Grrrr! Crabs!

[fcbayern]

So yesterday I bought home some Xenia and while it was acclimating I noticed Patrick the sand sifting starfish slowly working his way past a rock. He suddenly jerks under the rock and I could see he was bring dragged in so I grabbed him and wrestled him away from the big black crab that I recently noticed hiding in there which put up quite a fight and was surprisingly strong.

I whipped the rock out and after a battle that ended up with a football sized rock being smashed to pieces I found THREE live crabs.

Worryingly I lost a beautiful juvenile imperator angel a few months ago that I found wedged in a rock in a peculiar way. I had a similarly difficult fight to pull it it of the rock and assumed at the time it had died and been pulled in by a crab or had had a moment of stupidity and committed fishy suicide. Now I have a feeling the crabs have murderous tendencies! Anyone have similar experience? My Dad and I have kept marine for years and always had hitchhiker crabs in tanks without incident. Are there any fish that get rid of them? I seem to be fighting a losing battle.


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[Sniperhank]

Though I can't directly tell what kind of crab that is, I do have an understanding of how crabs work. They are opportunists, but slow and not well equipped hunters. A crab that you will find in your tank isn't going to be able to hunt any healthy fish. When fish get stuck in rocks, there is normally either something wrong before hand or that the rocks shifted and collapsed some as the fish swam through.
Now, in terms of the crab and your starfish...there is a good chance that a 'non-reef safe' crab was starfish hungry. Healthy or sick, a slow moving critter can become a meal if they are hungry enough.
So, what to do? Take a tall glass. Tilt it on the side of a rock. Toss some food in. Check it in the morning. You'll know if there are any of these guys left.
Are all crabs bad for your tank? No. That doesn't mean that I trust them or keep anything besides a couple of small hermits in my reef tank. Even then I end up with a smaller snail population eventually. When opportunity strikes.

 

[fcbayern]

Cheers for the reply. Odd you mention snails, when I set the tank up I had about 15 snails and one really big one and they all disappeared so that might explain that as well!

 

[fcbayern]

I did try the glass thing for a few weeks to catch them but to no avail, got lucky with the first couple one went behind an internal filter and I netted him out the other one inexplicably went out into the open!