Adults are quite variable in colour; some are muted brownish-red, some mottled or spotted brown, pink, or yellow.

[citation needed] This crab lives among the rocks at the often turbulent, windy shore, just above the limit of the sea spray.

[7][8] G. grapsus has been observed in an apparent cleaning symbiosis taking ticks from marine iguanas on the Galápagos Islands.

Steinbeck records:[11] These little crabs, with brilliant cloisonné carapaces, walk on their tiptoes, They have remarkable eyes and an extremely fast reaction time.

In spite of the fact that they swarm on the rocks at the Cape [San Lucas], and to a less degree inside the Gulf [of California], they are exceedingly hard to catch.