The Helmet Crab has an almost pentagonal-shaped body. The carapace, claws and legs are covered with stiff bristles. The lateral margin of the carapace has six large, jagged points on each side. The crab is predominantly yellow-green with darker claws.
Helmet Crabs eat the softer, sweeter portions of marine eelgrass. They also scavenge for dead invertebrates and fish, or will attack and eat other crabs. Herring Gulls will hunt Helmet crabs at low tide by wading through the eelgrass. When a seabird finds a Helmet Crab, the bird flips it over, cracks the crab’s shell with its beak, and eats the insides. Helmet Crabs are also hunted by bottom fishes and the Giant Pacific Octopus.
Helmet Crabs live on rocky and sandy ocean bottoms up to 130 feet (39 m) deep. They prefer sub tidal areas in or near dense marine eelgrass beds, but have also been found in heavy algal coverings in rocky areas. They range from Norton Sound, Alaska to Monterey, California, but are rare south of Puget Sound. On the western side of the Pacific Ocean, they can be found from Siberia to Japan.