A horseshoe crab. A prehistoric organism with an incredible life story, now facing adversity up and down the East coast of the United States. Prentice Stout, to whom this article is dedicated, pointed to the female’s tail, carapace, eyes, and then showed me its legs, highlighting the difference between the male and females. We were stood in the warm, shallow water of Point Judith Pond on a hot summer day. The tide was on its way in and it would be time to head for home soon. But I was hooked.
More closely related to spiders than crabs, horseshoe crabs are one of the oldest organisms on earth. This means that they evolved into their current form thousands of years ago and have remained that way ever since. They even look a little pre-historic with a large, rounded, protective carapace shaped like a horseshoe. Their tail extends to a narrow point behind them and allows them to steer through the water. Having ten eyes might also seem like they are all seeing, but only two of these are compound eyes on top of their exoskeleton. The others are simple eyes allowing them to discern light and dark which helps them re-orient themselves in the water in case a rogue wave overturns them. The most distinct and visible difference between male and female horseshoe crabs is found on their legs. The first pair end in a boxing glove shape on the male crabs, while on the females it is the same as the rest of their appendages.
Atlantic horseshoe crabs live on the eastern coast of the United States of America – or the western boundary of the Atlantic Ocean. Interestingly enough, Pacific horseshoe crabs also only live on the western boundary of the Pacific, and in the Indian Ocean. They can be found the full length of these boundaries in the northern hemisphere and prefer sandy habitats. In June, during full moons, masses of horseshoe crabs can be found converging on vast stretches of sandy beaches to breed. In the USA, red knot birds feed on their freshly laid eggs and can also be found at these times. However, recent pressures, such as human development and climate change, on the crab populations have begun to alter this ecological relationship between the two species.
One of the most unique qualities bestowed upon the horseshoe crab is that of its blood. Coloured blue because of the copper-based protein, their blood coagulates – thickens – as soon as it is exposed to gram-negative bacteria, such as E. coli. These bacteria are particularly difficult to deal with in the medical industry when trying to ensure that medical instruments and medicines are safe to use. Dr Frederick Bang made the discovery in the 1960s and since then bleeding crabs has become a large industry. Throughout the years management plans have evolved to try and increase survival rates and monitor the horseshoe crabs. Unfortunately for the crabs, they are frequently used as bait for other fishing industries and for a while there was little incentive to keep bled crabs alive. More recently however, a synthetic version of limulus amebocyte lysate (the name of the product used to test for bacteria) has been developed.
Horseshoe crabs are weird and wonderful, they tell us a lot about the ecosystems within which they live, as well as about past times. If you would like more ‘insights, perceptions, and a few truths about a critter that is as much a part of our lives as American Idol or Mocha Lattes’ then please read Anthony D. Fredericks book Horseshoe Crab; Biography of a Survivor.
At the very least, appreciate the service they provide us and their amazing, persistant nature. As long as we don’t screw it up, I’m sure their future will be longer than ours.
