Surprising Relatives of Horseshoe Crabs

horseshoe crab

Story at-a-glance -

  • Horseshoe crabs are not a true crab; they’re members of the phylum of arthropods, which includes three major classes: insects, arachnids and crustaceans.
  • The horseshoe crab is so unique that it belongs to its own class — merostomata, which means “legs attached to the mouth”
  • Horseshoe crabs’ closest living relatives include scorpions, spiders, ticks and mites

By Dr. Becker

Horseshoe crabs are prehistoric-looking creatures for good reason. They're one of the oldest species on the planet and lived on Earth more than 300 million years ago — even before the dinosaurs.

Though they resemble a crab, with their hard exoskeleton and 10 legs underneath, horseshoe crabs are not a true crab at all. They're members of the phylum of arthropods, which includes three major classes: insects, arachnids and crustaceans.

The horseshoe crab is so unique that it belongs to its own class — merostomata, which means "legs attached to the mouth."1

Surprising Relatives of Horseshoe Crabs

Horseshoe crabs are one of the few species that have survived three mass extinctions that eliminated most other marine and land animals living at the time. So it's not surprising that their closest relatives are now extinct.

This includes sea scorpions, which roamed shallow seas (and possibly land) prior to the time of the dinosaurs, along with trilobites, a group of extinct marine arthropods. Today, horseshoe crabs may seem to be total loners — not quite spider, not quite crab and not quite insect — but they do have some modern-day relatives.2

Scorpions

Scorpions, which are arachnids, are among the horseshoe crabs closest living relatives. They, too, lived hundreds of millions of years ago. Scorpions can grow to be up to 8 inches long (primarily the flat rock scorpion native to southern Australia) and once lived primarily in the sea.

It's thought that their ability to adapt to life on land enabled them to survive extinction events that occurred millions of years ago. Unlike horseshoe crabs, scorpions are poisonous and their stings quite painful.

Although scorpion stings are typically harmless to healthy adults, children, the elderly and pets should seek medical attention for a scorpion sting. Further, stings from about 30 of the 1,500 known species of scorpions can be fatal even in healthy adults.3

Spiders

Spiders, also arachnids, are also related to horseshoe crabs and lived up to 400 million years ago. While there are only four species of horseshoe crab, there are more than 50,000 known species of spider.

Most of them are harmless to humans, but a few — namely recluse spiders, widow spiders and hobo spiders, can be dangerous. Whereas horseshoe crabs live in the ocean and along coastal beaches, spiders live on land in webs or trees or on the ground.

Mites and Ticks

Mites and ticks also lived many millions of years ago and represent another one of the horseshoe crab's living relatives. Both mites and ticks are blood-sucking parasites (although some mites are plant-sucking) that attach themselves to animals (or, for some mites, plants) to feed.

Many mites are microscopic, but if you happen to see a tick up close it's easy to spot the resemblance, however distant, to their horseshoe crab relatives.

Horseshoe Crabs Have Nine Eyes — and Other Fun Facts

Horseshoe crabs are named for the horseshoe shape of their head, which is also the largest part of their body and home to their largest set of eyes.

These interesting creatures have nine eyes in all that are located throughout their body and used to find mates, determine movements and detect changes in moonlight. They also have special light receptors near their tail for this purpose.4

A horseshoe crab's tail, called the telson, is another one of its distinguishing features. Although it looks intimidating and dangerous, similar to a stingray's barb, it's nothing of the sort. The telson isn't used for stinging and it's not poisonous; it's there to help the horseshoe flip over if it finds itself on its back.

Horseshoe crabs like to eat worms and clams, and spend much of their time on tidal flats or deeper ocean waters, travelling to coastal beaches to breed. In the U.S., horseshoe crabs have been spotted all along the east coast and on the coastline as far west as Texas. 

They have a long lifespan compared to many of their relatives; they can live to be more than 20 years old. However, they don't reach sexual maturity until 10 years or so.

Horseshoe Crabs Are Incredibly Important, and Their Numbers Are Declining

Horseshoe crabs fulfill an important need in the ecosystem — their eggs are an incredibly popular food source for birds, reptiles and fish. Most horseshoe crabs are eaten before they even reach the larval stage, and some shorebird species depend on the eggs for more than 50 percent of their diet.5

Horseshoe crabs also have unique blue copper-based blood, which contains limulus amebocyte lysate, a substance used in medicine. According to the Florida Fish and Wildlife Conservation Commission (FWC):6

"The substance, which coagulates in the presence of small amounts of bacterial toxins, is used to test for sterility of medical equipment and virtually all intravenous drugs. Research on the compound eyes of horseshoe crabs has led to a better understanding of human vision."

FWC notes that horseshoe crab numbers are declining throughout much of the species' range. The International Union for Conservation of Nature and Natural Resources (IUCN) Red List classifies horseshoe crabs as "near threatened."

The primary 21st century threats to horseshoe crabs include overharvesting, as they're used to catch eels, as well as habitat loss.7 Development on beaches interferes with horseshoe crab breeding, which is why efforts are now underway in some areas to identify and protect horseshoe crab nesting beaches.

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