The Keystone Species the Whole World Needs
This week’s blog was written by Emma C., a Bucktails alumni. Emma is an avid student, ballet dancer, and nature lover. When not at dance or school, she enjoys hiking, photography, painting, watching musicals and movies, learning interesting histories, and especially reading. She is particularly interested in science at this time, advancing to the state science fair last year. She was extremely excited to attend the Academy because as she puts it, nature is her, “safe place” where she feels at home. Her favorite quote is by the brilliant scientist Rachel Carson, “One way to open your eyes is to ask yourself, ‘What if I had never seen this before? What if I knew I would never see it again?'”
When we typically envision a crab, the image of a small, clawed creature hurriedly scurrying sideways down a sandy beach pops into view. However, the species Limulus polyphemus–commonly known as the horseshoe crab–does not fit so well into this mold, being actually more closely related to scorpions and spiders. The large invertebrate is quite a sight to behold, with its distinct shape, long tail, and pointy accessories. It caused a great stir one beach day on Martha’s Vineyard, MA, as I observed a dead crab wash up onshore and the ensuing battle that unfolded between the present seagulls over this valuable food source. Later when the treacherous war ended and I was better able to view the crab, I was amazed at the number of other living things still present on its shell. An entire separate ecosystem seemed to exist on that one small surface. As I would later learn more about this brilliant species, the horseshoe crab is responsible not only for the small ecosystem on its own back but helps maintain larger ecosystems throughout the ocean.
The horseshoe crab is an ancient species. An iteration of this animal has been around for more than 300 million years–making it older than the dinosaurs–and has survived nearly unchanged for about the last 200 million of those years. Due to its age, this prehistoric species is often referred to as a “living fossil”. The horseshoe crab’s body is split into three parts. The prosoma (head)–this is what gives the crab its name, due to its rounded and u-shaped appearance that resembles a horseshoe–, the opisthosoma (abdomen), and the telson (tail). It has ten legs that enable the organism to walk along the seafloor and a hard exoskeleton that protects its soft underside. The horseshoe crab also has distinctive light blue blood that is one reason they are oh so valuable to humans.
In terms of the fishing industry, horseshoe crabs are commonly used by humans as bait for eel and whelk (often in whelk pots). However, more recently the primary use of these animals by humans has shifted, and with this shift has come much more devastating consequences for horseshoe populations. Today, horseshoe crabs are an absolutely essential part of the biomedical and pharmaceutical industry. Contained within their previously mentioned light blue blood is a substance called limulus amebocyte lysate. When this substance comes in contact with a contaminant called endotoxin, it immediately responds by releasing an array of defense molecules to neutralize the pathogen. This is great for horseshoe crabs and is one of the reasons they have been able to survive so long. For pharmaceutical companies, it provides an amazing way to detect this very contaminant within their products.
Endotoxin is a highly dangerous form of bacteria that can find its way into medical products such as vaccines, injectable drugs, and artificial knees and hips with deadly results. Pharmaceutical companies around the entire world are reliant on the blood of horseshoe crabs for this LAL test (referring to limulus amebocyte lysate). Each year, about half a million of these crabs are captured, bled, and then released back into the wild. However, often these crabs will not survive the ordeal or will die soon after release. This invasive procedure has led to an extreme drop in their populations that has never been seen before.
These harvesting methods can kill the crabs in many ways. Most obviously, too much blood loss can kill the organism. However, this does not seem to be the leading cause of death. These processes, including capture, transportation, and blood draws, expose the crabs to extreme stress, often changing their behavior when they are released back into the wild and ultimately killing them. In addition, the actual physical transportation and storage can kill the crabs. Often they are held in what for them are extremely high temperatures, putting their health at major risk. When being transported in large plastic containers, the crabs are stacked on top of each other, and many are crushed under the weight or accidentally impaled by another crab. The removal of crabs from water during capture can also be particularly lethal, as their gills are not properly adapted to handling out-of-water conditions.
Although it may seem as though the benefits of collecting the blood of these crabs outweigh their population loss, it is important to take into consideration the effects these drops have on other species and entire ecosystems as well. Horseshoe crabs are classified as keystone species, meaning they play a large role in maintaining the overall prosperity of their ecosystem and the species contained within. This classification makes sense, as the horseshoe crab wears a great deal of hats. The organism serves as a bioturbator in its ecosystem, meaning it contributes to the aeration of marine sediments providing oxygen to the benthic ecosystem. They also serve to regulate the population of benthic invertebrates–organisms that live in or on the bottom of bodies of water-consuming a wide range of these organisms such as crustaceans and gastropods. In addition, as I observed on the beach, they are a host to many epibionts like barnacles and blue mussels. Finally, they are a food source for many marine animals including different kinds of shorebirds, sand shrimp, and fish in their juvenile form, and seagulls, alligators, and loggerhead turtles in their adult form. Drops in population can be particularly devastating to shorebirds, who use the crabs as a food source during their most vulnerable times of migration.
Decreases in horseshoe crab populations affect all of the aforementioned organisms, which is why environmental conservationists are so very worried about the health of this species. In recent years, regulations have been put on biomedical and fishing companies limiting the number of crabs they can harvest. In addition, experts are continually working on developing more sustainable methods for the collection of horseshoe blood, or on developing entirely new tests for endotoxin not dependent on this blood. However, despite these efforts, fears still remain about the sustainability of these blood collection practices. Again, we as people are asked the question of how much can we and should we take from nature? This balance is an especially contentious issue with the horseshoe, as it is literally used to save people’s lives. Particularly in this pandemic world where the horseshoe crab could be essential in the production of the Coronavirus vaccine, experts are faced with developing a management plan for these crabs that is ethical, sustainable, and realistic.