I was wrong. They're bullies, only attacking smaller and weaker neighbours. Not only snails, proper crab food, but other crabs, hermits, and, I think, bubble shells and polychaetes.
|Ready for action.|
The aquarium has been a good place for them; in five months, the big male has grown from under 1/4 inch across the carapace to somewhat over an inch, with extended pincers twice that again. I've provided them with clean, cold water, gourmet crab dinners, a handy rock pile to live under; there was no need to beat up on the other residents.
But. Their neighbours included two shore crabs, male and female, grown up in the tank. The female, a large adult, disappeared. I didn't even find her remains. A few weeks later, the smaller male was gone, too.
I kept finding maimed and dying hermits. And the bubble shells that had lived all winter in the tank couldn't be found. The nudibranch got eaten. Even a big polychaete worm, one of the ones that can grow up to a foot long, even in a small tank, just wasn't there any more. And now there were only two black-clawed crabs; the female had seemingly evaporated.
But everything seemed right; I checked temperatures, salinity, acidity, current, filtering; all normal. So a couple of weeks ago, I brought home another three shore crabs to liven up the tank with their antics.
Two days ago, the largest (still smaller than the large black-clawed crab) turned up with three legs and a pincer gone on one side. Who would do this to him? Only another crab, and suspicion centered on that big black-clawed male, sitting so peacefully under his stone. While I pondered this, a second shore crab lost two legs.
So tonight, I've hunted down and arrested both black-clawed crabs. They're in solitary confinement, for now; the first trip to the beach, they'll go with me. And won't come home.
|Small black-clawed crab, 1/2 inch across the carapace.|
Now that I've got them out where I can watch them, I see that the small crab is terrified of the large one, and keeps a good distance away. He probably has good reasons.
The two traumatized shore crabs got extra goodies, in a bowl away from competition, and seem to be recovering.