3.2 BENTHIC INFAUNA 



Soft substrates, because of their mobility, are 

 most suitable for occupation by infaunal organ- 

 isms. The infauna are usually subdivided into 

 three groups: the microfauna, the meiofauna, and 

 the macrofauna. The distinction between the 

 groups is occasionally blurred, but basically the 

 microfauna arc the smallest benthic animals (pass- 

 ing through a 0.062mm mesh); the meiofauna are 

 small, but somewhat larger benthic animals 

 (usually those which pass through a 0.5 mm mesh 

 but are retained on a 0.062mm mesh), while the 

 macrofauna are larger (those retained on a 0.5mm 

 mesh). This size distinction also corresponds 

 broadly with some major taxonomic size breaks, 

 so that the operational definitions of microfauna, 

 meiofauna, and macrofauna do not reflect a purely 

 arbitrary decision. Among the meiofauna, those 

 taxa which never grow large enough to be retained 

 on a 0.5 mm mesh are termed the permanent 

 meiofauna. Macrofaunal juveniles which are still 

 small enough to be within the meiofaunal size 

 range are termed the temporary meiofauna. 



Microfauna include all protozoans. Ordinarily, 

 the most numerous are foraminifera and ciliates; 

 these groups can be extremely abundant on some 

 intertidal flats, but their ecological roles are poorly 

 understood. Because foraminifera produce a calci- 

 fied test which is left behind at the animal's death, 

 geologists and paleontologists have studied them 

 more intensively than have marine biologists. 



The meiofauna of intertidal sand flats differ 

 considerably from the meiofauna of intertidal 

 mud flats. Sands are by definition coarser, which 

 means that larger interstitial spaces exist between 

 adjacent particles. The meiofauna of sands are 

 largely interstitial organisms, well-adapted to 

 moving among these sediment grains. Gastrotrichs 

 and turbcllarians arc essentially restricted to the 

 sand environment (Mclntyre 1969). The intersti- 

 tial spaces in sands provide oxygenation to deeper 

 sediments so that the meiofauna of sands are dis- 

 tributed over a broader range of sediment depths, 

 extending in abundance to 10cm or more. In mud 

 flats, the meiofauna are restricted to surface sedi- 

 ments. Nearly all of the individuals occur within 

 the top centimeter or in the oxygenated zone 

 which may extend slightly deeper or shallower 

 than 1 cm. Here, most of the meiofauna are epi- 

 bcnthic forms, found on top of the sediment 

 surface, or burrowing forms found just below the 



surface. These mud animals tend to have large, 

 stocky bodies, whereas meiofauna from sands are 

 smaller vermiform animals, adapted to moxing 

 among the grains (.Mclntyre 1969). 



On the intertidal flats of North Carolina, 

 nematodes are the most numerous meiofaunal 

 taxon. This pattern of abundance is apparently 

 typical of shallow marine sediments world-wide. 

 Harpacticoid copcpods are ordinarily the second 

 m(jst abundant meiofaunal taxon. Other meio- 

 faunal taxa of importance in North Carolina flats 

 arc the gastrotrichs, turbcllarians, and gnathosto- 

 mulids. Coull and FIceger (1977) in studying the 

 meiofauna of sand and mud flats of South Caro- 

 lina found that seasonal progressions of harpac- 

 ticoids occurred regularly on the mud Hats, 

 whereas harpacticoids in a sandy habitat showed 

 less predictable patterns of variation in abundance. 



The ecological role of the meiofauna is not 

 clear. At one time, most meiofauna specialists 

 believed that these groups represented a trophic 

 dead end in estuarine food chains. Several more 

 recent studies have demonstrated that various 

 consumers feed upon the meiofauna. For instance, 

 grass shrimp, Palaemonetes, greatly reduce the 

 abundance of nematodes and a meiofaunal poly- 

 chaete probably because they are preying upon 

 them (Bell and Coull 1978). Balanoglossus, a 

 common macrofaunal species in North Carolina 

 sand flats, consumes nematodes very eifectively 

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

 Pers. Comm.). Coull and Bell (1979) reviewed all 

 studies that demonstrate that certain meiofauna 

 do serve as food for higher-level consumers and 

 found that most of these studies were done in 

 muddy sediments. Attempts to demonstrate con- 

 sumption of meiofauna in sands have almost al- 

 ways failed. Coull and Bell (1979) suggested that 

 the meiofauna in muds are much more available 

 to consumers because they are densely packed 

 into the surface sediments where a consumer can 

 gather them readily by merely ingesting the top 

 centimeter of sediment. In sands, on the other 

 hand, a consumer must process a large volume of 

 sediments to harvest the sparsely distributed 

 meiofauna. Not many organisms are adapted for 

 this sort of sediment processing, which is energet- 

 ically expensive. Sand meiofauna may often be a 

 trophic dead end, whereas mud meiofauna may 

 regularly be eaten by various consumers (normally 

 by those considered to be detritivores or deposit 

 feeders). 



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