POPULATIONS OF HORSESHOE CRABS, LIMULUS POLYPHEMUS , 

 ON THE NORTHWESTERN ATLANTIC CONTINENTAL SHELF 



Mark L Botton' and John W Ropes^ 



ABSTRACT 



This report analyzes the distribution and abundance of horseshoe crabs, Limulus polyphemus (L.), on 

 the Middle Atlantic continental shelf, based on Northeast Fisheries Center (NEFC) bottom trawl and 

 ocean clam surveys of the past two decades at depths beginning at 9 m. Crabs were collected from 

 North Carolina to southern New England, with the highest abundance and frequency of occurrence 

 between Virginia and New Jersey. Approximately 90% of the minimum estimated standing stock size 

 in that area, 2.3-4.5 million individuals, was located at depths <30 m. The geographic distribution 

 may reflect proximity of this shelf region to the principal spawning areas in Delaware and 

 Chesapeake Bays. Seasonally, horseshoe crab abundance on the shelf declined during those months 

 when spawning in estuaries peaked. Crabs were caught at depths to 290 m, the limit of sampling; 

 most of the animals caught at depths >100 m were off Cape Hatteras, North Carolina. Despite the 

 presence of horseshoe crabs in estuaries as far north as Maine, New York is the northward limit on 

 the shelf. This suggests that inshore populations in New England may be relatively isolated from each 

 other and from the large Middle Atlantic shelf population. 



Generalizations about the natural history, behav- 

 ior, and ecological importance of horseshoe crabs, 

 Limulus polyphemus (L.), are primarily based on 

 studies of the shallow-water phase of its life cycle 

 (Shuster 1979, 1982; Wells et al. 1983). The popu- 

 lations in the mid-Atlantic region are most acces- 

 sible in late spring and early summer, when 

 adults spawn en masse on sandy estuarine 

 beaches. Knowledge of behavior (Shuster 1950; 

 Rudloe 1980; Barlow et al. 1982; Cohen and 

 Brockmann 1983), orientation (Rudloe and 

 Herrnkind 1976; Botton and Loveland in press), 

 morphometries (Shuster 1955; Riska 1981), sedi- 

 ment disturbance (Woodin 1978, 1981), and pre- 

 dation (Smith and Chin 1951; Smith et al. 1955; 

 Botton 1984a, b) is all based on studies of shallow- 

 water or intertidal individuals. Population esti- 

 mates have been restricted to shallow-water 

 adults (Baptist et al. 1957; Sokoloff 1978; Rudloe 

 1980; Shuster and Botton 1985), with the excep- 

 tion of Botton and Haskin (1984), who surveyed 

 the population on the inshore New Jersey conti- 

 nental shelf. 



Perhaps because of the spectacular intertidal 



^Division of Science and Mathematics, Fordham University, 

 College at Lincoln Center, 113 West 60th Street, New York, NY 

 10023. 



^Northeast Fisheries Center Woods Hole Laboratory, Na- 

 tional Marine Fisheries Service, NOAA, Woods Hole, MA 

 02543. 



Manuscript accepted July 1987 



FISHERY BULLETIN: VOL. 85, NO 4, 1987. 



mass spawning phenomenon and accessibility, es- 

 tuarine populations have received a dispropor- 

 tionate amount of attention by ecologists. In con- 

 trast, most of the animal's life is spent 

 sublittorally. An adult female may spawn com- 

 pletely over several successive high tides; in gen- 

 eral, repeated breeding is more characteristic of 

 males (Rudloe 1980). Juveniles, during their first 

 and second summer, are abundant on intertidal 

 flats (Shuster 1955, 1979), but the remainder of 

 the species' 14-19 year life span (Ropes 1961) is 

 spent subtidally except for the annual spawning 

 migration. 



This report summarizes latitudinal and bathy- 

 metric distributions of horseshoe crabs on the 

 northwestern Atlantic continental shelf. North- 

 east Fisheries Center (NEFC) bottom trawl and 

 ocean clam surveys during the past two decades 

 have provided extensive data on the abundance 

 and distribution of horseshoe crabs, principally 

 north of Cape Hatteras, NC (Ropes et al. 1982; 

 NEFC unpubl. data). Seasonal and annual trends 

 in abundance are also discussed in the present 

 report. Concern for evaluating the general popu- 

 lation characteristics of horseshoe crabs parallels 

 expanding commercial exploitation of the species. 

 Animals are presently harvested to extract blood 

 for the Limulus amoebocyte lysate (LAD test, 

 and as bait in eel (Anguilla rostrata), conch 

 (Busycon sp.), and other fisheries (Pearson and 

 Weary 1980); the vast majority of the fishing ef- 



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