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Environment

How Many Horseshoe Crabs Are In Florida? Growing Volunteer Effort Aims To Find Out

horseshoe crabs in grass
Horseshoe crabs -- one of them tagged -- on display at a volunteer training session at Historic Spanish Point in Osprey, Florida in September 2019. Kerry Sheridan/WUSF

With a curved carapace, angry-looking eyes and a spiky tail, horseshoe crabs look like nature’s armored tanks, crawling in the sand at the shore.

They’ve been around for 450 million years, but just how they’re doing in Florida is a mystery, especially after last year’s devastating red tide event.

Now, there’s a growing, volunteer-led effort under way to tag and count Florida’s horseshoe crabs for the first time. It began in 2015 in Levy County, and has grown to include 16 counties, mainly in north and central Florida. Sarasota just joined.

Armando Ubeda and Lisa Taylor
Armando Ubeda, a marine biologist with Florida Sea Grant, and volunteer Lisa Taylor get ready to count horseshoe crabs. Kerry Sheridan/WUSF

Volunteer Linda Waugh said although she’s “a little creeped out” by horseshoe crabs, she wants to help because she’s curious about how red tide may have affected them.  

Researchers, too, want to know if horseshoe crab numbers are declining, like they are in some northeastern states. Having a better idea of population trends will help wildlife managers know how to protect them.

“In Florida, we don't know how the population is doing, which is really why we're here today,” said Berlynna Heres, Statewide Florida Horseshoe Crab Watch Coordinator for the Florida Fish and Wildlife Conservation Commission, during a volunteer training session in September in Osprey.

“Our program is volunteer-based, because there's just no funding otherwise. And so what you're doing is solving a huge problem. Your willingness to help is so incredibly valuable.”

About 20 volunteers came to the three-hour session at Historic Spanish Point, led by Heres and marine biologist Armando Ubeda with the Florida Sea Grant.

This is the only effort of its kind in the nation that allows regular people, after three hours of training, to capture, tag and release a live animal.

Fish and Wildlife tags in a row
Horseshoe crab tags will help scientists track their movement and population over time. Larger tags are for females, which are bigger than males. Kerry Sheridan/WUSF

First, volunteers learned the basics of horseshoe crab biology. (Fun fact: horseshoe crabs are not crabs at all — they are actually more closely related to spiders and scorpions.)

Handout shows horseshoe crab anatomy
Volunteers learn about the basic biology of horseshoe crabs. Kerry Sheridan/WUSF

Horseshoe crabs are also crucial to human survival. Their unusual blue blood is used to test for bacteria in vaccines, injectable medicines and implantable medical devices. So anyone who has had a shot in the last several decades has been protected by a horseshoe crab.

Harvesting horseshoe crab blood is a billion dollar business, but horseshoe crabs in Florida are not taken for this purpose. Heres said there are no plans for a biomedical industry in Florida, at least not yet.  

Volunteers learn to safely lift the animals by holding the edges of their carapace (or shell), never the tail.

In preparation for tagging out by the water, volunteers practice using an awl to make a hole in cardboard. When it comes to the actual animal, this hole will be made in the lower left side of the horseshoe crab’s shell.

man holds awl and horseshoe crab
Marine biologist Armando Ubeda shows how to make a hole in the carapace, so a tag can be inserted. Kerry Sheridan/WUSF

Once the hole is made, volunteers learn to insert a numbered tag, which will stay on the animal and allow scientists to track its movements, and eventually project the local population numbers.

Male horseshoe crab with tag
The first tagged male horseshoe crab in Sarasota. He gets released in the water, moments later, and is spotted the next day, still trying to mate. Kerry Sheridan/WUSF

Marine biologists have identified two general times of year – fall and spring around peak high tide times -- when horseshoe crabs are expected to come to shore in the largest numbers to mate.

three people look at a cluster of horseshoe crabs
Volunteers check out horseshoe crabs on the shore near the Indian Beach area of Sarasota. Kerry Sheridan/WUSF

Males hook themselves onto the females as they swim to shore. The females deposit tens of thousands of eggs on the beach, and the males fertilize the eggs. Most of these eggs serve as a vital food source for seabirds and turtles, but some do make it to adulthood.

In late September, Ubeda led the first tagging trip with three volunteers at a site near New College, in north Sarasota.

three people walk in water
Armando Ubeda with Florida Sea Grant leads volunteers into waist-deep water to look for horseshoe crabs

Twenty-three mating pairs were spotted here in the spring of 2018 near a seawall. That was before red tide.

On this outing, volunteers wade waist-deep into water near the seawall, and see two, but only capture one for tagging.

"We actually saw two... but during the survey you can only count them during the one pass-through," said volunteer Lisa Taylor.

"The second one we were unable to count. He got away from us!"

According to Ubeda, it's too early to know the true effects of red tide on these normally hardy creatures.

“We're trying to figure out what's going on. We believe that red tide had a big impact on the numbers. They went down and hopefully will soon come back," he said.

A map of Florida counties where volunteer programs count horseshoe crabs. Credit: FWC
A map of Florida counties where volunteer programs count horseshoe crabs. Credit: FWC

“I think that's why it's even more critical now to do this after the big event, just to see if they can actually rebound."

A day later, at another beach site nearby, they see many more, and tag 20. They also spot the sole male they tagged on the first day. It’s a good sign, suggesting he’s healthy and still trying to mate.

Heres said the long-term plan is for all coastal counties in Florida to get involved in the horseshoe crab count. It’s an effort that could take many years.

four horseshoe crabs in sand
One female and three male horseshoe crabs mate on a beach in Sarasota. Kerry Sheridan/WUSF

Another training session for volunteers in the Sarasota area is expected to take place in the spring.