Horseshoe crabs-The Living fossils

There are a few animals that could be described as living fossils; sharks, crocodiles, lizards. But none are as similar to their fossils from millions of years ago as the horseshoe crab, (Limulidae) and although its shares its name with crabs, it is more closely related to spiders and scorpions. They date back 250 million years and have not really changed at all. Below is a horseshoe crab and and fossil of a close ancestor.

There are four species of horseshoe crabs, Limulus polyphemus is found on the eastern coast of North and Central America and three other species, Tachypleus gigas , Tachypleus tridentatus and Carcinoscorpius rotundicauda are found around the indo-pacific. All of them are similar in terms of habitat, morphology, and uses in medicine.

Uses in medicine

The horseshoe crabs’ role in medicine is huge, but not very well known. When pharmaceutical companies sterilise their products they use heat in the form of steam, this is very effective in killing the bacteria but the cell membranes of Gram-negative bacteria (Bacteria easily decolourised during the Gram staining procedure) sometimes remain intact. The membranes contain a product called endotoxin (Lipopolysaccharide) which is toxic. Originally solutions were injected into rabbits, if the rabbit exhibited a fever the batch was deemed pyrogenic (Fever causing) and was rejected.  In the 60’s however, Dr Frederik Bang found that injecting bacteria into the bloodstream of a horseshoe crab caused massive clotting. This was later shown to be caused by endotoxin. Eventually Dr Bang, working with Dr Jack Levin was able to isolate the clotting to the blood cells and then to a substance named Limulus amebocyte lysate (LAL). LAL is still extracted from the blood of horseshoe crabs and thousands are caught each year to  be bled and then re-released.

Morphology

Horseshoe crabs are composed of 3 main body “segments”. The Prosoma (head), Opisthoma(abdomen), and a Telson (tail spine). They have 5 pairs of legs and two small feeding pincers called Chelicerae which place the food into the mouth which is in the centre of the underside of the body. They feed on small invertebrates. They have two lateral eyes which are compound eyes, meaning they are made up of many simple eyes rather than having one more complex eye. It does have 5 more eyes on the top of its shell, two median eyes, one endoparietal eye along the central ridge of the head and two rudimentary lateral eyes next to the other lateral eyes.

Habitat

The specific adult habitat is not well known but they can occupy depths of 200m however they are mostly found at around 30m. During spawning they congregate around bays with sandy beaches. The most famous beach for horseshoe crab spawning is Delaware Bay in New Jersey, USA. The newly hatched crabs will stay in the shallow intertidal zones until they are older, though still juveniles, when they will move to the deeper sea until they reach maturity (approx.5 years old) and will return to the beaches to spawn.

Conservation

Like a lot of marine species that are exploited by man the horseshoe crab’s population is in decline. Though most that are caught for their blood are returned some do die in transit although it has improved since it began. There are many schemes to help conserve the crabs all round the world including the Just flip ’em! Program which, as the name suggests, encourages people to turn over horseshoe crabs that have been stranded on their back.