Horseshoe crabs belong to the group Arthropods, which include insects, arachnids and crustaceans. Despite its name, the horseshoe crab is not a marine crab; it is in its own class among arthropods called Merostomata. In fact, horseshoe crabs prey upon soft-shelled crabs.
There are four species of horseshoes, three that inhabit eastern Asia from Japan to India, and one species (Limulus polyphemus) found on the Atlantic coast of the United States and Central America.
The commonly recognized U.S. species has a unique relationship with migratory birds and a variety of marine animals in its habitat that shows how interdependent all of these animals are on one another, helping to maintain balance within the ecosystem.
Horseshoe Crab Eggs Provide Food for Other Animals
Female horseshoe crabs emerge from the surf in evenings during high spring and summer tides, especially at the full and new moons, to lay eggs in a series of depressions in the sand near the high water line. Here the eggs hatch and the larvae feed on a store of yolk.
A number of birds and other animals rely on the eggs and larvae of horseshoe crabs to live. This is a natural relationship that does not adversely affect the horseshoe crab population. Only when man interferes with the balance are the other life forms affected.
- Sea turtles. Besides humans, sea turtles are probably the horseshoe’s chief predator. They will actively prey upon horseshoe crabs by flipping them over and attacking their vulnerable areas to get to unlaid eggs. It is possible that the loggerhead turtles in the mid-Atlantic area depend on horseshoe crab eggs for their long-term survival.
- Migratory birds. According to the Ecological Research & Development Group, at least 11 migratory bird species eat horseshoe crab eggs as their main food source when they stop over in the Delaware Bay area during their northward migration each year. Up to a million shorebirds converge here for two to three weeks and feed on eggs that have been churned up from the sand by waves and the abundance of spawning activity.
- Fish and other invertebrates. The eggs that are exposed on the beach when other female horseshoes churn up nests laid earlier by other females, and that haven’t already been eaten by birds, will be carried out by the surf and become available as food for fish and other invertebrates. Examples of marine animals that eat these eggs are striped bass, white perch, American eel, flounder, crabs and whelks.
Marine Life That Survive On the Horseshoe Crab
As young horseshoe crabs pass through stages of growth, they molt, and grow about 25 percent after each shedding. As they mature and reach adulthood, shedding less frequently and growing up to a foot in diameter, a host of organisms use the living horseshoe crab for their own survival:
- Crustaceans like mud crabs, sand shrimp and spider crabs, which are quite small, can live inside the crevices of the molting shells.
- A flatworm called the Limulus leech lays its eggs in the horseshoe crab’s gills.
- Sea sponges, starfish, barnacles, oysters, tube worms, and bivalves like mussels and slipper shells attach themselves to the horseshoe’s outer shell.
What if the Spawning Habitat Disappears?
With increasing coastal development, consider the consequences of a loss of habitat where horseshoe crabs breed and grow. These creatures require protected beaches for egg-laying, such as those in a bay or large cove. They require that the sand on those beaches have specific properties for nesting. The young will spend their first years in shallow waters nearby, which must have a specific salinity ratio and an abundance of food. Considering the number of animals who rely on the horseshoe’s eggs alone, if the habitat were lost, and eggs became scarce, the balance in the ecosystem would most definitely be impacted.
Sources:
- “Horseshoe crab.” The Ecological Research & Development Group. <horseshoecrab.org/nh/hist.html>
- “Horseshoe crab.” The New Encyclopedia Britannica, 15th ed., Vol. 6. Chicago: Encyclopedia Britannica, Inc., 2007.
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