Creature Feature by Dr Gerald Goeden: August 2013

Talk about old! No, not me; the horseshoe crab.

The horseshoe crab (Limulus) is a living fossil. It hasn’t changed much over the last 230 million years (Triassic period) and other relatives were alive 400 million years ago (Devonian period). Their fossils were left on ancient sea beds. Even though it is called a crab, it is more closely related to spiders, ticks, and scorpions.

The horseshoe crab can grow up to 60 centimetres in length (including tail); the female is usually about 25% larger than the male. They can start to reproduce when they are about 10-11 years old. They probably live to a maximum of 30 years.
Horseshoe crabs have interesting bodies. In some ways they are like a spider wearing an armored bulldozer that can push through sand and keep predators away.

There bodies are divided into three main parts:
The head has six pairs of legs underneath. The first pair is equipped with special claws for grasping food. The second pair are called walking legs but have claws too. In males they are hooked for holding onto the female. The last four sets of legs, each with claws, are designed for the heavy work and push the animal ahead through the sand. The mouth is located where all the legs are attached to the body.

The abdomen is where the book gills are located. These primitive gills are like flat pieces of heavy paper that open and close as the horseshoe crab breathes. The oxygen is carried by blood that is blue (copper) and not red (iron).

The tail (telson) does not sting but can be raised up so that the point stops fish from attacking from above. The scientific name for the group is Xiphosura (sword tail).

All the better to see you with! Horseshoe crabs have two large compound eyes on top of the shell each made up of about 1000 small ‘eye parts’. There are simple eyes just behind the compound eyes and another set of eyes on the front of the shell near the middle. Another pair of eyes is under the front of the shell and yet another pair is near the mouth. With all these eyes scientists believe they can only see black and white.

Photo of horseshoe crab on steroids.

Horseshoe Crabs face two problems today: The first is that they compete with man in coastal areas. Loss of habitat is now a major problem for these strange and ancient animals. Second, they are a popular food in Asian countries and these slow, living fossils are easy targets for fishermen.