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Several locals have reported “unusual amounts of dead horseshoe crabs” washing up on Massachusetts beaches in recent days.
However, officials say the spectacle is no cause for concern, as they’re just molted crab shells – not dead crabs.
Hundreds of these horseshoe crab exoskeletons have been spotted along the shores of Falmouth, Weymouth, and Nahant, among other Bay State beaches, according to the Massachusetts Division of Marine Fisheries.
It’s a sign that the crabs are molting and shedding their outer shells, which is typical during late summer and early fall, officials said.
“They’re a little bit offshore, buried down, hunkered down, molting, and then those old shells, the discarded ones, come in with the tides,” said Dr. Kathy Tuxbury, senior veterinarian at the New England Aquarium.
Horseshoe crab exoskeletons washed up on Massachusetts beach.
Why do horseshoe crabs molt and exit their old shells?
Atlantic horseshoe crabs molt multiple times a year throughout the first three years of their lives, and then once a year until they reach maturity at around 9 to 11 years old, the Massachusetts Division of Marine Fisheries said.
They shed their hard outer shells to have room to grow, officials said.
“They’re growing their new shell while they’re getting rid of the old one,” Tuxbury said. “And they’re a little bit soft when they’re done molting, so they have a period where they need to be careful from predators and damage and whatnot.”
The end of August and early September is the peak of horseshoe crab molting season, and officials expect more exoskeletons to pile up on Massachusetts beaches in the next few weeks.
“It is that time of year. I spent a lot of time on Cape Cod, in Wellfleet, and there are tons of exoskeletons in September – a really common time – even into October,” Tuxbury said.
Horseshoe crabs typically go through 16 to 17 molts throughout their lives, according to the National Wildlife Federation. They can live for more than 20 years.
A horseshoe tossed up onto the wrack line along Sandy Neck Beach along Barnstable Harbor. The ancient creatures date back 445 million years ago. Photo taken on August 17, 2025
How to tell the difference between a dead crab and an exoskeleton
You can distinguish between a dead crab and an exoskeleton by looking for a slit along the front rim of the shell, officials said.
That’s where a crab would have split its exoskeleton before crawling out.
A deceased crab would also weigh more than a hollow exoskeleton, Tuxbury noted.
A horseshoe crab exoskeleton on a Massachusetts beach.
This article originally appeared on Cape Cod Times: Horseshoe crab shells are washing up on MA beaches. Here’s why
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