The Atlantic horseshoe crab (Limulus polyphemus) is considered a keystone species due its critical role in coastal ecosystems. This species has been found in the fossil record dating back 445 million years. They are a crucial component for their ecosystems, and their impact creates a keystone effect, due to the systems and organisms that rely on them (Rudkin & Young, 2009). Their ecological significance is seen largely during their spring spawning. During these thousands of horseshoe crabs come to shore to lay eggs on beaches. These nutrient-rich eggs become a vital food source for numerous organisms. This leads to both marine and terrestrial systems being heavily impacted by the Atlantic horseshoe crab.  The eggs laid during these spawning events provide a source of food that is especially important to migratory birds.

Each spring, migratory birds traveling along the Atlantic Flyway depend on horseshoe crab eggs as a food source to help restock their energy supplies. These birds often migrate thousands of miles to reach their breeding grounds. On these flights, they rely on the eggs of the Atlantic horseshoe crab to replenish their strength and build up their fat reserves. Declines in horseshoe crab populations can have major negative effects on migratory birds (Reshetiloff, 2009). Without reliable access to this food source, many migratory bird populations face significant challenges in surviving their long migrations. 

Beyond just their ecological role, horseshoe crabs are also valuable to humans. Their blue blood contains the compound, Limulus Amebocyte Lysate (LAL), which is used to test vaccines, drugs, and medical devices for bacterial contamination (Liu & Passaglia, 2009). Additionally, horseshoe crabs have cultural and economic importance, as they are used in some fisheries for bait and are a focus of conservation and ecotourism efforts. By sustaining ecosystems and organisms throughout multiple niches and supporting human health, the Atlantic horseshoe crab truly represents a keystone species that is depended upon by entire ecosystems to help them remain stable.

Hundreds of Atlantic horseshoe crabs gather during a spring spawning event (The Nature Conservancy)

References:

Rudkin, D.M., Young, G.A. (2009). Horseshoe Crabs – An Ancient Ancestry Revealed. In: Tanacredi, J., Botton, M., Smith, D. (eds) Biology and Conservation of Horseshoe Crabs. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-89959-6_2

Reshetiloff, K. (2017, May 18). Migratory birds shore up appetites on horseshoe crab eggs. Bay Journal. Retrieved from https://www.bayjournal.com/columns/bay_naturalist/migratory-birds-shore-up-appetites-on-horseshoe-crab-eggs/article_072a00d6-9696-5f4a-a48c-427ff747ac88.html

Liu JS, Passaglia CL. Using the horseshoe crab, Limulus Polyphemus, in vision research. J Vis Exp. 2009 Jul 3;(29):1384. doi: 10.3791/1384. PMID: 19578331; PMCID: PMC2798857.