How Horseshoe Crab Research Saves Millions

Horseshoe Crab Research 

On Vision and Bacteriotoxin Detection

The American horseshoe crab, Limulus Polyphemus, is one of the oldest creatures on earth, and the animal continues to play an indispensable role in biomedical research. Not only does their blood contain special cells that scientists use to detect bacteriotoxins in our medicines, but their eyes also contain a neural network that has provided much insight about physiological processes operating in our visual system, such as light adaptation and lateral inhibition.
On Detecting Bacteriotoxins
Horseshoe crabs have been around for roughly 445 million years, and have apparently learned a trick or two on how to survive. One of their unique adaptations is developing blood with remarkable antibacterial properties. Although we haven’t been around nearly as long, we have learned quickly how to harness these properties in ways that save our own lives – in detecting bacteriotoxins in medicine and medical equipment to prevent infections from happening from the start.
The most obviously unusual aspect of crab blood is that it is bright blue, a consequence of using copper-based hemocyanin to transport oxygen where vertebrates use iron in hemoglobin. Instead of white blood cells to fight infection, many invertebrates have amebocytes, and Atlantic horseshoe crabs (Limulus polyphemus) have evolved these to such a peak of refinement that they are of enormous medical value.
Horseshoe crab amebocytes coagulate around one part in a trillion of bacterial contamination. Even better, the reaction takes 45 minutes, not two days as with mammalian equivalents. Coagulan, the chemical that makes this possible, is used for testing medical equipment and vaccines prior to use, without which many more people would die from preventable infections. Unfortunately, coagulan synthesis is in its infancy so a quarter of a million crabs are harvested each year for their blood, as shown in this video:

Unfortunately, overharvesting in North America has led to a decline in horseshoe crab populations, with worrying implications if this continues. In order to sustain the species, the harvesters take 30% of the blood from each crab, after which they are returned to the ocean. While this portion of blood is meant to be an amount the crabs can survive, it is estimated that 10-30% are affected enough that they don’t. Moreover, among female crabs that do recover, they often breed less after being bled. However, at $15,000/lb plenty of people still think the crab blood is worth bottling.

On Vision Research – Restoring Sight to the Blind
The horseshoe crab remains an attractive model for vision research because the animal is large and hardy for an invertebrate, its retinal neurons are big and easily accessible, its visual system is compact and extensively studied, and its visual behavior is well defined. Moreover, the structure and function of the eyes are modulated on a daily basis by a circadian clock in the animal’s brain. In short, the visual system of horseshoe crabs is simple enough to be understood yet complex enough to be interesting.
Electroretinogram (ERG) recordings measure with a surface electrode the summed electrical response of all cells in the eye to a flash of light. They can be used to monitor the overall sensitivity of the eye for prolong periods of time. Optic nerve recordings measure the spiking activity of single nerve fibers with an extracellular microsuction electrode. They can be used to study visual messages conveyed from the eye to the brain as well as circadian-clock messages fed back from the brain to the eye. Intraretinal recordings measure with an intracellular microelectrode the voltage fluctuations induced by light in individual cells of the eye.

They can be used to elucidate cellular mechanisms of retinal processing.
By understanding how horseshoe crab visual systems function on this more simplistic level, it is possible to delve into implementing a similar model for those who are vision impaired or blind. Research continues on seeing if this is a viable option. (Department of Biomedical Engineering, Boston University, USA)

Works Cited

Department of Biomedical Engineering, Boston University, USA. “Using the horseshoe crab, Limulus Polyphemus, in vision research.” J Vis Exp. 2009 Jul 3;(29). pii: 1384. doi: 10.3791/1384.