The circulatory fluid of the horseshoe crab is proving so useful for making medicine that we may be bleeding the population dry. Karen Boswarva reports
OCEANSCIENCE
Glorious blood
The circulatory fluid of the horseshoe crab is proving so useful for making medicine that we may be bleeding the population dry. Karen Boswarva reports
Apologies if you are squeamish, but blood is a fascinating substance! Human blood is a tissue( rather than a liquid) comprised of plasma and cells with an approximate ratio of 55:45. It makes up 7-8 % of our body weight. The plasma contains all the good stuff – nutrients, hormones, electrolytes, clotting factors and antibodies, and picks up the waste. The cells consist of red and white blood cells, and platelets. Red blood cells contain all the gases diffused from our lungs. All that goodness, contained within a closed system of arteries, veins and capillaries is pumped round our body by the heart.
But that’ s not news to us. As scuba divers we learn a lot about the various gases that flow through our bloodstream. You may be asking what this has that got to do with marine science? Well, let me introduce you to a group of extraordinary marine animals, one that is heavily relied upon by mankind despite most of us never coming across them, unless you’ ve walked the beaches of North America or Southeast Asia during mating season. The horseshoe crab is neither a crab nor a crustacean, it is a chelicerate – the same sub phylum as sea
24
spiders. They appeared in the Triassic( over 250 million years ago) and are often referred to as living fossils, that they look much the same as their fossilised ancestors.
Horseshoe crabs have an open circulatory system and no red blood cells. Instead of blood, the gills oxygenate a bright blue circulatory fluid called hemolymph. It consists of plasma and cells
that fill the body cavity, directly bathing the organs. The Atlantic horseshoe crab( Limulus polyphemus) is one of four extant species, it is found in coastal waters of North America from Maine to the Yucatán Peninsula, where breeding adults migrate to estuaries and sandy beaches to mate. In doing so it is naturally exposed to big changes in oxygen, but. L. polyphemus is physiologically adapted to tolerate environmental hypoxia( low oxygen concentrations).
The hemolymph [ circulatory fluid, analogous to blood ] of L. polyphemus contains hemocyanin, a copper-based
Horseshoe crabs mating
A female Indo-Pacific horseshoe crab, Tachypleus gigas
“ Instead of blood, the gills oxygenate a bright blue circulatory fluid called hemolymph”
respiratory protein that enables oxygen transport. L. polyphemus can survive in low oxygen conditions because of hemocyanin. The protein enables a reverse Bohr effect, increasing oxygen’ s affinity to hemolymph to maintain an adequate supply of oxygen to tissues and organs. That’ s the opposite of what occurs when we hyperventilate.
L. polyphemus oxygenates hemolymph by performing intermittent ventilation. Switching between active gill movement and apnea, the heart and gills perform a coupled synchronisation. This action stops the circulation of deoxygenated hemolymph during apnea and equally increases oxygenation and circulation when oxygen is in demand. How perfect!
But there is a dark side to this poor creature’ s tale that sounds like it’ s straight out of a sci-fi movie. The haemolymph is harvested extensively by biomedical industries, as it contains large granular amoeba-like cells with clotting enzymes that coagulate when toxins are detected. The Limulus amebocyte lysate( LAL) test is a standard and widely used method for detecting the smallest amounts of bacterial endotoxins within medicines and medical devices, ensuring they are free from contamination. It is believed that up to one million L. polyphemus are caught each year for“ biomedical-bleeding”. While most are released back to the ocean afterwards it remains largely unknown what impact the bleeding has on survivability. What is known is that populations are in decline( 2-9 % per year), possibly due to the combined effect of a changing climate on the environment, habitat loss / change and overharvesting. While scientists advocate for protective measures, a listing under the USA’ s Endangered Species Act is yet to be agreed.
In 2025, scientists at the University of South Florida tried to understand the potential impacts of biomedical-bleeding, by comparing the oxygen supply capacity and routine metabolic rate of L. polyphemus that had been bled with those which hadn’ t. Positively, the research showed that the horseshoe crabs appeared to have a buffer or the ability to recover quickly from a 10 % loss in hemolymph. However biomedical harvesting may take up to 30 % and the process of capturing and obtaining hemolymph likely causes the animals much stress.
Wouldn’ t it be fantastic if we could create a synthetic source of haemolymph? One less pressure may see horseshoe crabs surviving for another 250 million years. �