Saturday, November 21, 2009

The hidden life of crabs

Crabs are amazing critters! I am continually being surprised by the three (that I know of) in my aquarium. Let me introduce them; there's "Big Green", a green shore crab about an inch across, then "Snowflake", and "Babe".

Babe was a stowaway. The first time I saw her*, she was barely visible, maybe a couple of millimetres wide. Now, a few months later, she is about a quarter inch across the carapace. And she can bench-press dozens of times her own weight.

Babe and clam

In this photo, she had just come out from underneath the clam behind her, lifting the clam out of the sand on her way. There is a larger clam, fatter and heavier; she can pick that one up, too. The other two crabs move rocks  twice their length toe to toe and many times their height, covered with barnacles and seaweed.

And then, there's Snowflake. He's** a white crab, probably a juvenile shore crab. He was about 1 cm. (less than 1/2 inch) across the carapace when I brought him home. One day, he turned up maimed; he must have been fighting with Big Green. He was missing both pinchers and one front leg.

Snowflake's molted seven-legged shell.

Crabs eat with their pincers, delicately picking off fragments of meat or veggies, and bringing them up to their mouths.  They dig with them, through shells and sand, looking for food, or use them to snatch floating foodstuffs out of the current. How was Snowflake to eat without pincers?

Quite well, I found. He used the tiny mouthparts as he would normal pincers. I was surprised to see him walking around carrying chunks of algae with them, and nibbling away. It was slow going, but every time I looked, he was eating, so he was probably getting enough.

Drawing from Museum Victoria.

For grabbing food, and for self-defense, he used his remaining legs. And he kept his distance from Big Green.

Crabs regenerate broken limbs; often we see a crab with one large pincer, and one small one, a replacement still not full grown. Would Snowflake regenerate three all at once? I watched carefully; it looked, last week, as though the stub of her leg was a tiny bit bigger, but that was all. Then this morning ...

Big Green was in a funny mood last night. He was roaming around and around, trying to climb the walls, jumping off high seaweed, burrowing in the sand. This morning, I checked to see how he was doing. He'd been busy; he had torn up my carefully planted seaweed and eelgrass; they were floating loose. Rocks had been rolled away, three big clams dug out of the sand. And the sand had been mounded up on one end of the aquarium, leaving a bare half-inch at the other end.

I was straightening up, carefully so as not to damage tiny residents, when I came across Snowflake's molted shell. (Photos above and below.) I didn't see any sign of him, even in the sand as I raked it into an even field. He turned up, finally, around supper time. And he now has pincers!

Two small pincers. And eight legs, as he should have.

They must have been forming inside the old carapace, and liberated when he crawled out of it. And, I counted the legs; there are eight again!

Discarded shell, showing stubs of leg and pincers.

The pincers are still undersized. I'll be interested in finding out how long they take to grow.

What else I found, doing the clean-up work, I'll leave till tomorrow. And I hope Big Green doesn't go on the rampage again tonight.

*"She", just because I hate calling something with personality "it". If I turn out to be wrong, I'll apologize and switch.
**Snowflake is a male; the abdomen curled under the shell is sharply triangular, and flat. Female crabs have rounder, fatter abdomens.


  1. Brilliant - my favourite post of all I've read today - and I've read a lot! Not sure why, just fascinating I guess.

    We have corn snake that moults every few months - always an interesting time.

  2. Thanks, Mark,

    You made my day!

  3. I've always known them to regenerate old limbs, but had no idea they'd do it so quickly. Thanks for the update!


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