Horseshoe crabs: Xiphosura

This one surprised even me. I knew crustaceans were arthropods but… arachnids? Well, therein lies the confusion. Horseshoe crabs are not crustaceans. They belong to the order Chelicerata, and are close relatives of an extinct group of arachnids known as sea scorpions. They are considered “living fossils” as they have not changed much in the 450 million years that they’ve been on this planet.

Unlike other arachnids, horseshoe crabs actually have 5 pairs of legs, in addition to a set of chelicerae. Like other arachnids, they have two body segments, which are obscured by the heavy, armor-like carapace. The relation to arachnids was only recently confirmed last year during extensive genetic analysis. 

Horseshoe Crab Fun Fact! Horseshoe crabs have four eyes; 2 compound eyes and 2 simple eyes that can detect ultraviolet light. Their compound eyes do not provide much in the way of forming images, but they have the largest rods and cones of any known animal, meaning they are capable of detecting the slightest amount of light during the night.