Friday, May 22, 2015


"All bark and no bite," I said, about the black-clawed crabs in my tank, because they'll threaten me with amputation, but then sit meekly in my hand, all threats forgotten.

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.


  1. Oh not the nudibranch

  2. As voracious as they are, I'm surprised they're not more successful than they are at the beach. I've seen thousands of green and purple shore crabs, but have never knowingly seen one of these. I attended a Nature Vancouver talk last year on green shore crabs, and how they start off looking all mottled (a few of your previous posts have pointed out how perfectly it helps them blend with their background), but as they grow larger, only the drab ones seem to be found. When studied in the lab, it turned out that they don't lose their peppered colouring as they age - in fact, they keep it their whole life. But while as small crabs, a mottled colouring helps camouflage, the peppering actually makes larger crabs stand out, such that they are picked off by predators. I wonder if that is at play with these larger black-clawed crabs.

  3. Interesting, Tim! It could be; their big pincers, waving about, definitely would attract attention.

    They're not social crabs, like the green shore crabs, or the hermits; each one, in my tank, found his own shelter, well hidden from the other two, and ventured out into the open only to grab food and retreat to his cave again.

    I delivered them to the beach today, and watched as each hid under a different stone. When I came back to shore an hour later, I looked for them there, but couldn't find them.

  4. I hope to be able to attend those Nature Vancouver talks, when they start again in September. Last year was just too difficult to make it.

  5. Upupaepops; I know. I was mad.

  6. On top in the tank, I wonder where they will fit into the pecking order when they return to the ocean. - Margy

  7. Margy, At least they have a head start; well fed from babycrabhood. But now they'll have to deal with gulls and fish.


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