Monday, February 18, 2008

Conundrum at the Beach: the unliving undead.

Sally Lightfoot, Grapsus grapsus (sort of).

Child: “Is it alive?”

Adult: “No, it's…”

Child: “It’s dead?”

Adult: “No, it’s…”

It’s incomprehensible. An exuvium, the shed exoskeleton of an arthropod, in this case a Sally Lightfoot crab.

“... a molt, a shed skin, a… To grow, they shed their skeleton. But it’s on the outside, not inside like your bones.”

“But it looks alive.”

"Because they do a fine job of leaving their old body without breaking it. They open a suture, the pleural suture at the back between the carapace and abdomen, and pop out backwards. See, here." The adult picks up the exuvium and lifts the top of the carapace, revealing the branchial chambers above the legs. Even the chitinous gill covers remain inside. It isn’t as empty as one would hope for an unambiguous explanation. Besides, how can you leave your body behind and still be a crab?

Adult: “Here’s how you know if a crab is alive. If it is, it runs away, or swims away, or reaches up and pinches you, depending on what kind of crab it is. This one, the Sally Lightfoot, is a run-away kind. If it were alive it would have disappeared before we even saw it. Because it's still here in the open, we know it's a molt.”

Child: “But if it was dead it wouldn’t run away either.”

“No, but it would smell bad.” Holds exuvium to child’s nose. “Smell bad?”

“It doesn’t smell good.”

“It would smell a lot worse.”

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