FISHERY BULLETIN: VOL. 82, NO. 2 



noted in two samples and unidentified shells in 

 three. The most numerous item was Foraminifera 

 (n = 21), and no other item was found more than 

 three times. 



In a laboratory experiment, a 20.3 cm (prosomal 

 width) male horseshoe crab ate one 40.6 mm surf 

 clam; the same crab consumed two clams, 43.8 and 

 42.4 mm, several days later (see Botton 1982 for 

 procedural details). A 27.9 cm female ate two 

 clams, 46.0 and 36.2 mm. Clams of this size are 

 manipulated by the walking legs so that the ven- 

 tral shell margin is held against the gnathobases. 

 The chitinous gnathobases chip the ventral mar- 

 gin, eventually resulting in the fracture of one of 

 the valves. Cracking of the valves continues until 

 the crab is able to remove the meat from the shell 

 using the pincer-tipped walking legs or the 

 chelicerae. Ingestion of the shell of 4 cm S. solidis- 

 sima is apparently incidental. 



DISCUSSION 



A latitudinal gradient in horseshoe crab abun- 

 dance along the New Jersey coast during the 

 spring and summer months was recognized as a 

 decrease in abundance with distance north from 

 Delaware Bay, and an onshore-offshore gradient 

 was apparent in northern New Jersey. The transi- 

 tion between areas of high and low density takes 

 place between Great Egg Harbor Inlet (Ocean 

 City ) and Absecon Inlet (Atlantic City). Horseshoe 

 crabs were more abundant inshore in the late 

 spring and early summer than in the late summer 

 and fall. 



Why are adult L. polyphemus concentrated in 

 southern New Jersey, at least during the spring 

 and summer? Since Delaware Bay, in southern 

 New Jersey, contains the largest spawning popula- 

 tion of horseshoe crabs in North America (Shuster 

 1982), we believe that the distribution on the New 

 Jersey continental shelf may be related to the mi- 

 gration of deep-water crabs to those beaches for 

 reproduction. However, horseshoe crabs spawn 

 elsewhere in New Jersey and are widely distrib- 

 uted on the middle Atlantic continental shelf 

 (Shuster 1979); based on electrophoretic evidence 

 (Selander et al. 1970), there is gene flow between 

 widely separated populations. 



Hydraulic surf clam dredges are efficient 

 samplers of large benthic infauna (Meyer et al. 

 1981), but an evaluation of this dredge as a means 

 of capturing L. polyphemus is lacking. Given its 

 sluggish habits, it is unlikely that gear avoidance 



by horseshoe crabs significantly affects our re- 

 sults; indeed, much more active lady crabs, 

 Ovalipes ocellatus, are caught in large numbers 

 ( Meyer et al. 1981; Haskin, unpubl. data). However, 

 in the absence of direct observations, it is perhaps 

 best to consider our results as relative, rather than 

 absolute abundances of horseshoe crabs off New 

 Jersey. Because the temporal sequence of sam- 

 pling varied yearly and because the effect of time 

 on abundance was statistically significant, we do 

 not encourage speculation on year-to-year vari- 

 ability based on these data. 



The horseshoe crab is a dietary generalist; 

 based on the limited number of animals dissected, 

 molluscs, arthropods, and polychaetes are the 

 major food items. Although Foraminifera were 

 numerous, they are probably ingested inadver- 

 tently while digging out infauna. Opportunistic 

 foraging was shown from the Stone Harbor group, 

 which fed almost exclusively on M. edulis. Smith 

 (1953) noted that crabs could locate discrete 

 patches of soft-shell clam, Mya arenaria, but 

 the behavioral basis for patch selection is un- 

 known. 



Horseshoe crab predation may be an important 

 source of juvenile surf clam mortality. In aquaria, 

 crabs ingested only the meats of 4 cm S. solidis- 

 sima\ this implies that this species may be more 

 important as food than is apparent from visual 

 stomach content analysis, which relies heavily on 

 shell remains. Young S. solidissima may have 

 been underestimated because many small (0.5-2.0 

 mm) shells were categorized only as "unidentified 

 bivalves." Further studies of the food habits of 

 horseshoe crabs, and of the abundance and diets 

 of other predators, are necessary to evaluate the 

 importance of predation in the survivorship of 

 juvenile surf clams in New Jersey. 



ACKNOWLEDGMENTS 



We thank the many captains, mates, and Rut- 

 gers University assistants for their help in the 

 field, Michael Friedman for his aid in the statisti- 

 cal analysis, and Carl Shuster and three anony- 

 mous referees for helpful comments on the manu- 

 script. This research was supported by grants from 

 the National Marine Fisheries Service (contract 

 03-4-043-356) and the Surf Clam Inventory Fund 

 of the New Jersey Department of Environmental 

 Protection to the junior author, and from the 

 James and Anna Leathern Fund. This is publica- 

 tion #D-32503-l-83 of the New Jersey Agricul- 

 tural Experiment Station. 



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