"Living Fossils" - Horseshoe Crabs

Earlier this year when I was in Hong Kong I went up to Sai Kung Town, a little fishing port in the New Territories, where I came across a curious sight. Down by the waterfront, which is lined with fish mongers shops and seafood restaurants, there are all sorts of weird and wonderful creatures populating the banked tanks full of gently percolating saltwater. Outside one of the fish monger’s stalls on the wet pavement there was a group of unusual looking creatures – a type of animal which I’d never seen before but knew of and was intrigued by: horseshoe crabs.

Horseshoe crabs are one of the world’s oldest marine animal species. They are thought to have been living in the warm shallow coastal seas for at least the last 300 million years – that’s 100 million years before the dinosaurs roamed the Earth! In that vast period of time the shape of the continents and the oceans have changed dramatically and thousands of other species have come and gone, evolved and perished, but the horseshoe crab has outlasted them all and has changed little itself in all that time. This odd looking creature is arguably one of the most successful – and useful (as I’ve since found out) – species on our planet.

Horseshoe crabs are a type of invertebrate (animals without backbones) classified as Arthropods (joint-legged animals). Other perhaps more familiar animals in this group are lobsters, crabs, insects, spiders and scorpions. Their bodies are divided into three parts: the prosoma, the front part consisting of the head and thorax which is covered by a hard shell or exoskeleton; the opisthosoma, which is attached to the prosoma by a hinge; and, the telson, or sword-like tail which they use to right themselves if they get flipped over onto their backs. They use their legs for feeding as well as moving. They don’t have jaws as such but instead have gnathobases, or bristles located near the base of the legs that they use for walking. The legs which they use for searching out food are called chelicerae. The gnathobases tear and shred the food they eat, usually clams or worms which they grub up from the seabed using their chelicerae as they move along. They have two primary compound eyes located on the top of their carapace or shell, plus several secondary eyes, some of which are located under the carapace too. As they grow they moult their outer shells, increasing in size by 25-30% with each moult. It takes about 8 to 10 years for horseshoe crabs to reach maturity. It’s not known how long they live, but some scientists believe that horseshoe crabs can reach ages over 20 years old.

Given that they have existed for so many millions of years with relatively little change horseshoe crabs are often described as “living fossils.” They are in fact the closest living relative of the trilobite, an ancient marine animal which flourished for around 270 million years before finally disappearing during the mass extinction at the end of the Permian era around 250 million years ago. Trilobites were highly diverse and geographically spread quite widely across the globe. In contrast, horseshoe crabs are limited to certain temperate regions of the Earth’s oceans, although they may have been more diverse and more widespread in the past. Nowadays there are four extant species: Limulus polyphemus inhabits the western Atlantic and Gulf coasts from North to Central America; Tachypleus gigas and Carcinoscorpius rotundicauda populate the Indo-Pacific region from the Bay of Bengal to Indonesia and Borneo; and, Tachypleus tridentatus (presumably the sort which appears in the accompanying photos that I took at Sai Kung) are found from the Philippines up to the southwestern seas of Japan.

Like the trilobites, horseshoe crabs are also found in the fossil record. In 2002 a remarkable horseshoe crab fossil was discovered in Bavaria, Germany – in which, not only the horseshoe crab itself but also its final track in the seabed was preserved. The horseshoe crab somehow fell into what was probably an anoxic lagoon (i.e. – there was no oxygen at the bottom), and so the creature having valiantly righted itself then managed to move some 9.7 metres before it finally succumbed to suffocation. The fossil and its preserved “death march” is thought to be around 150 million years old.

Living horseshoe crabs are also remarkably useful animals to humans and have been utilised in various ways for hundreds of years. Most notably they have been used as a source of food, for tool making (their tails making good spear tips), a nitrogen rich fertiliser, and as animal feed. More recently they’ve also been utilised in medicine too. The study of horseshoe crabs has greatly aided eye research and significantly advanced pharmaceutical testing. Plus studies and derivatives of the material which makes up their exoskeletons (chitin – a polysaccharide, or sugar polymer) has also aided practical advances in medical research (anti-bacterial sponges, dressings for burns, artificial blood vessels, contact lenses, blood cholesterol control, to name but a few), as well as improving water treatment and filtration processes, anti-fungal aids to agriculture, dietary supplements, and cosmetics, even toothpaste!

For more information on horseshoe crabs see the University of Delaware's Sea Grant Program website.

Afterword:  (17 November 2012) Earlier this week I went to an excellent RGS lecture given by Richard Fortey, senior paleontologist at London's Natural History Museum and FRS. He was speaking about his latest book "Survivors: The Animals and Plants that Time has left behind" (Harper Collins, 2011), in which he discussed a whole host of weird and wonderful (and some quite ordinary) creatures which have managed to survive the various mass extinction events in the Earth's long history. He is an expert on Trilobites and naturally enough began his lecture with a look at the Horseshoe crab.

You can find out more about his books on the GoodReads website.