Here is an interesting thing… horseshoe crabs are not crabs at all. They are arthropods, invertebrates with jointed legs and an exoskeleton. While crabs are also arthropods, it is the arachnid group that includes the spiders, scorpions, and ticks that the horseshoe crab is most closely related to.  They differ from arachnids in numerous ways. Though horseshoe crabs have two body parts, they have 10 legs, not 8 like the arachnids.  They also possess gills and have a long tail called a telson which they use to upright themselves when they have been flipped over. Horseshoe crabs may look scary but they are totally harmless, something that can’t be said of spiders, scorpions and ticks. They can easily be picked up and the pincer looking legs do not clamp down like crabs – they tickle. But, NEVER PICK THEM UP BY THE TAIL!

Horseshoe crabs are marine creatures that move between the open Gulf of Mexico and coastal estuaries. They are bottom dwellers crawling through the sand and mud seeking smaller invertebrates to feed on. We usually encounter them in the spring and fall when they are in the estuaries to lay eggs. The larger females approach the beach during high tide of the full and new moon to lay her eggs above the high water mark. Females can lay as many as 80,000 eggs in a season. The smaller male has a leg that has been modified into a hook to hold on to her, usually riding her back on the way to the beach. Numerous other “satellite” males follow hoping for a chance to fertilizer the eggs. When she reaches the beach, she digs a shallow depression where she releases her eggs.  The males then rush in to release their gametes.

The eggs that aren’t eaten by predators remain buried two to four weeks before hatching into larvae and catching the following high tide back to the sea. The larvae will develop into tiny crabs that look just like the adults and remain in this juvenile stage until sexual maturity at about 10 years. Being arthropods, they too shed their exoskeletons periodically and these empty shells (molts) can be found on the beach.

This passive ancient mariner is declining throughout its range in part due to the valuable contribution horseshoe crabs have made to the biomedical industry. If you have ever gotten a shot, this creature has played a role. Their blood is blue, not red, because of a copper-based compound that reacts with bacteria – limulus amebocyte lysate (LAL). Scientists use a LAL test to assure there are not bacteria in injectable drugs or medical devises. See this amazing creature that’s been around nearly 200 million years before the dinosaurs and its role in medicine.

Horseshoe crabs were once plentiful on Pensacola Beach but are hard to find these days. They use to breed here in large numbers  See the Citizen Science section under Coastal Barrier Island if you would like to help document nesting.