Thursday, March 04, 2010

True or false?

I've been  pondering crabs, hermit and "normal". In some ways, the hermits are built like shrimps, with a curling tail end, and a cluster of appendages behind long antennae. But then again, they're crabs, with a carapace over a head/thorax combo, and big pincers.

How does the taxonomy go?

First, they're arthropods, in the same phylum as the insects: they have a segmented body, paired appendages, an exoskeleton, and they molt.

Next level down, cutting out the insects, Subphylum Crustacea. Antennae, gills, five segments, five pairs of appendages. A carapace. This covers a raft of critters, from barnacles to lobsters, some 42,000 species.

Hermit carapace.

Below Subphylum is Class; now we're in Malacostraca.  Head, thorax, abdomen; appendages everywhere. 18,000 species. Crabs, lobsters, shrimps, isopods, krill (what whales eat). Hermits fitting in nicely.

Superorder Eucarida. Head and thorax fused into the cephalothorax. Eyes on stalks.

Order Decapoda, which means "Ten feet". These include the pincers (chelipeds); besides that, there are the two stalked eyes, two pairs of antennae (yes, the crabs have them, too, but they're small.), and three sets of mouthparts. We're down to 10,000 species. Still the shrimps, lobsters, crabs.

And here's where the split comes, at the Superfamily level. There are "true" crabs, the Brachyura, and "false" crabs, the Paguroidea or hermits, plus a few others. The "true"crabs have a short, flat body, with the abdomen reduced to a flap folded over the reproductive organs. There are about 4,500 species.

The Paguroidea make up another 500 species. These are the "false" crabs.* Maybe I'd prefer to call them the "true"hermits; they're at least as honest as the flatties. Except for their habit of stealing other critters' housing.

Crab carapace. Dungeness crab, Cancer magister. Discarded home, but useless for hermits.

Ok, I think I've got that sorted out. I'll go on to more crab stuff tomorrow.

*Update; Tim notes, in the comments, that the hermits have only 6 walking legs. They're still decapods ("ten feet"), but the last two pair are reduced to hooks that hold the shell they borrow. So it's: first pair, pincers (chelipeds); second, regular leg with claw; third, no claw; fourth and fifth, shell hooks. In "true" crabs, it's: first pair, pincers; second to fourth, walking legs; fifth leg, swimming leg.


  1. The other famous example of a false crab are the coconut crabs.

    The other main distinguishing feature between the true and false is that the false (including hermits) actually only have six walking legs.

  2. Thanks. Yes, the last two pairs, at least in hermits, are reduced to hooks to hold the borrowed shell. Maybe I should have noted that in the post.

  3. Really cool photo of the hermit carapace's rare to see those intact.


If your comment is on a post older than a week, it will be held for moderation. Sorry about that, but spammers seem to love old posts!

Also, I have word verification on, because I found out that not only do I get spam without it, but it gets passed on to anyone commenting in that thread. Not cool!