Stay in the know about wildlife, water quality, and ecosystems on Sanibel and Captiva Islands and in Southwest Florida
Conserving Southwest Florida’s Horseshoe Crabs
By Elsa Wilson, shorebird intern
Have you ever come across a horseshoe crab (Limulus polyphemus) while walking on the beach? These strange looking creatures aren’t the most charismatic critters, but they play a surprisingly crucial role in the natural world and in biomedicine.
Despite their name, horseshoe crabs are more closely related to spiders and scorpions than crustaceans — and they’ve been crawling on our ocean floors for hundreds of millions of years. Since their features and genetics have remained virtually unchanged over this time, they’re considered “living fossils.”
During high tides in the spring and fall, horseshoe crabs come to shore to spawn. Each female creates a small egg chamber to lay her eggs (up to thousands!), and then one or more males fertilize the eggs. The almost microscopic crabs hatch from the eggs after two weeks and spend the next 10 years growing and molting until they reach maturity. Horseshoe crabs can live up to 20 years.
What’s So Special About Them?
Horseshoe crab eggs and the crabs themselves provide food for multiple species, most notably migrating shorebirds. Red knots (Calidris canutus), a medium-sized sandpiper, migrate between their breeding grounds in the arctic tundra and their wintering grounds in South America twice a year during spring and summer. Their journey is about 9,000 miles long, making it one of the longest migration paths of any bird species in the world. When horseshoe crabs in the northeast began to be overharvested in the 1990s, red knot populations began dropping at an alarming rate. Scientists figured out that the eggs were essential for red knot survival during migration, especially at major stopover sites like the Delaware Bay. Without horseshoe crab eggs, red knots run out of energy and can starve to death.
Horseshoe crabs not only help shorebirds survive — they also help us. Horseshoe crab blood contains something called the Limulus amoebocyte lysate (LAL), which is used to test for sterility in medications, needles, and biomedical devices. In the presence of bacteria, LAL coagulates, serving as an indicator to the presence of bacteria. If you’ve ever taken medicine, gotten a vaccine, or had an implant, horseshoe crabs played a big role in ensuring your safety and health!
The Atlantic States Marine Fisheries Commission (ASMFC) has protections in place to ensure that the harvest of east coast populations remain at a sustainable number. Gulf Coast horseshoe crabs are not protected by the ASMFC, so they are of special conservation interest.
What Can You Do to Help Conserve Florida Horseshoe Crabs?
On Aug. 19, SCCF staff and partners from the U.S. Fish and Wildlife Service and other organizations attended a training session hosted by Florida Horseshoe Crab Watch (FHSCW), a partnership between the Fish and Wildlife Research Institute and the University of Florida. The main goal of FHSCW is to learn more about the Florida horseshoe crab by training volunteers to assist biologists in surveying, tagging and re-sighting crabs. Not much is known about the status of Florida horseshoe crabs, and citizen science has proven to be an effective way to learn more.
Since 2015, data collected by biologists and volunteers has helped the Florida Fish and Wildlife Commission learn more about horseshoe crab populations, which has led to increased protections. While other horseshoe crab populations, such as those in the northeast U.S., are known for massive spawning events in the late spring, Southwest Florida horseshoe crabs may behave differently, coming to shore at different tides or moon stages, and in much smaller numbers.
Anyone can help us better understand our SWFL horseshoe crab populations by reporting horseshoe crab sightings, especially spawning crabs.