Horseshoe crabs are marine invertebrates that live in and around shallow water with soft or muddy sands. They have not changed much since 450 million years ago and so are commonly referred to as ‘living fossils’.

Special gills found behind the back legs (called ‘book gills’) exchange respiratory gases and can also be used for swimming. When these crabs swim, they actually do so upside down - though only at 0.3 miles per hour.

Horseshoe crabs do not have haemoglobin in their blood, but instead use haemocyanin to carry oxygen. Because of the copper present in hemocyanin, their blood is bright blue. This blood is used in the medical industry for a variety of purposes and is often harvested from wild crabs; the mortality rate of this process for the crabs ranges between 5-30% depending on the methods used.

(Photo found here)

Horseshoe crabs (L. polyphemus) are arthropods that live primarily in and around shallow ocean waters on soft sandy or muddy bottoms.They are commonly used as bait and in fertilizer. Horseshoe crabs are considered living fossils. They resemble crustaceans (see this post), but belong to a separate subphylum, Chelicerata, and are therefore more closely related to spiders and scorpions. The entire body of the horseshoe crab is protected by a hard shell. They have two large compound eyes and multiple smaller simple ones atop the carapace. Beneath the carapace they look quite similar to a large spider. They have five pairs of legs for walking, swimming and moving food into the mouth. The long, straight, rigid tails can be used to flip themselves over if they are turned upside down. During the breeding season, horseshoe crabs migrate to shallow coastal waters. Males select a female and cling onto her back. The female digs a hole in the sand and lays her eggs while the male fertilizes them. The female can lay between 60,000–120,000 eggs in batches of a few thousand at a time. Unlike mammals, horseshoe crabs do not have hemoglobin in their blood, but instead use hemocyanin to carry oxygen. Because of the copper present in hemocyanin, their blood is blue. Their blood contains amebocytes, which play a role similar to white blood cells for vertebrates in defending the organism against pathogens. Amebocytes from the blood of L. polyphemus are used to make Limulus amebocyte lysate, which is used for the detection of bacterial endotoxins. Harvesting horseshoe crab blood involves collecting and bleeding the animals (see this post), and then releasing them back into the sea. Most of the animals survive the process


guys if you ever feel like you have bad hearing just remember that in the (shipped) gold standard I thought that patrick said “The time my dad called me a horse shoe crab, and I asked him if throwing me back into the sea would bring our luck back” and I was like wtf patrick’s dad