Parasite?

[RevJDA]

My tank has been cycled for at least four months (75 gal w/live rock, live sand, a few fish, and a few easy to keep corals and a bubble anemone). About 4-6 weeks ago I began noticing what appeared to be "puffs of silt" (milk-like) coming from my live rock. At first I thought it was a fish disturbing light sediment on the rocks. Then, about a week ago I discovered a small creature in the crack of a rock that was puffing out this stuff.

This creature looks like a small snail w/o a shell--a slug. It is about a quarter inch in size, rounded, light colered (gray-white), no really discernable markings, and it almost glides across the rocks. We rarely see it as it stays in the cracks of the rocks. It doesn't seem to have bothered the corals or polyps in my tank. And I have checked out pictures of bristle worms. It doesn't look a thing like a bristle worm.

What is this little creature? Should I be worried about it?

[JHemdal]

Hi,

Without a photo is is impossible to say, but my general opinion is that if an unknown organism isn't found closely associated with another one in some way, it probably is NOT parasitic. Parasites (or even predators) are rarely found far from their host (or prey).
Just by your description, I'd say this could be an opistobranch (snail with an internal shell) or a nudibranch (shell-less snail). The puffs that you see are probably a result of its feeding activity - and are most likely just detritus kicked up by the animal's movement (crevices trap a lot of fine particles).
Watching a reef aquarium really closely often results in seeing weird things that can't be easily explained - a lot like a person who focuses too much on some minor health symptom and then starts to suffer from hypochondria as a result<grin>.

Jay Hemdal

[RevJDA]

Thanks for your reply. I wish I could get a picture of it. I have digital camera but I'm not sure it would zoom in close enought. Besides, it really doesn't stay in one place very long and we don't see it very often.