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Horseshoe Crabs, Red Knots And Humans: How One Species Affects Them All


The red knot is a shore bird species that visits the shores of Cape May on its migration route. This species has the longest migration route of any species that visits our shores. It travels from tropical and temperate conditions in the winter all the way to the Arctic for breeding, a journey that totals more than 19000 miles (Schwartz, 2016). In order to survive this journey the red knot makes a series of stops along the way in order to refuel. The red knot’s stop at the Delaware bay is the most crucial as it is the final stop before they arrive in the Arctic. The red knot when stopping at the Delaware Bay area is extremely dependent on horseshoe crab eggs as a food source. They need a sustainable food source to help them refuel on their long journey. This stopover is also important because once they reach the harsh conditions of the Arctic, food is low and they must be in good health in order to successfully breed. Horseshoe crab eggs are easy to digest and very high in lipid content which is essential for the red knot to survive it’s intense migration route. The red knot is currently listed as near threatened on the IUCN red list and unfortunately, their populations are continually decreasing.

Image 1: Horseshoe crab eggs along the Delaware Bay. Photograph taken by Madison Olszewski

Horseshoe crabs are an incredible species that has managed to live on this earth for 450 million years. Due to a number of factors, horseshoe crab populations have significantly decreased. Between 1990 and 2005, horseshoe crab populations were depleted by 88% and the egg densities overall have dropped by 98% (Niles 2009). This in turn has led to a decrease in red knot numbers by 70%(Niles 2009). It is easy to identify that red knot and horseshoe crab populations are very closely related but the issue lies in identifying the causes of this population decline. Some contributors to the population decline in horseshoe crabs involve the overharvesting of the horseshoe crab both for bait and for medical purposes.

Overharvesting of horseshoe crabs very quickly became a major issue. Between the years of 1992 and 1997, the amount of horseshoe crabs harvested increased from 100,000 to around 2 million (Niles 2009).This intense increase in horseshoe crabs harvested for bait led to a major decrease in horseshoe crab spawning and thus an immense decrease in the number of horseshoe crab eggs being produced. After this significant decrease in horseshoe crab populations, we saw a decrease in shore bird populations. Red knot populations from the years of 2003-2007 were 66% less than counts from 1998-2002 (Niles 2009).

            In recent years, horseshoe crabs have been harvested for much more than just bait. Horseshoe crabs are extremely important for medical research and development. Horseshoe crab blood has incredible antibacterial properties. Horseshoe crab blood is a very distinct blue color which is caused by hemocyanin which is copper based and helps transport oxygen throughout the horseshoe crabs body (Luntz, 2014). Amebocytes play an integral role in fighting off infections in invertebrates but the horseshoe crab has very specialized and effective amebocytes. Their amebocytes are fast reacting and can effectively react in 45 minutes, whereas in mammal species the reaction could take around two whole days (Luntz, 2014). This fast reaction is due to coaglun and this chemical has been utilized for medical testing such as testing vaccines. This has led to the harvesting of a quarter of a million horseshoe crabs. In attempts to help mitigate the overharvesting issue scientists began to only take 30% of the horseshoe crabs blood and then would release the horseshoe crab back into its habitat. While they are believed to be able to survive with this much blood, we do not know much about what happens when they are released back into their habitat but it is believed that around 10-30% do not survive (Luntz, 2014). Perhaps the biggest concerning factor for the red knot would be that females that recover from losing this much blood often breed significantly less. This then directly impacts the abundance of horseshoe crab eggs.

Climate change has significantly impacted the relationship between the red knot and horseshoe crabs. The horseshoe crabs mating is dependent on many different factors. The lunar cycle, water temperature and wave action all play a role in the mating behavior of the horseshoe crab (Schwartz, 2016).The red knot is not dependent on factors such as water temperature and wave action and depends mainly on the lunar cycle for its migration. This means as water temperatures heat up earlier in the season, or as the wave action intensifies due to climate change, horseshoe crabs could mate earlier. This causes a timing mismatch between the horseshoe crab and the red knot. If the horseshoe crab believes it is time to mate due to warmer water conditions, and the red knot has still not migrated to the Delaware bay area, then when the red knot eventually does reach the area there will be a much lower supply of horseshoe crab eggs for the red knot to consume.

Horseshoe crabs mating along the Delaware Bay. Photograph taken by Steven Olszewski

The final big question then is “what can we do?’ We can identify the decline in horseshoe populations as a threat for red knot populations and we can identify major conservation threats for the horseshoe crab so it is important we begin to take steps to mitigate the conservation threats horseshoe crabs are faced with. Mitigating the effects of climate change is the most complicated but by minimizing our carbon footprint we help to end this timing mismatch between horseshoe crabs and red knots. Regulations have been set in place to help regulate the overharvesting of horseshoe crabs. By 2004, horseshoe crab harvests had been restricted so that the annual harvest was around 600,000 as opposed to 2 million that may have been harvested before (Niles 2009). Further research regarding the survival rate of the horseshoe crab is required in order to evaluate to what level of blood can be drained from the horseshoe crab so that it may survive. There are a lot of factors negatively affecting this incredible symbiotic relationship between the red knot and the horseshoe crab and in order to keep both species populations from declining we must focus our attention on these conservation threats. 

Madison Olszewski, George Mason University

Intern at Cape May Whale Watch and Research Center

IUCN. Calidris Canutus: BirdLife International: The IUCN Red List of Threatened Species 2017: E.T22693363A111379432. International Union for Conservation of Nature, 1 Oct. 2016.
Niles, Lawrence J., et al. 2009. “Effects of Horseshoe Crab Harvest in Delaware Bay on
Red Knots: Are Harvest Restrictions Working?” BioScience, vol. 59, no. 2, pp. 153-64., doi:10.1525/bio.2009.59.2.8.
Schwartz, S. 2016. Knot your average bird: a case study of the rufa red knot in the face of climate change. Animal Law 22: 172-173.
Luntz, Steven.,2014. How Horseshoe Crab Blood Saves Millions Of Lives | IFLScienc