Finding numerous dead horseshoe crabs on beaches is common. While this sight can be concerning, natural biological and environmental factors often explain their presence. These occurrences are frequently part of their life cycle, though external factors also contribute to their stranding and mortality. Understanding these reasons provides insight into the ecology of these ancient marine creatures.
The Spawning Phenomenon
Horseshoe crabs migrate to sandy beaches for spawning, peaking during new and full moon high tides from March through July, especially May and June in regions like the Delaware Bay. They prefer protected estuarine beaches with lower wave energy to reduce stranding risk. Large numbers congregate along the shore to lay and fertilize eggs, with females laying an estimated 88,000 eggs annually in multiple clusters.
The density of crabs during spawning increases mortality. Reproduction is strenuous, and being overturned by waves causes exhaustion. Horseshoe crabs use their telson to right themselves, but damage hinders this ability.
Stranded crabs, unable to return to water, face desiccation, especially above the tide line in warm conditions. Physical injuries, like being trampled in dense huddles, also contribute to fatalities. Beach strandings during spawning cause around 10% of adult deaths in areas like the Delaware Bay.
Environmental Challenges
Beyond spawning risks, environmental challenges also cause horseshoe crab deaths and stranding. Strong storms and rough seas dislodge, injure, or push crabs onto beaches, leading to mass strandings even outside spawning periods.
Extreme tidal conditions also contribute to stranding. High tides can carry crabs too far inland, leaving them vulnerable when water recedes. Low tides might expose them to unbearable conditions.
Prolonged exposure to high temperatures, especially during low tide, can be lethal for stranded crabs, causing rapid desiccation. While horseshoe crabs can survive out of water for days if gills stay moist, direct sun and heat are detrimental. Low oxygen levels (hypoxia) in the water, sometimes from algal blooms or pollution, can also kill them, leading bodies to wash ashore.
Other Natural Causes of Mortality
Other natural factors also contribute to horseshoe crab mortality, though typically on a smaller scale than spawning and environmental events. Horseshoe crabs grow by molting, shedding their old exoskeletons. This is a vulnerable period, and some may die if they get stuck. What appears to be a dead crab is often a shed molt, distinguishable by its lighter weight and an opening where the crab exited.
Horseshoe crabs have a natural lifespan, living up to 20 years or more, with some estimates suggesting up to 40 years. Older individuals may die from age-related factors. Predation also occurs, with crabs consumed by birds or larger marine animals, though intact dead crabs from predation are less common. Diseases and parasites can affect horseshoe crab health, leading to individual deaths that wash ashore.