Mating Season in Long Island Sound Is Prime Time for Horseshoe Crab Researchers

Jun 17, 2014

Horseshoe crab population numbers have declined, in part because fishermen harvest the animals to use as bait.

It’s mating season for Long Island Sound’s horseshoe crabs. Every year, a group of biologists from Sacred Heart University scour Connecticut’s beaches to track and tag these ancient creatures. I met up with one group in Milford, under a full moon at midnight, to learn more.

Here’s a quick primer on basic horseshoe crab biology. The animals are more closely related to spiders and scorpions than they are to crabs. They have ten eyes. Their tail is just a tail — it’s not a stinger — and they’ve been around for about 450 million years.

Then there’s the blood, which "is actually a bluish color because it has a copper base to it, rather than iron,” said Jennifer Mattei, co-director of Project Limulus, a program out of Sacred Heart University that studies horseshoes.

"Another thing that people don’t know," Mattei said, "is that they can thank their good health to the horseshoe crab." That’s because their blood is used to test vaccine purity. It coagulates when encountering bacteria. So if you got a flu shot last year, federal regulations required that it was tested using horseshoe blood. 

"Actually horseshoe crab blood has been put up on the space station in a little kit that the astronauts use. If they have a sore throat, they can swab this little machine to tell them if they have a bacterial infection or a viral infection," Mattei said.

Over 16 years, Project Limulus has tagged more than 88,000 crabs in Long Island Sound — largely with the help of citizen volunteers.

A group of horseshoe crabs washed up on the beach.
Credit U.S. Fish and Wildlife Service
"The male fertilizes those eggs externally, probably through a cloud of sperm."
Jane Brockmann

When we met up last Friday, there were graduate students and undergraduates. Curious children and professors, from all over the country. It was high tide, and since spawning normally coincides with high tide and a full moon, that’s why we were out so late.

Jane Brockmann, professor emeritus at the University of Florida, works in the northern Gulf of Mexico, and was one of the professors searching for horseshoe crabs. She said female horseshoes lay about 60,000 eggs over the course of three days.

Pairs of horseshoe crabs can stay together for months. Brockmann explained the males use their pincers to attach to the larger female’s shell (see below). The female carries the male around until she’s ready to lay her eggs in the sand, “and the male fertilizes those eggs externally, probably through a cloud of sperm,” she said.

A mating pair of horseshoe crabs. The male attaches in the back and externally fertilizes the eggs laid by the female. Mated pairs can stay together for weeks.
Credit U.S. Fish and Wildlife Service

Researchers tagged about 20 crabs and spotted around a dozen more. They recorded the width of the animal’s shell, the shell’s condition, and the gender.

Jennifer Mattei said keeping track of all this data is important. Over the last 20 years, she said she’s seen Long Island Sound population numbers decline, in part because fishermen harvest the animals to use as bait.

A horseshoe crab fossil.
Credit Carbon NYC / Flickr Creative Commons

Mattei said horseshoes are resilient. "They’re called living fossils," she said. "They’re very generalist in terms of their needs and environmental behaviors… They’ve survived five mass extinctions, and we’re trying to make sure they survive us."

In 2006, Connecticut set up three "no-harvest zones" for horseshoes. The animals take about ten to 12 years to sexually mature, so it’s likely the scientists at Project Limulus won’t see whether that’s helping to boost population numbers until the end of this decade.