As we head into fall, the light changes, the leaves turn, and it is all beautiful but brings new stresses on our bodies that aren’t yet adjusted to the shifting temperatures. That often means the onset of fall colds, which serve as reminders to get the dreaded annual flu shot. While I never look forward to getting a shot, I am fascinated by the science behind vaccines. (This is the connection to the sea, which I’ll explain.)

Another sign of fall is the whipping up of winds that bring an array of floating items to the shore. Among the easily identifiable swirl of logs and leaves, there are sometimes odd, spiky objects breaking the surface. These are the shells of horseshoe crabs (Limulus polyphemus), which are exceptionally good at floating just beneath the surface once the animal has died or has shed its shell. Horseshoe crabs are particularly strange creatures for many reasons, including the fact that they are essentially prehistoric creatures that are somehow still surviving in today’s marine environment.

Perhaps their survival is part of why they are helpful in getting us through the oncoming winter. One of the components of their blood is used in vaccines we rely upon to keep us healthy. Horseshoe crab blood has a unique ability to form a clot around harmful bacteria and thus isolate it so that it doesn’t cause an infection. The blood can do this because it contains a compound called LAL (Limulus amebocyte lysate). This compound is super sensitive to components of the membranes of these bacteria called endotoxins. These not only make horseshoe crabs sick but they can also make humans sick.

Bacteria are everywhere, so discovering an indicator that identifies these harmful types is pretty incredible. Because of this, scientists have used LAL to test things like vaccines that are injected right into the bloodstream to make sure that they aren’t contaminated. Without a test such as this, the purpose of a vaccine could completely backfire and make someone very sick!

The process of harvesting the blood of horseshoe crabs is not a simple process or uncomplicated endeavor. The industry is very highly regulated with limits on size and quantity so as not to overfish the resource. In addition, the process for extracting the blood is very precise. Lab workers use a needle to extract a limited amount of blood from just underneath the shell, aiming to leave enough blood in the animal for it to survive. Having made incisions into the shells of hundreds of lobsters during my graduate work and watching them recover, I can attest to the hearty nature of crustaceans. Also, because horseshoe crabs are also used as bait in several fisheries, there have been collaborations between harvesters and laboratories to extract blood before these animals are used as bait.

The crabs you’re likely to see at this time of year are not crawling around on the shore but rather the washed-up shells, as they have made their way into deeper waters for the colder weather. But come spring, they are an amazing sight as they come up onto protected beaches to mate and lay their eggs. If you have a chance to see this, you’ll have a new appreciation for how special these creatures are and how they may just have helped make you a little healthier.

Susan Olcott is the director of strategic partnerships at Maine Coast Fishermen’s Association.