Some type of circular tube worm?

[mikethefishman]

I have a 125G tank, so far this is the only one I've seen in the tank. I don't know what it is but it seems like it's been growing for awhile without me noticing it. Not sure if this is a good thing or not. I think if there are many others it should be pretty easy to spot now that I know what to look for. These things are decently sized and I don't think they can hide in some rock hole without me noticing it now. Is this thing a bad pest?

 

[ISpeakForTheSeas]

I don't know for sure, but my guess is that it's a large vermetid snail species. If so, you may want to get rid of it, as they feed with mucus webs that can irritate corals, and vermetids are known to spread rapidly (I've never heard of rapid spreading with the larger species, but they seem to be much rarer than the smaller ones, and people tend to remove them as soon as they see them - so they might just not be in the tanks long enough to spread like the smaller ones).

Out of curiosity, roughly how big is it?

Welcome to Reef2Reef!

 

[mikethefishman]

I'd say the whole thing is about a bit bigger than a quarter.

 

[EeyoreIsMySpiritAnimal]

I'd say the whole thing is about a bit bigger than a quarter.

Large vermetid snail.

 

[mikethefishman]

Just tried to remove it and I'd say it's impossible to do in the tank, I'd have to take out the rock and hammer it away.
I boiled some water and shot some hot water directly into the opening a few times. Then I covered the opening with some glue. Seems to be fully covered. Not sure if this is good enough.