feeds by scavenging. Ilyanassa obsoleta also ingests 

 sediments and assimilates energy by this other 

 feeding method. Like may trophic distinctions, 

 that made between scavengers and deposit feeders 

 is blurred by the diversity of feeding options actu- 

 ally employed in nature. A pure scavenger may 

 not exist among the infauna of an intcrtidal flat. 



3.3 CONTROLS ON INFAUNAL 

 POPULATIONS 



Infaunal species often interact strongly 

 enough to set patterns of distribution and abun- 

 dance on an intertidal flat. Such interactions can 

 be either direct or indirect. The major form of in- 

 direct interaction among the infauna is what 

 Rhoads and Young (1970) have termed "trophic 

 group amensalism." They demonstrated that de- 

 posit feeders, because of their high level of mobi- 

 lity, tend to increase the water content of the 

 sediments they occupy. This creates a loose, floc- 

 culent layer on the sediment surface which is 

 easily resuspended and mixed into the water 

 column with the slightest degree of water turbu- 

 lence. Wave action and even the relatively slow 

 motion of tidal currents are sufficient to produce 

 such resuspension of silts and clays. 



Large quantities of sediment suspended in the 

 water column tend to clog up the filtering appara- 

 tus of suspension feeders. Consequently, wherever 

 deposit feeders are abundant, suspension feeders 

 tend to be eliminated. This process, involving one 

 trophic group and the negative effects it produces 

 upon another, is an example of trophic group 

 amensalism. Deposit feeders are more common as 

 a group in muddy sediments than in sands because 

 the finer particles (muds) have greater surface 

 area for microbial attachment and because some 

 of these fine particles are detritus itself. In other 

 words, food levels for deposit feeders are far 

 higher in muds than in sands, and the density of 

 deposit-feeding species increases accordingly. Be- 

 cause suspension feeders cannot feed or grow at 

 normal rates where deposit feeders are abundant, 

 only in sandy areas can one normally find high 

 densities of suspension feeders. This pattern of 

 complementary distribution of suspension feeders 

 and deposit feeders has frequently been observed 

 in natural infaunal distributions (Sanders 1958, 

 Young and Rhoads 1971) and appears to be exhi- 

 bited on the intertidal flats of North Carolina. 



Brenchley (1978) has carried the ideas of 

 Rhoads and Young (1970) a step further by recog- 

 nizing that any species which moves around in the 

 sediments will increase water content and stimu- 

 late resuspension of sediments. A mobile suspen- 

 sion feeder can produce this same effect; however, 

 deposit feeders tend to be more active and mobile 

 than suspension feeders, probably because most 

 deposit feeders must forage for their food while 

 suspension feeders need only sit and wait for 

 water currents to carry in their me;il. Usually, it is 

 a deposit feeder which will induce high rates of 

 sediment resuspension, as Rhoads and Young sug- 

 gest. Brenchley (1978) also showed by laboratory 

 experiments that stationary species tend to stabi- 

 lize the sediments and that the total level of sedi- 

 ment instability produced by any assemblage is an 

 additive result of all the various species inputs. 

 Seagrasses, which baffle wave action and increase 

 sedimentation rates, and large suspension-feeding 

 clams such as cockles have a substantial stabilizing 

 effect on the sediments which can lead to coexis- 

 tence of suspension feeders and deposit feeders. 

 Some long-lived and relatively large suspension- 

 feeding bivalves like the stout razor clam {Tagelus 

 plebeius) and the hard clam {Mercenaria merce- 

 naria) may play this role in North Carolina flats. 



In addition to interacting indirectly through 

 effects on sediments, infaunal species can also 

 interact directly. Adult infauna can and do feed 

 upon the larvae of potential colonizers. Woodin 

 (1976) has emphasized the importance of these 

 "adult-larval interactions" in determining which 

 types of species can coexist in marine infaunal 

 communities. Suspension feeders often capture 

 larvae of many infaimal invertebrates while they 

 are still present in the water column and before 

 settlement. Deposit feeders, on the other hand, 

 have an effect upon larval survivorship just after 

 settlement. Deposit feeders consume larvae 

 directly as part of their diet and also cause mor- 

 tality of new recruits by their physical activities 

 in processing the sediments. Larvae that survive 

 these "rites of passage" may then grow to adult- 

 hood in the community, if they avoid subsequent 

 predator attacks. Woodin (1976) argues that these 

 adult-larval interactions are strong enough in any 

 dense infaunal assemblage to limit the member- 

 ship of that assemblage to only a few of the 

 species that could potentially live there. In other 



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