Crabby Monday - Bone Crushers

Welcome to the second Tides and Trails installment of Crabby Monday!

Pacific shores are stalked by the Pacific Rock Crab, Cancer antennarius, pictured here, and by its close relative, the Red Rock Crab, Cancer productus. Both species are denizens of the intertidal and shallow subtidal zones. Like many crab species, rock crabs are opportunistic and eat dead material along with a wide range of live animals: mussels, barnacles, snails, other crabs.

The robust pinchers of rock crabs tear through neoprene gloves and rip through human skin—it’s even said they can break a human finger. I haven’t met anyone who’s had a digit cracked by a crab, but I keep the power of their pinchers in mind when I rake my hands through kelp and probe rock crevices with my fingers while searching for tidepool creatures.

When the tide is out, rock crabs burrow in sand near seaweed and rocks to hide from seagulls. A rock crab’s bone-crushing power offers little protection from agile gulls that dodge a crab’s flailing pincers. When seagulls find a crab, they flip it onto its back and pierce its belly armor with quick jabs of their bills. A crab that a few minutes earlier may have cracked open a snail shell to eat the living creature within is in turn devoured alive by gulls. Maybe this is where the term "crabby" originated?

Though adult rock crabs are a uniform reddish brown, juveniles display a staggering variety of colors and patterns. The reason for this color polymorphism of juvenile rock crabs is an open question—one of many mysteries marine biologists are trying to solve. By tackling specific problems like why color polymorphism exists in juvenile rock crabs, science moves toward answering some of the largest questions of all: How did life arise and evolve in the oceans, and which forms of life will survive in the world’s changing seas?