This species can often be seen following 

 the leading edge of flood tides over tidal 

 flats feeding on resuspended detrital 

 material and carrion. The hermit crabs, 

 Paqurus longicarpus and P_. pol licaris , 

 are abundant locally, Paqurus longicarpus , 

 found occupying Ilyanassa and Littori na 

 shells, and P^. pol licaris , preferring 

 Polinices shells, are omnivores scavenging 

 on living and non-living animal material 

 as well as detrital material on the sedi- 

 ment surface. The lady crab. Ova li pes 

 ocellatus , is frequently seen on the sand 

 flats of Cape Cod where it hides buried in 

 sand with only its eyestalks exposed. 

 Spider crabs ( Libinia emarginata and L^. 

 dubia ) and fiddler crabl ( Uca pugilator 

 and L[. pugnax ) are also locally abundant, 

 although the former two species are more 

 characteristic of eelgrass beds, while the 

 latter two species are in greatest abun- 

 dance near or in salt marsh habitats. Var- 

 ious smaller amphipods and isopods also 

 occur in both mud and sand flats. These 

 species typically burrow slightly below 

 the sediment-water interface and have been 

 categorized as infaunal organisms (see 

 Appendix I). 



3.3 BENTHIC INFAUNA 



Broad designations, based on organism 

 size, are used to distinguish among groups 

 of infaunal organisms. Confusion arises 

 because of this approach although size 

 groupings tend to correspond to taxonomic 

 groupings. Organisms that pass through a 

 64 ^m mesh sieve are termed microfauna, 

 those retained on a 300 to 500 um mesh are 

 called macrofauna, and all others are 

 designated as meiofauna. In addition to 

 the arbitrariness of sieve-size selection 

 in determining the various infauna groups, 

 many organisms pass from the meiofauna! 

 category to the macrofaunal category as 

 they grow. 



Because of the small size of micro- 

 and meiofauna and difficulties in sampling 

 them, our knowledge of these groups is 

 fragmentary and speculative. Microfauna 

 include the protozoans, especially the 

 ciliates and foraminiferans. They are 

 abundant, particularly in fine sands 

 with strong reducing properties and numer- 

 ous sulfur bacteria (Fenchel 1967). Most 



microfauna are found within several centi- 

 meters of the sediment surface although 

 Fenchel (1969) noted distinct species- 

 specific vertical distribution patterns 

 related to the redox-discontinuity layer. 

 Relatively little is known about the role 

 of microfauna in coastal ecosystems al- 

 though Barsdate et al. (1974) found that 

 detrital decomposition was apparently 

 stimulated and phosphorus cycling in- 

 creased in the presence of grazing proto- 

 zoans. Other workers have questioned the 

 overall importance of the microfauna in 

 the recycling of detrital materials 

 (Fenchel and J^rgensen 1977) recognizing 

 that microfauna may be a food source for 

 meio- and macrofauna. 



Meiofaunal populations comprise a 

 taxonomically broader group of organisms. 

 Tietjen (1969), for example, found that 

 nematodes, ostracods, harpacticoid cope- 

 pods, and turbellarian flatworms were 

 abundant in two shallow subtidal sites in 

 southern New England. Meiofaunal dis- 

 tributions are apparently controlled by 

 sediment composition. Turbellarians dom- 

 inate coarser sandy sediments and nema- 

 todes are in greater numbers in muddy 

 sediments, presumably because of the 

 increased amounts of detrital material and 

 microorganisms in muds. Most meiofauna 

 occur in the upper, well -oxygenated layers 

 of the sedim,ent (Figure 5) although nema- 

 todes have been recorded at greater 

 depths. 



As more information accumulates on 

 the marine meiofauna, biologists share a 

 greater appreciation for the ecological 

 importance of these organisms in soft- 

 sediment environments. In addition to 

 accelerating decomposition and recycling 

 of detrital materials (see Chapter 2), 

 these effects may be transmitted to higher 

 trophic levels in the detritus-based food 

 web (Tenore et al. 1977). A high degree 

 of interest has focused on the trophic 

 position of the meiofauna--questioning 

 whether they represent a trophic dead end, 

 are competitors with macrofauna for shared 

 food materials, or are a major food source 

 consumed by macrofauna. Recent evidence 

 points to the last hypothesis. Gerlach 

 (1978) estimated that foraminifera and 

 meiofauna represent 12% to 30% of the liv- 

 ing biomass in many marine sediments and 



25 



