Horseshoe crabs have got to be some of the most fascinating invertebrates around. Their strange shape and heavily armoured bodies make them so unique that I could not resist writing about them today. In fact, I’m surprised I haven’t written about them before.

They’re kind of cute, actually. Image by PookieFugglestein, CC0, via Wikimedia Commons

Horseshoe crabs are more closely related to spiders and scorpions than crabs, despite their name. They are an extremely old species, with horseshoe crab fossils being found as early as 450 million years ago. Today there are only four species of horseshoe crab left, three that live in Asia and one in North America.

Horseshoe crabs have a horseshoe-shaped shell that covers the body, and a spiny tail known as a telson. Their shell colour ranges from green to grey to brown. They range in size from a human hand to 60 cm in length. These creatures have nine eyes: a pair on the sides of their heads, five eyes on the top of their shells, and a pair near their mouths. They also posses a bunch of photoreceptors on their telsons. Despite the large number of eyes, horseshoe crabs don’t have very good vision (maybe that’s why they need so many?). They do, however, possess the largest cones and rods in the animal kingdom, 100 times the size of those found in human eyes.

The underside of horseshoe crabs is occupied mostly by legs. The crabs use these to move, as well as to push food into the animals’ mouths. The area behind the legs houses the horseshoe crabs’ gills, which let the crabs breathe, and also sometimes help them with swimming. The spiny tail of horseshoe crabs is not used for defence, but rather to flip the crabs back over in case of an accident that leaves them on their backs.

A great picture of the underside of a horseshoe crab. Image by Yale Peabody Museum of Natural History, CC0, via Wikimedia Commons

Another strange thing about horseshoe crabs is their blood. Most animals use hemoglobin to transport oxygen in their blood, but horseshoe crabs rely on hemocyanin, a copper-based compound. This makes the blood of horseshoe crabs blue, which is super cool. The blood is used in the medical industry to detect bacterial toxins. The only way to get the blood is to harvest it from the crabs, which means these creatures are caught, bled, and then released back into the wild. Unfortunately, this procedure stresses out the crabs, and this combined with the blood loss can lead to mortalities as high as 30%.

Horseshoe crabs coming to shore to mate. Image by Asturnut, CC BY-SA 3.0 via Wikipedia

For most of the year, horseshoe crabs live on the ocean floor to depths of 30m, digging in the sand for tasty worms and molluscs to eat. In the spring, horseshoe crabs crawl to the shallows to breed, at night and during high tide. After mating, females lay clutches of eggs in the sand, sometimes laying as many as 20,000 eggs. These hatch after five weeks, and go through a number of moults before they becomes sexually mature, at nine to twelve years of age.

Due to their slow rate of maturing, as well as harvesting pressures (both for their blood and for other purposes, such as bait), horseshoe crab numbers have been declining. Some management efforts are in place, but hopefully we can do enough to keep these crazy animals around.

Cover image by Marshal Hedin from San Diego, CC BY-SA 2.0, via Wikimedia Commons