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Thin Shelled Rock Crab, A'ama, Natal Lightfoot Crab

Grapsus tenuicrustatus


When living, the Thin Shelled Rock Crab is a deep purple, almost black, with delicate lacy white markings all over its carapace and legs. When it dies or sheds its shell to grow larger, the unoccupied shell turns bright red on exposure to the sun. Either way, it's a distinctive crab about which there is almost NOTHING on the web. A'ama is the Hawaiian name for this crab.


This crab is a denizen of rock jumbles at the edge of the surf. It scrambles around seeking food and hiding in crevices at the approach of danger. You can hear them clicking and creaking as they jam themselves into hiding places. On Kauai they were the most common crab, both on the rocks and as cast-off crab shells along the beach.


I combined three spottings in the photos above to include a pair of live crabs, a washed up newly dead one, and a sun-reddened cast-off shell.

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Scott Frazier
Scott Frazier 6 years ago

You're welcome, Irene!

Irene Brady
Irene Brady 6 years ago

oops! I know better than that! Thanks, Scott.

Scott Frazier
Scott Frazier 6 years ago

Moved to the arthropods category :-)

Irene Brady
Spotted by
Irene Brady

Hawaii, USA

Spotted on May 6, 2011
Submitted on Apr 6, 2016


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