permit the insertion of the snail's siiell margin. 

 Once opened, the clam is quickly consumed, 

 leaving a dead pair ot articulated shells marked 

 with a clear clue to the cause of the clam's demise. 



Many species of marine epifauna, such as 

 some barnacles on rocky shorelines (Connell 

 1970), possess a refuge from predation by gastro- 

 pods and other predators in large size classes. 

 Some sizes of prey are too large for the predator 

 to handle. Such refuges are important in insuring 

 the stability of prey populations in the face of 

 what are often very efficient predators. Little 

 work has been done to establish whether soft- 

 sediment infaunal species generally possess an 

 analogous escape from their predators as a result 

 of growing to large sizes. Whelks, however, take 

 all sizes of Mercenaria and Chione , even the 

 largest. Although hard clams can grow large 

 enough to become invulnerable to predation by 

 blue crabs, whelks more than compensate for the 

 decline in blue crab predation and can denude an 

 unvegetated sand flat of its hard clams in short 

 order. Whelks do not extend in distribution into 

 the high intertidal zone or into brackish waters, 

 so clams have a refuge there from whelk preda- 

 tion. 



Other predatory gastropods can occasionally 

 be found on the lower margins of intertidal flats 

 in North Carolina. Tulip snails, including the true 

 tulip {Fasciolaria tulipa) and the banded tulip {F. 

 hunteria), are residents of tide flats. Both of these 

 species prey upon the benthic infauna and possess 

 extremely varied diets (Paine 1963). Other gastro- 

 pod predators found in this environment include 

 the baby's ear (Sinum perspectivum) and the 

 moon snail {Polinices duplicatus). Moon snails in 

 North Carolina also have a broad diet which 

 includes some clam species. A moon snail con- 

 sumes a clam by using its radula to rasp a circular 

 hole near the umbo of the clam. These character- 

 istic holes often mark the shells of dead clams and 

 clearly indicate the cause of death. 



In addition to the blue crab and the horseshoe 

 crab, a number of additional types of crabs roam 

 an intertidal flat at high tide. In North Carolina, 

 perhaps the most abundant and evident of these 

 remaining species are the hermit crabs {Pagurns 

 longicarpus and Petrochirus diogenes), which 

 carry on their backs the shells of moon snails, 

 mud snails, and whelks. The larger the crab, the 



larger the shell it carries, such that little Pagurus 

 longicarpus is almost always found in the smallest 

 gastropod shells, those from mud snails (Finn 

 1973). Hermit crabs are apparently omnivores, 

 acting as both predators and scavengers in estu- 

 arine systems. Various small grapsid mud crabs 

 are locally abundant predators on intertidal flats. 

 As one moves closer and closer to the equator, 

 the importance of xanthid crabs gradually in- 

 creases. North Carolina has a number of tropical 

 faunal elements, including several xanthid crabs, 

 during the summertime. 



One major group of mobile epibenthic inver- 

 tebrates remains to be discussed: the shrimp. 

 While shrimp are generally considered residents of 

 slightly deeper waters where marine grasses are 

 common, many shrimp do forage up on intertidal 

 flats at higher tides. Grass shrimp [Palacmonetes 

 spp.) probably remain fairly well restricted to 

 vegetated habitats, but other shrimp types such as 

 the penaeids, alpheids, and mysids range widely. 

 The brown shrimp (Penaeus aztecus) is the most 

 common penaeid in North Carolina waters. Juve- 

 niles enter the estuaries in spring where they grow 

 and develop in shallow nursery areas usually in 

 relatively muddy habitats far up small tidal creeks. 

 By the end of summer when they have grown and 

 matured, they migrate into the deeper waters of 

 the sounds and ultimately into the ocean (Will- 

 iams 1955). Penaeid shrimp are apparently both 

 predators on small shallow-burrowing infauna and 

 also detritivores (Williams 1955). Most of their 

 energy is presumably derived from consuming the 

 microbial flora on Spartina and seagrass detritus. 

 Mysid shrimp, es\:)ec\A\\\ Neomysis americana, dxc 

 commonly found on intertidal flats south of 

 Chesapeake Bay feeding on detritus (I). ,\llen, 

 Pers. Comm. Univ. South Carolina, October 1979). 

 Snapping shrimp {Alpheus spp.) are often found 

 over flats. They are strictly predatory. 



A few other even more sul)tidal species of 

 mobile invertebrates can occasionally be found in 

 the lower intertidal zone of flats when the tide is 

 in. For instance, in fine sand sediments the sand- 

 dollar, Mellita quinqiiiespcrforata, can be an 

 abundant deposit feeder. Mellita forages just 

 below the sediment surface by plowing along at a 

 relatively rapid rate. It presumably consumes ben- 

 thic microalgae and organic deposits, but because 

 of its large size and high activity rate it must also 

 have an important impact on the juveniles of many 



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