Types of Horseshoe Crabs

Descendants of Palaeozoic sea scorpions called eurypterids, four species of horseshoe crabs survive today. These "living fossils," which are more closely related to spiders and ticks than to true crabs, have remained virtually unchanged for over 400 million years. The Atlantic Horseshoe Crab (Limulus polyphemus), and the three Indian Pacific varieties, Tachypleus tridentatus, Tachypleus gigus, and Carcinoscorpius rotundicauda, play a key role in modern medicine because of substances in their blood.

  1. Horseshoe Crabs: Living Fossils

    • Also commonly called horsefoot crabs, king crabs and saucepans, horseshoe crabs are the only living members of the ancient class Merostomata. Limulus polyphemus, found on Atlantic beaches of North America, and the three other species of the order Limulidae, found on the beaches of Indonesia and Japan, have a spade-shaped carapace and long sharp tail. Members of all species reach sexual maturity between 9 and 12 years, and may have a lifespan of up to 25 years.

    Horseshoe Crab LIfe Cycle

    • Horsehoe crabs generally spawn between May and June, with peak spawning between the full and new moon. On the beaches females lay thousands of eggs, which are then fertilized by males and buried in sand. At the full moon, hatched larvae crawl out of the sand, letting the tide carry them out to sea. Young horseshoe crabs undergo numerous moult cycles on their way to adulthood, when moulting slows and they attain their adult size.

    Horseshoe Crabs in the Food Web

    • Although horseshoe crabs are not considered edible by humans, adults are eaten by sea turtles, while the eggs are consumed by fish and shore birds. Since frequent molting cycles have ceased by adulthood, horseshoe crabs also play host to a number of smaller crustaceans and flatworms, which cling to their shells in a symbiotic relationship.

    Horseshoe Crabs in Medicine

    • Horseshoe crabs of all species have found an important niche in medical research. Limulus amebocyte lysate, or LAL, an extract of horseshoe crab blood, contains an agent which helps to ensure that injectable medications are free of endotoxins, a type of bacterial contaminant that resists sterilization. To obtain blood for making LAL, horseshoe crabs are caught, bled and released back to their habitat.