Is a Horseshoe Crab a Crab? Closer to Spiders and Scorpions

What a horseshoe crab really is

All crabs, spiders, insects and horseshoe crabs are arthropods: animals with jointed legs and a hard outer skeleton that they molt as they grow. Within the arthropods, though, there are several very different lineages. Crabs, lobsters, shrimp and barnacles are crustaceans. Spiders, scorpions, mites and horseshoe crabs are chelicerates, named after their first pair of limbs, small pincers or fangs called chelicerae.

Horseshoe crabs make up their own order, Xiphosura, and a single living family, Limulidae. The best-known species is the Atlantic horseshoe crab of North America. Its scientific name, Limulus polyphemus, has nothing to do with crabs either. When Carl Linnaeus described it in 1758 he called it Monoculus polyphemus, after the one-eyed giant of Greek myth, in a genus he also used for water fleas and other small crustaceans. Later naturalists moved it once its true nature became clear.

Body plan: spider, not crab

The difference shows as soon as you turn a horseshoe crab over. Its body has two main sections, a fused head and chest called the prosoma under the big front shield, and an abdomen called the opisthosoma, plus a tail spine. A spider has the same two-part plan.

On the front section are six pairs of limbs: the chelicerae, then five pairs of legs. A spider also has six pairs, its fangs, a pair of leg-like pedipalps and four pairs of walking legs. A crab works differently. It has two pairs of antennae, a pair of jaws called mandibles and several pairs of mouthparts, followed by five pairs of legs, the first of which are big claws.

Horseshoe crabs have no jaws at all. They grind food with spiny bases on their legs as they walk and push it into the mouth with the chelicerae.

Book gills and book lungs

Under the abdomen, a horseshoe crab has five pairs of flat book gills, named because each one holds up to about 200 thin folds stacked like the pages of a book. The crab also flaps them to swim.

Spiders and scorpions breathe air with book lungs, built from similar stacks of thin, leaf-like folds. Many biologists think book lungs evolved from book gills like these when the ancestors of arachnids moved from water to land. Crabs breathe with feathery gills tucked inside chambers beneath the shell, a quite different design.

Where horseshoe crabs fit on the family tree

Traditionally, horseshoe crabs were placed next to the arachnids as a separate group, closely linked to the extinct eurypterids, or sea scorpions, some of which grew longer than a person.

Genetic studies have complicated the picture. Several analyses of large DNA datasets have placed horseshoe crabs inside the arachnids, sometimes as the closest relatives of a small group of tropical arachnids called hooded tickspiders, which would make them marine arachnids. Other studies, and many experts, argue this result may be caused by known errors in the methods and keep horseshoe crabs as a separate branch beside the arachnids. Either way, a horseshoe crab is far more closely related to a European garden spider than to a blue crab.

Ancient fossils and the “living fossil” label

Horseshoe crabs have a long history. The oldest known fossil horseshoe crab, Lunataspis aurora, about 4 cm (1.6 in) long, was found in Manitoba, Canada, in rocks about 445 million years old, and it already looks much like a modern animal. Forms very similar to living species appear in rocks of the Triassic period, around 250–200 million years ago.

This is why horseshoe crabs are often called “living fossils”. The label needs care: the four living species are not the same species as those ancient animals, and the Atlantic horseshoe crab itself has no fossil record. Instead, the overall body plan has changed little over a very long time. The same idea is explored in Why is the coelacanth a living fossil?

The four living species

Only four species of horseshoe crab survive, out of many more known as fossils.

Atlantic horseshoe crab (Limulus polyphemus): the only species in the Americas, living from Maine to the Gulf of Mexico and on Mexico’s Yucatán Peninsula. It is listed as Vulnerable.

Tri-spine horseshoe crab (Tachypleus tridentatus): the largest species, found from Southeast Asia to China and southern Japan. It is listed as Endangered after steep declines caused by overharvesting and the loss of coastal habitat.

Indo-Pacific horseshoe crab (Tachypleus gigas): found on coasts of South and Southeast Asia.

Mangrove horseshoe crab (Carcinoscorpius rotundicauda): the smallest species, often found in brackish mangrove areas of South and Southeast Asia.

The Atlantic species split from the Asian line long ago, and the three Asian species are each other’s closest relatives.

Why the name “crab” stuck

The name is old and based on appearance. A hard, rounded shell, many legs and life on sandy shores made people think of crabs, and the front shield looks like a horse’s hoof. Fishermen and naturalists also called the animal a king crab or horsefoot.

Science has since shown how misleading the name is, but common names rarely change. The same thing happened with jellyfish and starfish, neither of which is a fish. Curiously, horseshoe crabs and true crabs do share one trait: both carry oxygen with copper-based hemocyanin, so their blood turns blue in air, as explained in Why is horseshoe crab blood blue? Spiders and scorpions use hemocyanin too, so the color says nothing about who is related to whom.