At first glance, sharks and horseshoe crabs have almost nothing in common. Horseshoe crabs – actually more closely related to spiders than crabs – don’t even have teeth and certainly aren’t the top predators of the ocean.
On the other hand, both sharks and horseshoe crabs are considered “dinosaurs of the deep” because they evolved about 400 million years ago, even before dinosaurs roamed the earth. Both species also are critically important to human health, and multiple lines of research are being conducted on their unique blood, eyes, immune systems and ability to regrow body parts.
The most frightening similarity for both species is the fact that they have few known predators – except for human beings.
But while sharks are among the most-researched animals in the world, much less is known about horseshoe crabs, particularly population trends that could predict problems for an animal that plays a key role in many of Florida’s ecosystems.
“Looking for a few good kids”
A native of inland Connecticut, Ed Steponaitis first became involved with horseshoe crabs while taking a Florida Master Naturalist course, then discovered more at a two-day course in Wellesley, MA. The more he learned, the more concerned he became about the limited data available on local horseshoe crabs.
Horseshoe crab populations are concentrated in the northeast United States where they’ve been harvested for fertilizer, farm animal food, bait for crab traps and their unique “blue blood.” Most of those states have formal monitoring programs in place, he said, and many have implemented strict regulations on harvesting.
The Florida Fish and Wildlife Research Institute, however, relies on volunteers to track horseshoe crab sightings, and doesn’t have enough data to create models that could predict population trends. The problem, says Ryan Gandy, FWRI’s crustacean research biologist, is that there is no clear financial benefit to a commercial fishery or significant interest from recreational anglers in Florida.
“With a 2,000-mile coastline, it’s a very expensive process,” Gandy said. “Monitoring stone crabs on the West Coast costs $375,000 per year, and we’d need to double that for horseshoe crabs because they live on both coasts.”
That’s where Steponaitis comes in. He’s teamed up with Gandy, Tiffany Black, who runs the state’s horseshoe crab database from Cedar Key, and Randy Runnels, manager of the Tampa Bay Aquatic Preserves, to recruit students to monitor nearby horseshoe crab populations.
“It’s the kind of research that works for students from the fifth grade through graduate school,” Steponaitis says. “What we need most is consistency – dedicated teachers or students who are willing to track the crabs month in and month out, then year in and year out on the same beach.”
Population trends are difficult to determine because horseshoe crabs spend most of their lives in deep water except when they come to shore to mate and lay eggs. Extraordinarily long-lived animals, they don’t become sexually mature until they are eight to 10 years old, so populations of young animals could be growing or shrinking significantly without any way to track them. At the same time, much of the state’s shoreline has been hardened, destroying the beaches that horseshoe crabs need to reproduce.
The last major effort to track populations occurred in 2010, and concluded that horseshoe crabs are declining at a rate not experienced since the end of the last Ice Age, according to the U.S. Geological Survey. While harvests in the northeast played a role, climate change exacerbated the loss – and it extended to other animals that depend upon them.
For instance, 11 species of birds depend upon horseshoe crab eggs, including the red knot that migrates from the Arctic Circle to the southernmost tip of South America. Loggerhead turtles, another endangered species, are one of the horseshoe crabs’ major predators.
In Florida, most of the harvested horseshoe crabs are captured for “life science” programs, providing specimens for classroom study, dissection and aquariums. However, there is no data available for incidental catch that occurs when trawling boats catch them along with shrimp and other bottom-dwelling creatures.
Current law also allows harvesting for other reasons, including bait, although a permit is required, Black said. “We haven’t issued permits for horseshoe crab harvesting in recent years, but the law does allow it,” she said.
When, Where & How
It’s easy to participate in the ongoing Florida Horseshoe Crab survey (see Bay Soundings, Summer 2002), but to have a real impact, you’ll need to commit to collecting data on a regular basis. “Any data is good data, but what we really need is consistency,” says Ryan Gandy, FWRI research scientist.
Horseshoe crabs come ashore on beaches or sandbars, with the highest numbers seen after dark three days before and after a full or new moon at high tide. Spawning occurs year-round but peaks March through August. Horseshoe crabs need beaches or sandbars that are slightly above the water at high tide, where a female lays about 4,000 eggs in a shallow nest.
Data sheets are online. All you’ll need to participate is a clipboard, pencils to record data and a flashlight if you’re surveying at night. Be sure to wear clothes and shoes that can get wet.
Classes or organizations that commit to ongoing surveys can track their own data by contacting Tiffany Black at horseshoe@myfwc.com in advance for a code that will identify them.
Thousands of horseshoe crabs die every year when they’re stranded upside-down on a beach exposing their tender undersides to hot sun or birds. It’s easy to save them, just roll them over gently. Use the edge of their carapace, not their tails though. The tails – which are used as rudders, not spears – are very delicate and can break easily.
Horseshoe Crabs Critical to Medicine, Medical Research
Even though little is known about population trends in horseshoe crabs, they’re among the most well-studied invertebrates in the world and several Nobel prizes have been awarded to researchers based on their work with horseshoe crabs.
Nearly everyone in the nation is impacted by a test that uses the horseshoe crabs’ “blue blood” to test for bacterial contamination in commercial drugs and medical equipment. The use of horseshoe crab blood is mandated by the Food and Drug Administration for all injectable and intravenous drugs.
Horseshoe crabs are not killed for their blood. Instead, researchers capture them, remove about one-third of their blood and then release them near where they were caught. The industry estimates about a 90% survival rate but some other groups think it may be significantly lower.
Along with their blood, horseshoe crabs are an important source of chitin used in dressings and bandages to speed up wound healing by 35% to 50%. Research on their compound eyes also has helped scientists understand how human eyes function.