Background
Atlantic (American) horseshoe crabs are marine invertebrates with large brown shells and hard tails. Unlike true crabs, horseshoes have six sets of legs instead of five and do not have jaws or antennas. Interestingly, they have 10 eyes distributed throughout their body, located along the top shell (carapace), tail, and near their mouth. Horseshoe crabs inhabit inshore coastal water bodies from the Northern Gulf of Maine to the Gulf of Mexico ⁹. They occur in the greatest numbers in the Delaware Bay ¹⁶. Females grow to about 2 feet in length (including tails), while males are typically  smaller ¹¹. Males can be further distinguished from females by a set of legs that resemble boxing gloves, which are used for mating. Horseshoe crabs shed (or molt) and replace their shell multiple times during their lives as they grow. They are a long-lived species, maturing at ~10 years, and living up to 20 years ¹¹. They are currently harvested as bait for American eel and whelk fisheries, as well as for scientific and medical purposes ¹³ — their blue, copper based blood is harvested for a pharmaceutical product which can be used to detect certain bacteria in vaccines ⁹.

Spawning migrations typically occur in late spring, and horseshoe crabs are found in Massachusetts sound and bay habitats throughout the summer ¹. Spawning occurs along protected sandy beaches ⁸, peaking on full and new moons during May and June. During migration, horseshoe crabs use incoming tides to carry them to spawning beaches, often burrowing in sand to rest. Eggs are buried in nests made just above mean tide line ¹². Factors that may signal spawning include photoperiod (the length of day), water temperature, tide height, and wind direction ⁹. Horseshoe crabs eat marine invertebrates such as razor clams, surf clams, soft-shelled clams, and various species of worms ^1,11. Predators of juvenile horseshoe crabs include crabs, shrimp, and small fish, while larger horseshoe crabs are consumed by sharks, sea turtles, and seagulls ¹⁶. Horseshoe crab eggs play an important ecological role by providing food sources for many organisms, including migratory birds such as red knots ⁶.

Spawning populations of horseshoe crabs have declined throughout their Atlantic range; in particular, populations along Cape Cod, Massachusetts have declined by as much as 80% and spawning activity by 95% since the mid-1980s ¹⁶. Although drivers of these declines are not well known, potential causes include loss of habitat and food, changes in water conditions, increased predation, and harvest. Massachusetts has designated areas prohibiting harvest (closures) in an effort to restore populations, however horseshoe crabs continue to decline ^10,16.

Climate Impacts
Climatic change, combined with recent human impacts such as overharvesting, has been shown to drastically alter horseshoe crab populations ⁴. Horseshoe crabs rely on environmental cues, such as water level, temperature, and the cyclical patterns of tides, to initiate important life stages ³. In particular, larval growth and development vary with temperature and salinity ⁷. Additionally, projected increases in temperature will likely change juvenile movement patterns as well adult spawning success ⁹. Sea level rise and changes in hydrology from increased precipitation and storm surge will also impact important beach and marsh spawning habitat ¹⁵. Horseshoe crabs prefer sand that is at least 4 inches (10 cm) deep for spawning ¹⁷.

As sea levels rise, horseshoe crabs will likely have to spawn further upland to successfully reproduce above the mean high tide line. However, the extent to which beach loss and degradation due to sea level rise and erosion will impact horseshoe crabs is still largely unknown ¹⁸. In Delaware Bay, horseshoe crab habitat suitability was found to be lower with a 3.9-ft sea level rise compared to a 2-ft sea level rise, although this did not take into account the potential for beach migration ¹⁹. Wind and wave activity have also been shown to impact horseshoe crab spawning success, which may change in the future ¹⁴. Hurricane Sandy was largely responsible for destroying over 70% of optimal habitat for horseshoe crabs on the New Jersey side of Delaware Bay ²⁰. Horseshoe crabs use a diversity of beach habitats, which may provide options for successful spawning, provided these habitats are not eroding or inundated from sea level rise ¹⁴. Coastal infrastructure designed to protect inland areas from sea level rise and storms can prevent beaches from migrating inland and cause erosion to the foreshore of beaches, further reducing spawning habitat ²¹.