Wednesday, July 21, 2010

Size Matters

One way to group male snow crabs is to use their relative claw size. Male opies can be small-clawed adolescents or large-clawed adults. Their claw size distinction is based on the height of their claw, or chela, compared to the carapace width.

It seems pretty straight forward, but the difference in claw size represents where each male snow crab is in his life cycle. Crustaceans molt/shed their hard exoskeleton in order to grow. Once they shimmy out of their old exoskeleton, they're really soft and vulnerable. But because they're soft, they're able to swell their bodies by taking in lots of water so that once their new shell hardens they're larger.

Snow crabs have a final, or terminal, molt. This means that after the terminal molt, they will no longer grow. When the males terminally molt, their claws become disproportionately larger compared to their bodies' growth. Why the larger claws? It's thought that larger claws (and larger muscles within them) give terminally-molted males a physical advantage when competing for females.

Not all male snow crabs terminally molt at the same time; some may skip a molt for a year or "choose" to stay adolescent for one more molt (it's not really well understood). Because of this, adult males come in a wide range of carapace widths. In order to determine which males are adolescent versus adult, I have to use a logarithmic discriminate function, which I won't get into too much, but I think it sounds impressive. It goes something like this:

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