They may look like crabs and were given the name horseshoe crab as they so closely resemble the common seashore animal, but in actual fact horseshoe crabs are arthropods, like spiders and scorpions, belonging to the order Chelicerata. They are closely related to the extinct eurypterids or sea scorpions, some of the largest arthropods that ever lived. They are four species living today:
- Carcinospinus rotundica: the mangrove horseshoe crab, found in south and southeast Asia.
- Limulus polyphemus: the Atlantic or American horseshow crab, found on the Atlantic coast of the United States, and through the Gulf of Mexico.
- Tachypleus gigas: the Indo-Pacific horseshoe crab, found in South and Southeast Asia.
- Tachypleus tridentatus: the Chinese or Japanese tri spine horseshoe crab, found throughout South and Southeast Asia.
The animals living today are very close to fossilized specimens from as long ago as 445 million years, showing very little evolution from their ancient relatives, a phenomenon called morphological stasis. An extremely well-preserved fossil found in mineral deposits from Illinois even showed the soft tissues of the internal organs and how little they have changed to today. Along with other primitive arthropods like the trilobites, the horseshoe crab evolved in the volatile shallow seas of the Paleozoic era, surviving the extinction of the dinosaurs to trundle through our waters today.
A perfectly preserved horseshoe crab, showing how little the anatomy has changed to today. Mineral deposits enabled the soft tissue to be so perfectly preserved (image from BBC news)
Ancient physiology: the biology of the horseshoe crab
The soft tissues of the body are entirely covered in a hard carapace, with their rudimentary eyes on top and their chelicerae, or mouthparts, underneath. Horseshoe crabs have several eyes, including two compound lateral eyes and eyes that can detect ultraviolet light, making them incredibly sensitive to light at night.
Horseshoe crabs have incredible bright blue blood, thanks to the molecule they use to carry oxygen around their blood. Unlike our blood, which contains haemoglobin, horsehoe crabs have a protein called hemocyanin, that contains copper rather than iron, as found in our blood, that gives their blood such a vivid blue colour.
Horseshoe crabs have pedipalps which they use to move across the sand foraging for food, which usually consists of worms and molluscs. The males also have special claspers that they use to cling to females during mating, fertilizing the eggs as they are laid into the sand. Dominant males take precedence in mating, but so called satellite males will linger around the mating area and may have some success fertilizing some of the eggs that have been laid. Anywhere between 60,000 and 120,000 eggs can be laid in one batch from one female, and these eggs are a vitally important link in the food chain. Many shorebirds will feast on these eggs to fuel their long migratory journeys to their breeding and feeding grounds.
The anatomy of the horseshoe crab (image from ourmarinespecies.com)
Those eggs that are nor snaffled by hungry birds will incubate for 2-4 weeks, after which the larvae will begin to emerge. As they grow, they will move several kilometers from their natal beaches into deeper waters on the continental shelf. They will molt numerous times as they grow larger, shedding their exoskeleton as many as 17 times. As well as their eggs being a nutritious meal for many animals, the adult animals are preyed upon widely, including everything from crustaceans to fish, sharks and sea turtles. They also make hospitable homes, with a miniature ecosystem living on them. Ghost anemones, rock barnacles, sea strawberries, seal lettuces, sponges, skeleton shrimp and builder worms all make their homes on a horseshoe crab.
Horseshoes and human health
Horseshoe crabs are not only vital for the integrity of their ecosystems, but they have also been essential in the advancement of the biomedical field. Research on their visual systems led to the development of new surgical sutures and burn dressings, and an even bigger discovery hid in their bright blue blood. Their blood is now so valuable that a quart of it can sell for as much as $15,000, all thanks to a very important molecule found in their blood.
As they forage through the sand, horseshoe crabs will come into contact with a staggering number of bacteria, and so they evolved a particularly efficient defense mechanism. Unlike us, horseshoe crabs don’t rely on white blood cells to keep themselves disease free. Instead, they utilize cells called amebocytes. When an amebocyte bumps into a pathogen, or a harmful chemical from a pathogen, called an endotoxin, it will attach to the foreign invader and release a chemical that causes the blood to clot, isolating the pathogen. This efficiency at halting bacteria in their tracks makes this substance, Limulus Amebocyte Lysate (LAL), incredibly valuable to the biomedical industry. Any medical product that comes into contact with human blood, such as vaccines or other drugs that are injected, needs to be completely free of bacterial contamination, or else there is a severe risk of septic shock. In the past, to test the safety of batches, researchers had to conduct time consuming tests in rabbits, injecting them with a test substance from the batch, and waiting to see if any septic disease developed. With the discovery of LAL, scientists could scrap this long winded work for a simple test of adding horseshoe crab blood to the sample, and seeing if a clot would develop.
Too valuable for their own good: the dangers facing horseshoe crabs
Unfortunately, to fuel the demand for their vitally important blood, hundreds of thousands of horseshoe crabs are bled each year. Although in some places attempts will be made to drain a small amount of blood and release the crabs back into the ocean, in Asia, the practice is to completely drain the blood from the animal, killing the animal. Even when best efforts are made in the US to keep the animals alive after the draining process, inevitably the trauma of handling and being out of the water will kill a large number of them. Somewhere between 15 and 30% mortality is estimated. Even when the handling process doesn’t directly kill the crabs, the injuries and disorientation can led to reduced spawning success and increased levels of disease. In addition to harvesting for the biomedical industry, horseshoe crabs are often caught for the bait industry for the catching of eels, whelk and conch. On the East coast of the US alone, 500,000 horseshoe crabs can be caught in a single season. This level of demand across their range is considered ecologically unstable.
Adding to the pressure of these harvests, across Japan and other parts of Asia the horseshoe crab is facing significant habitat loss, with vast spawning grounds being lost to human developments. Across their global range, climate change and rising sea levels are putting their essential spawning grounds at risk. This doesn’t just endanger the horseshoe crab, but also six species of shorebird in the mid Atlantic region of the US, that stop along the shore to feed. Losing that essential food resource along these stop off points on the coast also puts these birds at risk too.
Conservation optimism
Although the horseshoe crabs are fairly common and are rarely high on the conservation attention list, many scientists in the biomedical industry have understood that the harvesting going on at its current levels is unsustainable. In 1997, scientists at the university of Singapore began researching an alternative to LAL, trying to create an animal free endotoxin detection technology. Subsequent research has proved that this synthetic alternative is just as efficacious as the stuff extracted from horseshoe crab blood. Now there is a significant push from many scientists to move the pharmaceutical industry into using the alternative, animal-free option.
As well as this push away from horseshoe crabs in the pharmaceutical industry, the WWF also has the HSC Counts project, aiming to develop strategies for horseshoe conservation. They work especially in Hong Kong, trying to reduced the consumption of horseshoe crab meat by tackling the restaurant trade in Asia. Learn more about this project here: Horseshoe Crab Conservation | WWF Hong Kong
Awesome videos!
References
Avise, J.C. Nelson, W.S. and Sugita, H. (1994) ‘A speciational history of ‘living fossils’: molecular evolutionary patterns in horseshoe crabs.’ Evolution
Bicknell, R.D.C. Ortega-Hernandez, J. Edgecombe, Gaines, R.R. and Paterson, J.R (2021) ‘Central nervous system of a 310 m.y old horseshoe crab: expanding the taphonomic window for nervous system preservation.’ Geology
Garwood, R.J. and Dunlop, J. (2014) ‘Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders.’ PeerJ
Karpanty, S.M. Fraser, J.D. Berkson, J. Niles, J.L. Dey, A. and Smith, E.P. (2006) ‘Horseshow crab eggs determine Red Knot distribution in Delaware Bay.’ Widlife Management
Maloney, T. Phelan, R. and Simmons, N. (2018) ‘Saving the horseshoe crab: a synthetic alternative to horseshoe crab blood for endotoxin detection.’ PLOS Biology
Walls, E.A. Berkson, J. and Smith, S.A. (2002) ‘The Horseshoe Crab: 200 million years of existence, 100 years of study.’ Reviews in Fisheries Science
MASSIVE THANK YOU TO RACHAEL DA SILVA FROM THE UK FOR THIS WRITE UP! PLEASE FOLLOW HER ON INSTAGRAM AND HER WILDLIFE ARTWORK AT TILLY_MINT08
