Blue Zoo: Horseshoe Crab

Featuring one amazing marine animal per week.

The horseshoe crab (Limulus polyphemus) is an ancient species. Fossils indicate that types of horseshoe crabs have existed on earth for at least 450 million years! 

The horseshoe crab's ancient ancestors look almost identical to the modern species. Photo by David Goehring via Wikimedia Commons, Creative Commons License. 

Now horseshoe crabs can be found all along the East Coast of North America as far north as Maine, though other related species of horseshoe crabs also exist in the Pacific and Indian Ocean, near Japan and Indonesia. The biggest population can be found in Delaware Bay. They live on muddy and sandy bottom habitats and dig for small prey.

Horseshoe crabs gather on beaches in massive numbers to spawn. Photo by Hayden via Wikimedia Commons, Creative Commons License.

Horseshoe crab eggs are a source of food for other animals, especially shorebirds, which need to fatten up before making long migrations. In fact, a recent decline in the population of horseshoe crabs in Delaware has caused a huge decline in shorebirds. 

Though they are in the phylum Arthropoda (animals, such as insects and crustaceans, with jointed exoskeletons), horseshoe crabs are not closely related to true crabs. Instead, horseshoe crabs are in the class Merostomata; they are more closely related to arachnids, like spiders and scorpions.

On the underside of the crab molt (the shell left behind when the animal sheds and grows a new one), one can see the crab's many legs and the modified gill flaps, called book gills, below the legs. Photo by Curious Expeditions via Flickr, Creative Commons License.

Enormous groups of horseshoe crabs migrate to mating areas on beaches, usually during high tide and a full moon. The females lay their eggs in the sand and the males, sometimes several, then fertilize them.

Horseshoe crabs are critical to the environment and the food web, but they are also very important to humans. Horseshoe crab blood, which is blue instead of red because it is copper-based, is harvested from live crabs (usually without killing them so they can be returned to the wild afterwards) because it clots when it comes into contact with toxic bacteria. All drugs administered by IV are tested using the clotting compound in order to make sure they’re safe to use. 

All the legs end in pincers, but only the small claws used for feeding are called chelicerae. Underneath them, in the middle of the photo, is the mouth. Photo by Carol Vinzant via Flickr, Creative Commons License. 

They have been useful for developing other medical procedures like surgical sutures, as well. They have also been used in research on eyes and vision – the crabs have a total of nine eyes! They have the two main compound eyes on top but there are five more on top of their shell and two underneath. They also have light sensors on their tails.

A close-up of one of this crab's two compound eyes; it also has seven simpler, smaller eyes on its body. Photo by Carol Vinzant via Flickr, Creative Commons License. 

Horseshoe crabs are also harvested by fishermen for bait, but there is concern about recent population declines. Coastal communities have started developing sanctuaries where the crabs are protected, and there is hope that there will be horseshoe crab fishing regulations soon too. However, to implement truly effective protections, more research is needed on migration and reproduction patterns, which are still poorly understood.

Males gather around a female, in the middle, during spawning. Male horseshoe crabs are smaller than females. Photo by Michael Lusk via Flickr, Creative Commons License. 

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