With reports from people of “hundreds of dead baby crabs” strewn on beaches from B.C. to California, you’d think there was a crab-zombie apocalypse going on. But, like most things, there is more to the story once you delve further…

After recounting the details to Aquarium researcher Donna Gibbs, she didn’t hesitate to say that what people are finding are probably crab moults, not dead crabs. Moults are the discarded exoskeletons of crabs, and moulting is just a normal part of a crab’s growth process.

Here’s a brief rundown of crab moulting with help from Gregory C. Jensen’s new book Crabs and Shrimps of the Pacific Coast:

thumbnail 1 IMG_7527The rear edge of the crab, where the carapace (top shell) meets the abdomen splits, and the crab starts to back out (can take minutes to hours).thumbnail 2 IMG_7532

 

The unattached carapace gives the crab more room to back out (right).

 

thumbnail 3 IMG_7536The crab continues to pull itself out. Its new exoskeleton is soft, which makes it vulnerable to predators for the time being. At this time, the crab takes on seawater, which swells and enlarges the new exoskeleton (takes about three months to completely harden). It leaves behind an exact replica of itself, right down to the surface of its eyes and gills.

Here are some more pics of this fascinating natural phenomenon.

To tell a crab moult apart from a dead crab, just feel and take a whiff – a dead crab will be heavy and will really stink. If it’s a moult, you can easily lift the crab’s carapace up from the back and it will smell like fresh seafood.

As for the proliferation of crab moults, adult crabs tend to moult all at once – females in the spring and males in the late summer. Although it’s not unusual to see more moults this time of year, Donna says in her 30 years of monitoring Howe Sound she has seen a noticeable increase of crabs in the last two months. The reason is not clear, but it could be a mass migration of sorts.

In short, we’re happy to report that this isn’t a case of a crab-zombie apocalypse. Well, this time around…

Written by Karen Horak, writer-editor, content and digital experience at the Vancouver Aquarium.

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