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How to Measure the Carapaces of Mole Crabs

The tidal regions of the world are a hybrid ecosystem different from both dry land and the open water, with a number of species who live their lives only in the region between the high and low tide marks. One prolific species found in North America is a tiny crustacean called the sand crab or mole crab, because it burrows into the sand in the same way a mole burrows into garden soil. It is widely studied, in part because it is significant in the tidal food chain, and in part because specimens are easily found and measured.

Things You'll Need

Trowel
Small shovel
Calipers
Notebook

Instructions

- 1
  Dig your crabs in the tidal zone with a trowel or small shovel, and place them in a bucket filled with sea water from the same beach until you have collected as many specimens as you want.
- 2
  Remove each specimen individually from the bucket, and open your calipers.
- 3
  Adjust the calipers until they just span the carapace from front to back, taking care not to injure the crab by crushing its antenna in the calipers.
- 4
  Read and record the measurement, in millimeters. Repeat for all remaining crabs.
- 5
  Release the crabs at the waterline, where the waves are washing onto the beach. Remain nearby while three or four waves pass; this will deter predators as the crabs dig themselves back into the safety of the sand.

Animals & Science