This horseshoe crab life history was developed by the Atlantic States Marine Fisheries Commission (ASMFC) Horseshoe Crab Plan Development Team from available scientific literature and state natural resource agency documents. It has been edited to provide the reader with the basic information to understand the habitat requirements of the Limulus polyphemus species of horseshoe crab.
Horseshoe crabs are benthic (bottom-dwelling) arthropods that use both estuarine and continental shelf habitats. Although it is called a "crab," it is neither a decopod nor a crustacean; horseshoe crabs are grouped in their own class, Merostomata, which is more closely related to the arachnids. Horseshoe crabs, ranging from the Yucatan peninsula to northern Maine, are most abundant between Virginia and New Jersey, with the largest population of spawning horseshoe crabs in the world found in the Delaware Bay (Shuster, pers. comm., 1995). While adult horseshoe crabs have been found as far as 35 miles offshore, 74 percent of the horseshoe crabs caught in bottom trawl surveys conducted by the National Marine Fisheries Service (NMFS), Northeast Fisheries Center were taken in water shallower than 20 meters (Botton and Ropes, 1987a).
Migration from beaches where horseshoe crabs have been tagged vary from a few kilometers in Florida to almost 34 kilometers in Massachusetts (Shuster, 1982). Based on a tagging and recovery program, Rudloe (1980) concluded that the mean distance traveled from a breeding beach in Florida was 7.6 kilometers with a range of 3.5 and 40.7 kilometers. Similarly, Shuster (1950) reported tagged horseshoe crab movements of up to 33.8 kilometers in Cape Cod Bay, Massachusetts. Thompson (1998) reported a maximum distance covered by a tagged horseshoe crab was 4 kilometers in South Carolina. However, distance traveled may reflect movement of a fishing vessel rather than actual animal migration (Thompson, 1998).
In New Jersey and Delaware, horseshoe crab abundance decreases with distance north and south of the Delaware Bay (Botton and Haskin, 1984). Within the Delaware Bay, the largest concentration of horseshoe crabs traditionally was found along the Cape May shore of New Jersey (Shuster and Botton, 1985). Spawning densities of over 30 animals per square meter occurred on the New Jersey side of the Delaware Bay based on 1986 spawning counts along 15 meter segments (Botton, et al., 1988).
Since 1993, the majority of horseshoe crab spawning activity has occurred on the Delaware shores of the Delaware Bay (Swan, unpublished data, 1998). Annual variation in spawning concentrations may be the result of weather or habitat changes. In the Chesapeake Bay, spawning densities only exceed one per square meter on the most heavily used beaches, based on counts using similar techniques. During peak spawning, densities exceeded three per square meter on these preferred beaches (Maryland Department of Natural Resources, 1998). Rudloe (1980) and Thompson (1998) reported spawning densities in Florida and South Carolina as three and one animal per square meter, respectively.
Shuster (1979) suggested that each major estuary along the coast had a discrete horseshoe crab population, which could be distinguished from one another by adult size, carapace color, and eye pigmentation. However, no significant differences between the morphologic characteristics of discrete populations are evident, based on high variability both within and among populations (Riska, 1981). In addition, based on DNA evidence, gene flow does occur between widely separated populations, and considerable genetic variation also exists within and between populations of horseshoe crabs (Selander, et al., 1970). Saunders et al. (1986) found no evidence for genetic divergence between New England and mid-Atlantic populations based on mitochondrial DNA analysis. Larger animals and populations are located in the middle of the species' distribution (Maryland to New York) while smaller animals and populations are found in the southern and northern limits of its range (Shuster, 1982). Based on morphometric data collected in South Carolina, Thompson (1998) suggests that the greatest mean adult horseshoe crab size occurs in the South Atlantic Bight and that their average size decreases in size north and south of this area. Thompson (1998) hypothesized that larger individuals occur in the South Atlantic Bight due to optimal temperature and salinity for horseshoe crab development in this region.
The distribution of horseshoe crabs is chiefly defined by water temperature, salinity, and depth, with temperature being the limiting factor for northern ranges (Shuster, 1982). Horseshoe crabs are ecological generalists and can survive within a wide range of environmental conditions, including variations in salinity; however low salinities (less than four parts per thousand) are lethal (Pearse, 1928). The horseshoe crab is also tolerant of a wide range of oxygen levels. Physiological changes in the blood enable the horseshoe crab to survive hypoxic conditions when partially buried during spawning and when wintering on the continental shelf. In addition, they can survive hyperoxic conditions when exposed to air (Shuster, 1982).
The status of horseshoe crab populations along the Atlantic Seaboard is poorly understood due to the limited amount and inconsistency of information collected regarding stock levels. In addition, basic information regarding age and growth rates, recruitment, and population dynamics is lacking. Other than the NMFS trawl survey data, little information was collected until the late 1980s when independent spawning surveys and trawl surveys were initiated, primarily in the Delaware Bay region.
Concern over the perceived growing exploitation of horseshoe crabs has been expressed by state and federal fishery resource agencies, conservation organizations and fisheries interests. For more detailed information on stock assessment click here.