Throughout the month of February, which I am calling “Feral February,” I am going to do something a little bit different – I’m going to create a series of theme posts every week day about my favourite things in the world: Animals.

Today’s animal is the horseshoe crab.

Horseshoe crabs, also known as king crabs, are not actually crabs – they are more closely related to spiders than lobsters. These animals, which can reach the size of a large serving dish (60 cm or 23.5 inches), are the only living members of a group of animals that first popped up in the fossil record approximately 450 million years ago! They are some of the oldest and most successful animals on Earth.

Horseshoe crab. Photo courtesy of Malcolm Schuyl/ARKive. Source.

The horseshoe crab is easily identified by its name sake, a thick horseshoe-shaped shell that covers its head and thorax. It also has ten legs, spines on the abdomen, a long tail and nine eyes: two simple eyes at the front, a pair of compound eyes on either side and several light receptors near the tail. These animals are also scavengers, looking for food in the mud and sediment, like mollusks, worms, etc. The food is found using the first few pairs of legs, then transferred to their mouth, located in the middle of their abdomen, attached to the legs.

One of the most fascinating things about the horseshoe crab is its value to humans, specifically medical advances.

The horseshoe crab has blue blood because, unlike us, it does not use hemoglobin to carry oxygen, but hemocyanin, which contains copper and makes the blood blue. Scientists found out around 60 years ago that when horseshoe crab blood was exposed to bacteria, it immediately clotted in order to prevent an infection. In the years since, the purified clotting compound has become the standard screening test for bacterial contamination of a substance. A litre of the purified compound, called limulus amebocyte lysate, sells for approximately $15,000.

Daily dose of trivia:

The tail of the horseshoe crab, which looks extremely intimidating and used for defense, is actually harmless. Called the “telson,” the horseshoe crab uses it to flip over if it ever finds itself upside-down.