Crab molt comparison

Crabs are invertebrates that have a hard outer shell, or exoskeleton. This protective body armor is rigid and has flexible joints but does not get larger as the animal grows. So what does a crab do?

In order to grow it must get out of the old shell. Molting is a process where animals shed a part of their body and grow new ones. Birds molt old feathers and new ones grow in to replace them, snakes shed their skin, and elephant seals molt a layer of skin and hair. Crabs and lobsters shed the old and grow a new shell.

Crabs emerge from the exoskeleton covered with a new soft shell. This shell will stretch when the crab pumps seawater to expand to a larger size. Because it will take a few days for it to harden, they’ll need to find a safe place to hide.

Dungeness crab under water. Wikimedia commons.

Finding mole crab molts washed up on the beach is not uncommon. Normally I might just notice and keep walking. But on this day I decided to record it in my nature journal.

As I walked along the beach I noticed lots of Pacific mole crab (Emerita analoga) shells which are pretty common on this sandy beach. But as I looked more carefully, I noticed that there were other species- some spiny molecrabs (Blephariopoda occidentalis) mixed in, as well as small cancer crabs.

Crab molts comparison page

My mind became curious and I noted the approximate proportion of each type of crab in my findings, estimating about 85% Emerita. I measured the shells and recorded the measurements, and jotted down my questions as they popped into my mind.

What common things do you have in your nearby nature that you can stop to look more closely today?



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