Happy International Horseshoe Crab Appreciation Day! In celebration of these wonderfully weird animals, our Critter of the Month is among Chesapeake’s most ancient critters, the American Horseshoe Crab, Limulus polyphemus. While these remarkable creatures inhabit Gulf and Atlantic waters from the Yucatan to Nova Scotia, and with the epicenter of their breeding activity famously concentrated along the beaches of Delaware Bay, the Chesapeake Bay too serves as an important stronghold for these living fossils. Horseshoe Crabs may be found throughout the Chesapeake Bay’s waters wherever salinity exceeds 6 parts per thousand, playing a valuable ecological role.
The pair of Horseshoe Crabs in this artwork were observed by Julie Trommatter on the beach near the PhytoChop Observatory at the Horn Point Laboratory on the Choptank River. A modest number of spawning Horseshoe Crabs and their arguably cute juveniles, many just 1 inch in diameter, are regularly observed on this beach.
An Extraordinary Spectacle on Modern Shores
If you live anywhere near Delaware Bay, you have likely witnessed the extraordinary spectacle of the annual Horseshoe Crab spawning. When evening tides roll in during May and June’s full moons, thousands of these arthropods storm the beaches in a spectacular ritual. The first horseshoe crabs emerge at high tide looking like something out of a sci-fi novel, like prehistoric barnacle-encrusted helmets tossed by the surf, yet steadily making their way to the shore with determined purpose.
The larger females often arrive towing several smaller male suitors. Once ashore, each female may lay thousands of eggs, returning multiple times during high tides to deposit as many as 80,000 eggs over several weeks. From above, you can best appreciate their namesake feature, their horseshoe-shaped prosoma (front section) that houses vital organs, chelicerae that push food into their mouths, and 5 pairs of legs. Their opisthosoma (middle section) attaches by a hinge to the prosoma and contains gills for respiration, while their long, and fearsome-looking but actually harmless spike-like telson serves as both rudder and righting mechanism when they get knocked over in the surf.
Despite their common name, Horseshoe Crabs are not actually crabs at all, but are the closest living relatives of the ancient trilobites, more closely related to spiders and scorpions than true crabs. Fossil evidence of their distinctive body plan dates back to an estimated 450 million years ago, 200 million years before the first dinosaurs. The direct ancestor to today’s modern species probably originated in European waters some 100 million years ago. As ancient shallow seas disappeared, one group of horseshoe crabs appear to have migrated east to become today’s three Indo-Pacific species, while another group ventured west to establish the American species, Limulus polyphemus. This timeline highlights their remarkable persistence, surviving both the End-Permian mass extinction that wiped out more than 95% of marine species, and the end-Cretacious mass extinction that terminated the dinosaurs. Horseshoe crabs are therefore frequently regarded as “living fossils”.
Ecological Connections Across Space and Time
Our modern Horseshoe Crabs are woven into ecological relationships that can span continents. For example, in the lower Chesapeake Bay, adult Horseshoe Crabs were identified as preferred prey of Loggerhead Turtles in the early 1980s, though these turtles apparently shifted their diets to blue crabs and then to fin fish in the decades since, potentially in response to diminishing Horseshoe Crab abundance.
The best established ecological relationship involves Horseshoe Crab eggs serving as a vital fuel station for migratory shorebirds along the Atlantic flyway, particularly in Delaware Bay. The Red Knot, itself listed as a threatened species, exemplifies a possibly deeply connected evolutionary history. These long distance travelers overwinter in South America and migrate to Arctic breeding grounds, timing their Delaware Bay stopovers to coincide with peak Horseshoe Crab spawning. During their critical refueling stop, Red Knots may consume hundreds of thousands of Horseshoe Crab eggs, replenishing their fat stores essential for their final flight to their nesting grounds.
Blue Blood
Horseshoe Crabs also possess extraordinary blood, rich blue or blue-green in color due to hemocyanin, a copper-containing compound, rather than the iron-rich hemoglobin that makes our blood red. (Think of the color of old oxidized copper roofs.) The Horseshoe Crab’s unique blood also has properties that have made it highly valuable and sought after for medicine. Their blood contains specialized cells, amebocytes, that produce Limulus Amebocyte Lysate (LAL), a protein which clots rapidly when it encounters bacterial toxins (“endotoxins”). Since the 1970s, LAL has been the global standard for testing medical devices and injectable medicines including vaccines for endotoxin contamination. Growing demand for LAL is sparking research into synthetic alternatives, as sustainable supply from wild Horseshoe Crabs cannot meet future medical demand.
Conservation Challenges and The Science of Tracking Gene Flow
While surveys within the past decade found that Horseshoe Crab populations have been stable in Delaware and Chesapeake Bays, the species is listed as vulnerable to extirpation (local extinction) in some regions, particularly at the extreme end of their range where populations are smaller and more isolated. Primary threats include habitat degradation from coastal development and shoreline hardening that eliminates their access to the sandy beaches essential for spawning. Additional pressures come from harvesting for bait and biomedical use.
Scientists use multiple approaches to understand the movement of these ancient creatures. Tagging programs help track individual Horseshoe Crabs movements, for example, revealing that adults typically inhabit deeper continental shelf waters outside the breeding season before migrating to shallow spawning grounds. DNA analyses can be used to track genetic connectivity between regions, which is invaluable for managing populations. For example, genetic studies have revealed that populations can be genetically distinct, such as those from Delaware Bay and Chesapeake Bay, despite their close physical proximity. Interestingly, mitochondrial DNA, which is inherited maternally, shows limited connection between populations, while nuclear DNA, which is inherited from both parents, reveals more mixing, identifying that male Horseshoe Crabs exhibit more movement and genetic dispersal (the biological term for this is “vagility”). This sex-biased pattern in movements has important implications for their management. It means that if a local population disappears, natural recolonization may be difficult despite male movement, since females tend to return to their natal beaches.
Timing your Encounter with these Ancient Oddities
The peak viewing season of American Horseshoe Crab spawning runs from May through early June, with the best timing during evening high tides that coincide with full moons, and among the best viewing locations found on protected beaches in Delaware Bay, such as Slaughter Beach and the DuPont Nature Center at Mispillion Harbor.
If you missed the spectacular event this year, the best time to mark your calendar for next year is now! The Littoral Society runs a Volunteer Horseshoe Crab tagging program. In Maryland, the MD DNR seeks community support in identifying horseshoe crab spawning habitat and tag reports.
Looking for More Information?
Botton, M.L. (2009). The Ecological Importance of Horseshoe Crabs in Estuarine and Coastal Communities: A Review and Speculative Summary. 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_3
Smith, D.R., Brockmann, H., Beekey, M.A. et al. 2017. Conservation status of the American horseshoe crab, Limulus polyphemus: a regional assessment. Rev Fish Biol Fisheries 27, 135–175 (2017). https://doi.org/10.1007/s11160-016-9461-y=
Walls, Elizabeth A., J. Berkson, and S. A. Smith 2002. The Horseshoe Crab, Limulus polyphemus: 200 million years of evolution, 100 years of study. Reviews in Fisheries Science.
