words, the history of which species first abun- 

 dantly colonize an infaunal area is extremely 

 important and determines, in part, which species 

 will continue to occupy that area. 



In the intertidal sand and mud flats of North 

 Carolina, infaunal densities are rarely high enough 

 to produce exclusion of one type of species by 

 the adults of another. The activities of adult in- 

 fauna are nonetheless important in determining 

 infaunal abundances on these intertidal flats. One 

 group of deposit feeders, the tunnel feeders, are 

 extremely abundant on the high salinity flats of 

 North Carolina. This group processes and turns 

 over tremendous quantities of sediment, probably 

 causing the mortality of vast numbers of coloniz- 

 ing lar\ae. On intertidal sand flats, the enterop- 

 neust Balanoglossus aurantiacus is the most com- 

 mon funnel feeder. Although its density is often 

 only about one to four individuals per square 

 meter, the worm is about 75 cm long and processes 

 about 140cm^ of sediment daily during the warm 

 months (B. Duncan, Univ. North Carolina, Chapel 

 Hill, unpub. data). The funnel feeders such as the 

 Balanoglossus can be recognized by their cone- 

 shaped feeding funnels on the sand flats and by 

 the coiled extrusions of feces, which lie in mounds 

 spaced at some distance away from the feeding 

 funnels. 



By sampling the sediments falling down the 

 feeding funnels of Balanoglossus and the fresh 

 fecal extrusions, Duncan (unpub. data) demon- 

 strated that this enteropneust consumes the most 

 abundant mciofaunal taxon, the nematodes, very 

 efficiently. Balanoglossus probably also consumes 

 larvae and other infaunal taxa, and it certainly 

 causes the mortality of large numbers of other 

 larval and juvenile forms which it does not digest. 

 Such mortality effects are a common result of 

 large-scale sediment processing (Myers 1977a, b). 

 Partly as a consequence of this mortality, the in- 

 faunal densities on intertidal fine sand flats in 

 North Carolina are kept very low. Balanoglossus 

 is effectively excluding potential competitors while 

 at the same time processing sediments for its own 

 dietary needs. In addition to the fimncl-feeding 

 enteropneust, Balanoglossus, another major sedi- 

 ment processor coexists on inlcrlidal sand tlats in 

 North Carolina. Li'ptosvnapla tenuis, a vermiform 

 deposit-feeding echinodenn, is also very abundant 

 in this environment and has similar high rates of 

 sediment processing. Powell (197 7) estimated 



Lcptosynapta densities on one intertidal Bogue 

 Scjund, North Carolina, sand flat to be about one 

 individual per square meter and its sediment proc- 

 essing rate to be IGOOcm^ per individual per year. 

 This level of activity probably has a similarly high 

 impact upon the survivorship of other colonizing 

 infauna and helps to explain why the intertidal 

 sand flats of North Carolina contain relatively low 

 densities of infauna. 



.As the mud content of intertidal flats in- 

 creases, two other large-scale sediment processors 

 tend to replace Balanoglossus and Leptosynapta. 

 An<}thcr funnel feeder, the lugworm Arenicola, 

 and another enteropneust, Saccoglossus, are 

 extremely common on most mud flats in North 

 Carolina. Arenicola is well known world-wide for 

 its high rates of sediment reworking and for its 

 effects at limiting the abundances of most other 

 infauna (Reise 1978). .\gain the high density of 

 these deposit feeders may well help to explain the 

 relatively low density of other infauna on the 

 intertidal mud flats of North Carolina. .-Irt'n/co/a, 

 like Balanoglossus, is large, up to 0.5m in length, 

 and also lives fairly deep in the sediments, usually 

 below the 30-cm level. The feeding funnels of 

 Arenicola are occasionally distinguished from 

 those of Balanoglossus by the presence of Areni- 

 cola's long gelatinous egg case, attached at one 

 end to the burrow and trailing out into the cur- 

 rents. 



The infauna of intertidal flats in North Caro- 

 lina follows a very general pattern found broadly 

 among the benthic infauna of shallow bottoms. 

 The infauna on un\egetated flats is far less abim- 

 dant than the infauna of marine grass beds at the 

 same tidal iieight (Williams and Thomas 1967, 

 VVarme 1971, Thayer et al. 1975a, Orth 1977, 

 Reise 1977a, C. H. Peterson unpub. data for Bogue 

 Soimd, North Carolina). This difference in infatm- 

 al abundances is usually \ery great, often up to 

 two orders of magnitude. Three general explana- 

 tions exist to explain this rcpeatable abundance 

 pattern (Heck and Wctstone 1977). First, the 

 inlauna of grass beds may get more food because 

 as the grass blades baflle and slow the passing cur- 

 rents, the rate of organic deposition increases. 

 This (Mganic matter is food for several species of 

 sus])ension feeders while it is still suspended in 

 the water column. .-Xfler it is deposited on the 

 bottom, it Ijtcomes available to the deposit feeders 

 as an added food supplv. Second, densities of 



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