it is not a good idea to deliberately introduce them to a tank due to their obligate feeding, but since this guy is a hitchhiker and has only shown up a couple of months after the last live rock was introduced, I guess he may be finding nourishment in there. I hope so anyway.