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Ecological Disaster? Nope, Just Oregon Coast Crabs Mass Molting

Published 08/01/2018 at 12:01 AM PDT
By Oregon Coast Beach Connection Staff

Ecological Disaster? Nope, Just Oregon Coast Crabs Mass Molting

(Oregon Coast) – Sometimes it happens and it seems almost apocalyptic. Crab shells abound on the beaches, in such great numbers it's almost as far as the eye can see. What appears to be a mass die-off of crabs on the Oregon coast leaves endless shells on the beaches, looking as if something has gone very, very wrong. (Photo above by Tiffany Boothe, Seaside Aquarium).

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This winds up being one of the most frequently asked questions of officials and experts on the Oregon coast. It is, in fact, just a simple thing that nature does on occasion.

If you're walking along the Oregon coast in the late summer or in the spring your chances of spotting this spectacle are pretty good. You may indeed encounter it at any minute this time of year. But it's nothing close to a mass die-off of dead crabs at all. It's just that crab tend to molt all at the same time.

It's not just any crab, either. It's the famed and delicious Dungeness crab.

The females molt in the spring and the males do this in late summer. This tends to happen all at once, creating masses of crab shells littered on your favorite Oregon coast beach.

Scientists call it crab exuvia.

“It can look like a major disaster occurred to the crab population, but it’s as natural as kids outgrowing their old shoes,” ODFW said.

You can tell the difference between a shell from a molt and a dead crab by looking at the molt line.

Parts of this, by the ODFW's descriptions, sounds like something out of an alien movie, or a flick about zombies. Crabs sometimes molt by losing a lot of their bodies – old gills, antennae and even parts of the mouth. This larger kind of molt is called a “whole exuvia.”

“Every detail is there except the crab,” ODFW said. “In summer months, when Dungeness crab molting activity peaks, ODFW usually receives calls reporting 'many dead crab on the beach' which are almost always these molts.”

In their first two years of life, Dungeness crab molt up to six times a year. After that, the process slows down as they reach maturity and then the molting hits about once a year.

As the crab begins the molting process, the old exoskeleton starts to to separate from the new one beneath it. The new one starts absorbing water and becomes larger, which causes a kind of split at their molt line. The softer shell beneath gives the crab the flexibility to wiggle out of the old one and it eventually squeezes out backwards.

This softness, however, results in the crab being very vulnerable to predators. It hides in the sand for a while, waiting for the new shell to harden.

Ocean currents wash the crab shells onshore, leaving this wild sight.

By early fall, the crabs have long finished their molting process and begin to fatten up their meat, which is why often December is when crabbing yields the fattest, juiciest catches of crab.

Weird things can happen to crabs when molting, said Tiffany Boothe of Seaside Aquarium. She encountered a freak crab once with extra pinchers. (Photo above by Boothe).

"Sometimes a crab is injured while molting," she said. "This can cause strange deformities in the crab's shell, such as in this crab. Notice the crab's double pinchers on its right side." Oregon Coast Lodgings for this event - Where to eat - Map and Virtual Tour

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More Crab Molt Photos Below, courtesy Seaside Aquarium





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