Despite their name, horseshoe crabs—bizarre, ancient critters—don’t look like horseshoes, and they’re definitely not crabs. Horseshoes are actually arachnids, more closely related to spiders, ticks, mites and scorpions than they are to other decapods!
Horseshoe crabs first appeared over 450 million years ago, around that same time that land-dwelling arachnids crawled into existence. These living fossils survived through several mass extinctions virtually unchanged, but only four species exist today. Of these, Limulus polyphemus is the only horseshoe crab found in North America, spread along the Atlantic coasts from Maine to Mexico. The other three species are found in Southeast Asia.
Horseshoe crabs have a tank-like structure encased in a 3-part carapace: a front shell called the prosoma, a back shell called the opisthosoma, and a spike-like tail called a telson. They have five pairs of legs, and a pair of pincer-like feeding appendages called chelicerae.
Despite their archaic look, these creatures are highly adapted. Horseshoe crabs have two compound eyes (made up of thousands of individual photoreceptors), a pair of median eyes that are able to detect both visible light and ultraviolet, a single endoparietal eye, a pair of rudimentary lateral eyes, a pair of ventral eyes located near the mouth, and a cluster of photoreceptors on the telson! Although horseshoes do not have the clear, colour vision humans do, their eyes are a million times more sensitive at night than ours.
The spiked “tails” have several uses, and despite what most people think, stinging isn’t one of them. Instead, the telson primarily serves as a rudder when swimming, and to help the crab right itself after getting stuck on its back. In fact, you should never pick up a horseshoe crab by its tail, as it can harm the animal. Instead, gently pick it up by both sides of the prosoma.
During the breeding season, horseshoe crabs migrate to shallow coastal waters and form massive nesting aggregations on beaches. Delaware Bay in Northeastern US is the largest Atlantic horseshoe crab spawning zone on Earth, with hundreds of thousands of crabs coming to its shores every year in May and June. When mating, the smaller male crab attaches himself to the top of the larger female, and together they crawl to the beach. Eggs are laid in the sand, and larvae emerge from their nests several weeks later. Young crabs spend most of their time on the sandy bottoms of intertidal flats, above the low tide mark, feeding on various invertebrates.
Both larvae and fully grown horseshoes eat aquatic worms and mollusks. Adult crabs will also devour algae and carrion, but they predominantly consume clams and mussels.
Fishery and Other Threats
Horseshoe crabs are most known for their vibrantly blue blood – a consequence of using copper-based hemocyanin to transport oxygen where vertebrates use iron-carrying hemoglobin. This blue blood comes with remarkable antibacterial properties, which has made it highly valuable in medicine, particularly for making Limulus amebocyte lysate (LAL) – a compound used for testing the safety of vaccine and injectable drugs. In the 1970s, the FDA made the horseshoe crab test mandatory for experimental drugs and surgical implants, and at its peak the substance cost $15,000 per quart. Today this practice has mainly become outdated, and we now have a synthetic substitute.
However, horseshoe crabs are still harvested, mainly to be used as bait for fishing eels, whelk and conch. The eggs are eaten in parts of Southeast Asia, Johor and China. Today, their numbers are declining throughout much of the animal’s range. A lot of that has to do with fishing, but other threats include habitat loss, particularly in China where seawalls replace the spawning beaches, as well as water pollution.
Horseshoe crabs are an important part of the ecology of coastal communities. Their eggs are the major food source for migratory shorebirds, including the federally-threatened red knot. These shorebirds have evolved to time their migrations to coincide with peak horseshoe crab spawning activity, especially in the Delaware and Chesapeake bay areas. Adult horseshoes serve as prey for sea turtles, alligators, horse conchs and sharks.
Precious little is known about the Chinese, Indo-Pacific and mangrove horseshoe crab populations, so their conservation status is very difficult to assess. The Atlantic horseshoe crab is recognized as vulnerable by the International Union for Conservation of Nature (IUCN). In 1995, the nonprofit Ecological Research and Development Group (ERDG) was founded with the aim of preserving the four remaining species of horseshoe crab, primarily through the identification and protection of nesting beaches, and modification of fishing practices so as to reduce the amount of bait needed. There is still a lot of data missing on the lives of these unusual animals, and today the species is mainly managed through systematic monitoring.
Young horseshoe crabs can actually swim, but they do it upside-down!
The mangrove horseshoe crab is the smallest of the four living species, while the Chinese horseshoe crab is the largest.
Like many marine arthropods, horseshoe crabs breath through book gills, which are attached to the legs, and as they move they push water over the lamellae, which are thin membranes that allow gases to be exchanged with the environment. These gills are also useful for increasing surface area when swimming!