What's this nudibranch critter?

[Betta132]

I saw a bunch of these in a tank at the Austin Aquarium. It's one of their tanks that's basically a large reef tank, and there were a few dozen of these crawling around. Just random places... Coral, rock, wall, sand, and one on a snail. No corals appeared to have been eaten... I think they're harmless.
I asked one of the people who worked in that area, and he says he isn't sure what they are. I'm pretty sure they're nudibranchs... You can't see it, but they have the little antennae. This was one of the biggest, and it was slightly under an inch long.

 

[Sugar Magnolia]

The pic is pretty far off but from your description and the body shape I'd toss Elysia sp as an ID. They are harmless algae grazers.

 

[pagojoe]

It may be an arminid nudibranch, which feeds on soft corals. There actually are a couple species of Elysia that are that color, and it's about the right shade and shape to be a Plakobranchus (also harmless, like Elysia) but it should have "horns" that point out to the sides instead of up. If the rhinophores have bulbs on the tops and it lacks the exposed feathery gill in the middle of the back, it's an arminid and not good news. Pretty much any other option won't be a problem.

Cheers,



Don

 

[Betta132]

I don't remember if it stuck its antennae up or to the sides. Sort of in-between, I think... It didn't have bulbs on them, or any gills on the back that I could see.
So if the horns go out to the sides, it's fine, but if they go up, it's bad? Or is it the bulbs that are bad?

 

[pagojoe]

Pale color, elongated shape, no gill on the back and bulbs on the rhinophores = probable arminid nudibranch (soft coral eater). Straight or rolled rhinophores, a slit down the back, green inside the slit = algae eater, good guy.