Celebrate International Horseshoe Crab Day with HCT!

Why change when you’ve got a good thing going? It sure works for horseshoe crabs, the old school arthropod that has looked the same for millions of years. While “living fossil” doesn’t exactly sound like a compliment, it is indeed a testament to an animal that has cruised effortlessly through the eons. Simply put, horseshoe crabs are built to last.

With International Horseshoe Crab Day coming up on June 20, Harwich Conservation Trust (HCT) is delighted to celebrate these beloved beasties.

Naturalist Andrea Higgins will lead a Horseshoe Crab Walk on Wednesday, June 18, 2025 from noon to 1 p.m. The walk is suitable for all ages and participants will enjoy a beach stroll and a horseshoe crab search along the shores of Pleasant Bay. For more information about all of our great guided nature programs and to register for the Horseshoe Crab Walk, click here.

“I admire their resilience,” said Higgins. “It’s almost like they were perfect from the beginning of time. We can find horseshoe crabs in Cape Cod Bay, Pleasant Bay or Nantucket Sound that look almost the same as fossils that researchers are finding that date back 300 million years or so.”

Higgins is looking forward to the guided program and plans to share lots of cool crab info. “Walking the beach is always a beautiful adventure, and by the time we’re finished, all my participants will be wizards at telling the difference between male and female horseshoe crabs,” she said.

A deeper dive into horseshoe crabs

For visitors to Cape Cod who might not spend much time along the shore, there can be a “what the heck is that?” moment when they first encounter a horseshoe crab. And who can blame them? Suddenly they’re looking at a time-machine tank with a tail, and it’s alive!

But affection grows quickly for these helmet-shaped inhabitants of the sea. Especially when you realize they are harmless to humans. Horseshoe crabs don’t bite or sting, and that scary-looking tail isn’t a weapon: it serves as a rudder when the crabs are swimming, and can help right the crab if it gets tipped over. By the way, the tail is called a “telson,” and you should never pick up a crab by the tail as it can harm the animal.

Horseshoe crabs aren’t picky eaters, according to the Florida Fish and Wildlife Conservation Commission, which describes their diet this way: “They eat almost anything. They feed on small clams, crustaceans, and worms; however, they will also eat other animals and even algae. Because they have no mandibles or teeth, they crush hard food between their legs before passing it to their mouth. Like birds, horseshoe crabs also have gizzards for grinding food before it reaches their stomachs.”

Spring is spawning time

We are currently in the midst of peak horseshoe crab spawning season, which runs through May and June. That’s when you’ll see female crabs come ashore to lay eggs, as well as interested males, which might only be a third of the size of the females. A bit of a scrum might occur as males crowd around the female, seeking to attach themselves with modified claws and fertilize the eggs.

From late summer into early fall, folks might encounter numerous horseshoe crab shells on the beach, particularly along the south-facing shores of the Cape. These shells are evidence of molting, which is how horseshoe crabs grow.

According to the state Division of Marine Fisheries (DMF), “When a crab is ready to molt, the rim of the shell will split and the crab will pull itself out of this opening, leaving a perfectly intact shell of its former self behind. The crab’s new shell, which had been forming underneath the old shell, is soft at first. The new shell will then swell by taking in water before the new, larger shell begins to harden.”

Crabs molt multiple times per year over their first 3 years of life, according to DMF, then molt once per year until they reach sexual maturity (around 9 to 11 years). After that, they stop molting.

Horseshoe crab blood helps humans

An extract made from horseshoe crab blood is used by the biomedical industry to ensure that harmful bacteria is not present in drugs before they are released for human use.

The International Union for Conservation of Nature and Natural Resources (IUCN) has listed the American horseshoe crab as “vulnerable to extinction,” citing reasons that include “overharvesting for use as food, bait and biomedical testing, and because of habitat loss.”

In 2020, IUCN established June 20 as International Horseshoe Crab Day as “a celebration of the flagship species for coastal habitat conservation.”

For folks looking to see spawning horseshoe crabs, DMF suggests visiting these Cape Cod beaches within an hour of high tide: Stage Harbor (Chatham), Ridgevale Beach (Chatham), West Dennis Beach (Dennis) and Millway Beach, (Barnstable). But DMF also reports that you “can see horseshoe crabs at many other locations with sandy, protected beaches (sheltered from wind and waves).”

Or, you can sign up for HCT’s awesome Horseshoe Crab Walk on June 18. It will be a crabby blast!

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