tors have less impact and reproductive activity is 

 again intense. Low infaunal densities characterize 

 the coldest winter months. 



Although these relatively small opportunistic 

 polychaetes and amphipods are the numerically 

 dominant macrofauna on North Carolina's inter- 

 tidal flats, some of the less abundant but far 

 larger species often contribute an overwhelming 

 proportion to the biomass. On intertidal mud 

 flats, the stout razor clam, Tagelus plebeius, often 

 makes up 90% or more of the total community 

 biomass (Holland and Dean 1977 present some 

 South Carolina data). In muddy sand flats the 

 mud shrimp, Upogebia, commonly dominates the 

 community biomass, even though its density is 

 not especially high. Despite their taxonomic dis- 

 similarity, Upogebia and Tagelus are both suspen- 

 sion feeders that live in semipermanent burrows 

 constructed in the sediments. Long-lived, tube- 

 building polychaetes also contribute substan- 

 tially to total infaunal biomass: Amphitrite 

 ornata on mud flats and Diopatra spp. on 

 sand flats. These tube builders are surface deposit 

 feeders, although Diopatra also grazes on the epi- 

 biotic growth on its tube cap. 



3.4 THE MOBILE EPIBENTHOS 



In the mobile epibenthos category are in- 

 cluded some larger, especially active deposit 

 feeders, as well as a group of predators which 

 forage on and in the sediments of intertidal flats. 

 The most obvious of these epibenthic species are 

 fiddler crabs of the genus Uca. These crabs roam 

 the intertidal zone at low tide foraging for epi- 

 benthic algae and detritus, both of which they 

 apparently ingest and assimilate (Haines 1976b, 

 Haines and Montague 1979). Three species of 

 fiddler crab are found on the intertidal flats of 

 North Carolina: Uca pugilator, U. pugnax, and U. 

 minax. In sandy areas where there is no emergent 

 vegetation near the high tide line, U. pugilator 

 is found alone. On intertidal flats where Spartina 

 is found toward the high tide line, either U. 

 minax or U. pugnax is usually present. Uca minax 

 will be dominant where the root mats in the Sf>ar- 

 tina zone are most dense, whereas U. pugnax is 

 found in areas of low or intermediate root density 

 (Ringold 1979). All three species of fiddler crab 

 construct burrows near the high tide mark. Often 

 the round entry holes and piles of spherical 

 droppings are a common sight when one first 



steps out on the intertidal zone of a North Caro- 

 lina estuary. 



Only a limited number of the mobile epiben- 

 thic species forage from land out onto the inter- 

 tidal zone at low tide like Uca. Nevertheless, 

 several amphipod species exhibit this same behav- 

 ioral trait and can be extremely abundant on 

 intertidal flats in North Carolina and elsewhere. 

 Gammaridean amphipods of the genus Orchestia 

 are especially common grazers on blue-green algal 

 mats in North Carolina. Blue-green algae are 

 usually considered to be a poor food source and 

 difficult for herbivores to digest. Yet assimilation 

 studies on similar gammaridean amphipods have 

 demonstrated efficient utilization of blue-greens 

 (Brenner et al. 1976). Numerous insect larvae 

 (often larval dipterans), mites, and nematodes are 

 also found grazing both on blue-green algal mats 

 and on the wrack, which is the dried detrital 

 material cast up at the most recent high tide mark 

 on the shoreline. For anyone who has visited the 

 pristine shorelines of a North Carolina sound or 

 estuary in summer, it should not come as a sur- 

 prise that the larvae of flies are quite abundant 

 somewhere along the shore! 



While all these mobile epibenthic consumers 

 that forage down into the intertidal zone from 

 land are herbivores or detritivores, those that for- 

 age on the intertidal zone when the tide is in tend 

 to be largely predatory. The single most import- 

 ant of these species is the blue crab, Callinectes 

 sapidus. Virnstein (1977) used cages in the Chesa- 

 peake Bay to enclose and other cages to exclude 

 blue crabs so that he could estimate their impact 

 on the benthic infauna. These experiments dem- 

 onstrated that blue crabs cause substantial mortal- 

 ity among the benthic infauna, especially among 

 the shallow burrowers which can be most easily 

 excavated and consumed by the crabs. The sedi- 

 ments offer the deep-burrowing species some- 

 what better protection against predation. Ihin- 

 shelled bivalves such as Macoma balthica and 

 Mulinia lateralis and polychaete worms are the 

 most susceptible species to blue crab predation, 

 but even thick-shelled clams are susceptible as 

 juveniles. When young, the hard clam, Mercenaria 

 mercenaria, is heavily preyed upon by blue crabs 

 (Carrikcr 1959, Menzel et al. 1976). Blue crabs 

 cannot forage effectively in the presence of shell 

 debris and other hard objects which inhibit their 

 digging. Consequently, the abundance ol hard 



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