Extra crab appeared in my tank...from where?

[Joe31415]

I have a tank that's used for reefs (and inverts) only, to QT them before going into the DT. The tank was set up about 3 months ago with dry rock, so no hitchhikers there. The corals were all dipped before going into the tank, some dipped one or two more times since then.
A few weeks ago I added a trochus snail and three red leg hermit crabs.
This morning, I noticed one of the crabs, without it's shell, blowing around on the glass bottom. My intention was to find it's shell, bring it over to it and hopefully he'd climb back in. However, all three shells still had crabs in them. Last I checked, he had scooted back under a rock and I'm guessing/hoping he'll be fine until I get to to an LFS or petco and get some shells later today. The snail is in no danger as it spends 99% of it's time cruising back and forth at the waterline.

I guess my question is, is it possible that one of those shells had two crabs in? From what I'm reading, they really don't reproduce in tanks, so I'm guessing it wasn't that. The only other thing I can think of is that it's a hitchhiker from a coral and possibly not a crab at all...though it does look like what I'd expect it to look like outside of it's shell.

 

[SaltyT]

It sounds like one of your hermit crabs molted. The extra crab is probably just its old exoskeleton.

 

[Joe31415]

It sounds like one of your hermit crabs molted. The extra crab is probably just its old exoskeleton.

That's entirely possible. For one, I figured it was dead and tried to pull it out of the tank but it started moving, however I'm only 50/50 sure it was alive vs dead and just blowing around in the current.
Second, when I checked the other three shells, trying to find the empty one, one of them did see quite a bit redder. Not that I typically go around flipping their shells over, but it looked...newer.
I need to pick up some extra shells anyways since I don't have any spares in the tank, so I'll do that. And I'll keep my eye out for the extra one. If it's dead, I would like to remove it before it causes any issues in such a small tank.

 

[mdpitts]

I have a tank that's used for reefs (and inverts) only, to QT them before going into the DT. The tank was set up about 3 months ago with dry rock, so no hitchhikers there. The corals were all dipped before going into the tank, some dipped one or two more times since then.
A few weeks ago I added a trochus snail and three red leg hermit crabs.
This morning, I noticed one of the crabs, without it's shell, blowing around on the glass bottom. My intention was to find it's shell, bring it over to it and hopefully he'd climb back in. However, all three shells still had crabs in them. Last I checked, he had scooted back under a rock and I'm guessing/hoping he'll be fine until I get to to an LFS or petco and get some shells later today. The snail is in no danger as it spends 99% of it's time cruising back and forth at the waterline.

I guess my question is, is it possible that one of those shells had two crabs in? From what I'm reading, they really don't reproduce in tanks, so I'm guessing it wasn't that. The only other thing I can think of is that it's a hitchhiker from a coral and possibly not a crab at all...though it does look like what I'd expect it to look like outside of it's shell.

I found this by searching for the same thing. I bought one blue leg hermit crab and just found another one without a shell wedged up under some rock. I put some empty shells near it and if they are all empty in the morning and it’s still in the same spot I’ll mess with it and see if it’s a live one or a molt.

The weird thing is that there is still something in the old shell because when I went to pull it out of the tank the shell twitched and tried to move away. I can’t see the opening because it’s tucked into some rock.