Atlantic horseshoe crabs in the US
[Image credits: U.S. Fish and Wildlife Service Northeast Region, Public domain, via Wikimedia Commons]
Imagine a big, inverted saucepan with two holes for eyes, a few legs at the bottom and a long tail. That is what a horseshoe crab vaguely looks like. Don’t get carried away by the name! It is not a crab, but a marine invertebrate closely related to spiders and scorpions. While the earliest fossils of horseshoe crabs date back to 450 million years ago, there are four species in the world today. Three of them are found on the beaches of South and Southeast Asia. What’s fascinating about these archaic organisms is not just their strange looks or huge size (about 60 cm long and weighing around five kilos) but their blood.
In invertebrates, haemocyanin, a copper-containing protein, carries blood to different parts of the body and imparts a blue colour to the blood. Cells, called amebocytes, defend these organisms from pathogens, just like our white blood cells. When a pathogen like bacteria attacks, special blood-clotting proteins in the amebocytes help the blood to clot around the bacteria, protecting the organism from further harm.
In 1953, Frederik B. Bang, an American medical researcher, first observed this effect when he infected Atlantic horseshoe crabs (Limulus polyphemus) with marine bacteria. Later, scientists extracted Limulus amebocyte lysate (LAL), a chemical from the amebocytes, which reacted with bacterial endotoxins and resulted in the blood clot. By the 1970s, a test to detect bacterial toxins using this extract was commercialised in the USA. Even today, this test is performed on all medical implants and vaccines to ensure that there are no harmful bacteria or fungi on them.
Every year, thousands of horseshoe crabs are collected to draw their blood and extract Limulus amebocyte lysate and Tachypleus amebocyte lysate (from the Asian species Tachypleus tridentatus). After sucking the blood, they are released back into the oceans. However, conservationists have raised concerns that this process may be killing about three in ten crabs and reducing the egg-laying capacity of the females. Some also question if pharma companies are indeed returning these crabs to the seas or selling them as fishing baits. These factors, along with overfishing for consumption, have resulted in the dwindling numbers of horseshoe crabs.
In recent years, scientists have developed alternatives to the blood-soaked proteins. They have identified the Limulus clotting factor C, an enzyme found in LAL. Instead of drawing it from horseshoe crab blood, this clotting factor can be made in laboratories, thus sparing many lives. However, this alternative is yet to be approved by regulatory authorities and hasn’t yet been widely adopted. Until then, spare a thought for the blue-blooded horseshoe crabs when you take any vaccine or get a medical implant.
Editor's note - This article was originally published on Deccan Herald.