NEWPORT – Crab shells, hundreds of them, are washing up on Oregon beaches, but don’t worry – the crabs are only molting.
Adult crab populations tend to molt simultaneously: females in the spring, males in the late summer. Crabs need to break out of their old shell to grow, just like a child needs a new pair of shoes when they outgrow last year’s pair.
When preparing to molt, a crab’s old exoskeleton separates from the new one beneath. At this time the new exoskeleton absorbs water and become larger. This causes a split at the molt line.
The new and extremely soft crab now has the flexibility to back out of its old shell. During this incredible feat, crabs are extremely vulnerable to predators and for that reason it’s done quickly. Soon after the live crab has exited they bury themselves in sand to allow their new shell to harden.
Observant beachgoers are familiar with crab molts. You can tell the difference between a dead crab and the shell left behind from molting by checking the molt line. Since, the crab molts are is whole – the crab leaves even its old gills, antennae, and mouthparts behind – this determination usually requires a good look.
In summer months, when Dungeness crab molting activity peaks, the Oregon Department of Fish and Wildlife usually receives calls reporting “many dead crab on the beach,” which are almost always these molts or “exuvia.”
For more information and to learn how you can tell the difference between a molt and the shell of a dead crab.