Marine Curios #11 - order Xiphosura, or horseshoe crabs

(Photo: Wasu Watcharadachaphong/Shutterstock)

There are four extant species of horseshoe crab, an ancient marine animal which – despite the name – is more closely related to spiders than crabs. One species is found along the east coast of North America (Limulus polyphemus, the American horseshoe crab) and the three others are found throughout Southeast Asia, from the coast of China all the way round to India.

Horseshoe crabs date back some 480 million years to the Ordovician period of the Paleozoic Era, two hundred million years before the rise of the dinosaurs and around the same time as the first sharks are known to have roamed the oceans. Specimens very similar to today’s species were common in the Jurassic Period, around 200 million years ago.

Horseshoe crabs have ten eyes which can see ultraviolet (UV) light; seven pairs of legs and ‘book’ gills, which they use for both respiration and, occasionally, swimming.

In the 1960s it was discovered that their blue blood – which is copper rather than iron-based – is remarkably sensitive to endotoxins and is now extensively used to test for bacteria in intravenous drugs and on the surfaces of medical instruments.

A fossil of a member of the extinct *Mesolimulus* genus, present from the Triassic to late Cretaceous period, and virtually identical to modern horseshoe crabs (Photo: Raimond Spekking / CC BY-SA 4.0 via Wikimedia Commons)

Across Asia and America there is now an extensive and grisly industry bleeding horseshoe crabs – in the US, the 70 per cent which survive having had a tenth of their blood extracted are returned to the sea. In Asia, the survivors are sold on to restaurants or used for fertiliser.

As a result, populations in the US have dropped by as much as 90 per cent and in Asia, the species Tachypleus tridentatus (tri-spine horseshoe crab) is currently listed as endangered on the IUCN Red List of Threatened Species.

The same is thought to be true of both Tachypleus gigas (southern horseshoe crab) and Carcinoscorpius rotundicauda (mangrove horseshoe crab), although they are currently listed as ‘Data Deficient’.

Conservationists insist that there are other options that can be used to test for endotoxins and argue that the bleeding industry is unsustainable. They point out that the biomedical industry, in barely 50 years of harvesting horseshoe crabs, threatens to wipe out creatures that have existed before dinosaurs.