Primary Source: Owings, Meghan, Christopher Chabot, and Winsor Watson III. 2020. “Effects of the Biomedical Bleeding Process on the Behavior and Hemocyanin Levels of the American Horseshoe Crab (Limulus Polyphemus).” Fishery Bulletin 118 (3): 225–39. doi:10.7755/FB.118.3.2. https://spo.nmfs.noaa.gov/sites/default/files/pdf-content/fish-bull/owings.pdf
The Humble Horseshoe Hero
Don’t let appearances fool you; the humble Atlantic Horseshoe Crab, Limulus polyphemus, has been saving human lives worldwide for decades. How? The secret is in the crab’s blue blood. Most animals have the iron-based molecule hemoglobin to carry oxygen in their blood. However, because horseshoe crabs are living fossils that have remained relatively unchanged for 450 million years, they rely on the copper-based molecule hemocyanin to carry oxygen, giving their blood its distinctive color.; Horseshoe crabs haven’t had much need to change in so long thanks to their extremely adaptable immune system. Rather than use specialized cells like white-blood cells to hunt down invaders, the simple horseshoe crab has its blood cells produce a substance that coagulates at the earliest sign of a foreign entry. The result: near immediate blockage of any cracks or leaks, and defense from a wide variety of pathogens. Since 1977, the FDA has utilized this unique property of horseshoe crab blood to test the purity of injected medicines, assuring no harmful bacteria has inadvertently made its way into mass-distributed vaccines or other medicines. With the sheer number of vaccinations and other injected medicines taken daily, along with the ineffectiveness of alternative procedures, anyone who has been vaccinated in the past four decades can thank the horseshoe crab for an infection-free vaccination.
Being Bled Dry
The past 40 years of horseshoe crab blood haven’t come without their costs. Despite the incredible longevity of this historic species, countless stressors like habitat loss, climate change, and overfishing have had compounding effects that have greatly reduced the population across the Atlantic coast. Furthermore, while not inherently deadly, the harvesting of blood is greatly taxing on the health of the horseshoe crab, leading bled individuals to a greater risk of mortality upon being returned to the wild. With the COVID-19 pandemic bringing forth a surge of vaccine research and production, reliance upon the horseshoe crab is pushing harder than the species may be able to take. With this conflict in mind, researchers from New Hampshire sought to determine which stressors associated with the bleeding process influenced the amount of movement and hemocyanin (the crab’s copper-based equivalent to hemoglobin) levels in the bled individuals.
Dealing With Stress
When being bled, horseshoe crabs are subjected to higher temperatures than their native environment, time away from the water they live in, and the obvious loss of blood. As such, researchers tested the effects of these three stressors, both individually and in conjunction, on levels of hemocyanin and movement for male and female horseshoe crabs over several 3-week periods throughout 2016 and 2017. In these periods, baseline measurements were taken one week after being moved to a controlled observation tank. After this week, various treatments exposing the horseshoe crabs to one or more stressors were done to different groups of crabs, which were kept for another 2 weeks for measurement of lasting effects on hemocyanin levels and movement. Results showed loss of blood to be the singularly most harmful stressor, but also found that reduction of additional stressors, such as time out of water and increased temperature, significantly increased hemocyanin levels, leading to a reduced threat of mortality. Differences in hemocyanin levels were found both between sexes, as well as seasonally. Male horseshoe crabs consistently had higher levels of hemocyanin than their female counterparts, and overall hemocyanin levels were lowest in Spring to early Summer, and highest from late Summer to Fall. While our reliance on the bleeding of the Atlantic Horseshoe Crab has no alternative yet, the results of this study can help the pharmaceutical industry to make informed alterations to their bleeding processes that minimize the damage that bleeding does to the horseshoe crab population, ensuring that we have horseshoe crabs for generations to come.