clams and other benthic fauna is usually far great- 

 er at the margins of oyster beds, even those occur- 

 ring near the high intertidal zone (Figure 1). 

 Clammers have long recognized this pattern and 

 often exploit their knowledge by clamming most 

 intensively in such locations. 



Blue crabs are especially effective predators 

 because of their great tolerance of reduced salin- 

 ity. Most other predators, including those which 

 prey upon commercially important shellfish, drop 

 out rather rapidly along a gradient upstream 

 toward lower salinities. For instance, oyster drills 

 [Urosalpinx) and seastars {Asterias) cannot toler- 

 ate brackish waters, and their prey have some re- 

 fuge from prcdation in the upper portions of the 

 estuary. This is not true for the prey of blue 

 crabs, which are abundant throughout the estuary. 

 Because blue crabs dig into the sediments to 

 depths of 6 to 8 cm both to forage and to hide, 

 they actually kill more organisms than they con- 

 sume (Virnstein 1977). This, too, adds to their 

 influence on the abundance of benthic fauna. 

 They act, to some degree, as sediment processors 

 and cause physical as well as predatory mortality. 



The blue crab is common in all of the sounds 

 and estuaries of North Carolina. Intertidal flats 

 contain more juvenile crabs than adults, which 

 prefer deeper waters. At night and during the day 

 in spring and f4ll, even adult crabs can be found 

 foraging over intertidal areas. In warmer months, 

 large blue crabs tend to remain relatively inactive 

 in deeper waters during the day. Juvenile crabs 

 remain active during summer days even in shallow 

 waters. During winter, blue crabs migrate to deep 

 channels, this time to escape the extreme cold of 

 the shallows. Only from about mid-December to 

 mid-March are blue crabs rare on intertidal flats 

 in North Carolina. 



Oyster reefs are not the only structures in soft- 

 sediment environments that protect the infauna 

 from blue crab predation. Numerous polychaete 

 species build tubes which extend into the sedi- 

 ments. The tubes of some onuphid polychaetes 

 are especially large and extend vertically up to 

 10 cm into the water column. Two species of 

 tube builders, Diopatra cuprea and Americonu- 

 phis magna, arc quite common on intertidal sand 

 flats in North Carolina. Woodin (1978, in prepara- 

 tion) has demonstrated that infaunal densities are 

 far higher in the immediate vicinity of a Diopatra 



tube on intertidal flats at Assateague Island, 

 Maryland. Abundance of infauna declines rapidly 

 with distance away from a tube. By building 

 cages to exclude blue crabs, Woodin was able to 

 demonstrate that blue crab predation was lowest 

 near the tubes where the structures inhibit digging 

 and foraging but that at some distance away from 

 the tubes the blue crab controlled infaunal com- 

 munity abundance. This pattern also appears to 

 exist among the benthic infauna of North Caro- 

 lina's intertidal flats and is probably produced by 

 the very same mechanism. Blue crabs leave small 

 pits up to 8 cm deep on intertidal flats where they 

 have been foraging (Woodin, in preparation). 

 These pits persist only 2 or 3 days; so the usual 

 pock-marked surface of North Carolina's inter- 

 tidal flats is an indication of the high activity rate 

 of the blue crab there. 



Woodin (in preparation) also performed 

 experiments with another major mobile epiben- 

 thic predator of the infauna on intertidal flats, 

 the horseshoe crab, Lh?iulus polyphemous. These 

 large crabs dig broader pits than those made by 

 the blue crab, but the horseshoe crab pits are shal- 

 lower — only about 4 cm deep. Woodin (in prepa- 

 ration) showed by experiment that Limulus has 

 an effect on infaunal densities too but not nearly 

 as great an effect as that of blue crabs. Because 

 Limulus is abundant on the intertidal flats of 

 North Carolina, it is doubtless having a substantial 

 impact there, too. Horseshoe crabs are quite sea- 

 sonal in North Carolina, with noticeable abun- 

 dances in shallow water from about April through 

 October. This corresponds to the period when 

 large shallow pits are evident on many North 

 Carolina mud and sand flats. 



Another abundant group of mobile epibenthic 

 invertebrates on North Carolina's mud and sand 

 flats is whelks, composed of species of Busycon 

 (Magalhaes 1948). Three species of these large 

 gastropods are common at the lower margins of 

 intertidal sand and mud flats. The channeled 

 whelk (Busycon canaliculatum)ieeds mostly upon 

 carrion and is commonly captured in crab pots. 

 The two other whelks, the knobbed whelk (B. 

 carica) and the lightening whelk [B. contrarium), 

 are voracious, feeding mostly on clams such as 

 Mercenaria niercenaria and the dog clam, Chione 

 cancellata. These predatory whelks feed by using 

 the lip of their shells to rasp away at the margins 

 of a clam until enough of a gap is created to 



27 



