bivalves are more restricted to the near- 

 surface layers than are the burrowing an- 

 nelids. The deposit feeders exhibit a wide 

 range of feeding and mobility patterns 

 although three general life styles or 

 guilds are apparent. First is the surface- 

 feeding species. These organisms either 

 live in vertical tubes (e.g., spionid and 

 terebellid polychaetes) or burrow slightly 

 below the surface (e.g., some gammaridean 

 amphipods) feeding with appendages on or 

 slightly above the sediment-water inter- 

 face. The deposit-feeding clam, Macoma 

 balthica , an abundant species on northern 

 New England mud flats, also feeds off the 

 sediment surface with a long inhalent 

 siphon. The surface-feeding guild is the 

 most abundant group of organisms in tidal 

 flat habitats. Second in abundance are 

 the organisms that burrow through the sed- 

 iment, much like earthworms. This group 

 has the largest number of species (e.g., 

 members of the polychaete worm families 

 Capitellidae, Nereidae, Syllidae, Lumbri- 

 nereidae, Orbiniidae, Nepthyidae). Several 

 species build temporary burrow-like struc- 

 tures to the surface. Since many worms 

 live in anaerobic sediments, the burrows 

 aide in transport of oxygenated water to 

 the organism from the sediment surface. 

 Last are the "conveyor-belt species" 

 (Rhoads 1974), organisms that live head 

 down in the sediments (e.g., the polychae- 

 tes, Pectinaria gouldii and Clymenella 

 torquata ) feeding at depth and depositing 

 egested sedimentary materials on the sur- 

 face. While this feeding group is less 

 diverse and abundant than the other two, 

 the members are interesting because of 

 their impressive bioturbation activities. 

 Dense populations of Clymenella are known 

 to completely bioturbate (turn over) sedi- 

 ments to a depth of 20 cm (8 inches) 

 annually. One noticeable effect of this 

 extensive feeding activity is described by 

 Sanders et al. (1962) who state that the 

 presence of Clymenella on the Barnstable 

 Harbor, Massachusetts, tidal flats could 

 be detected by walking over areas and 

 feeling a spongy sediment underfoot. 



Suspension-feeding organisms include 

 bivalve molluscs and some species of 

 amphipods and polychaetes. Probably the 

 most abundant suspension feeder on New 

 England tidal flats is the small bivalve. 

 Gemma gemma . Densities exceeding 300,000 

 per m2 have been recorded and individuals 



are often found packed valve to valve in 

 fine-grained tidal flats. Even though 

 these are small organisms (about 3 mm), at 

 these high densities they are able to 

 effectively exclude other species of 

 suspension-feeding bivalves and surface- 

 feeding polychaetes from their habitats 

 (Sanders et al. 1962; Whitlatch unpub- 

 lished data). The clam, Mya^ arenaria , is 

 also abundant, especially in Maine, New 

 Hampshire, and parts of Massachusetts. 

 This species tends to be associated with 

 silty-sand sediments and is not usually 

 found in areas dominated by G. gemma . The 

 hard-shelled clam, Mercenaria mercenaria , 

 is generally restricted to sand flats in 

 southern New England. Abundant assemblages 

 of suspension-feeding amphipods are found 

 in northern New England (Croker 1977) 

 where they are primarily associated with 

 sandy beach habitats. 



New England tidal flat infaunal asso- 

 ciations are highly dynamic and many stud- 

 ies have noted pronounced seasonal changes 

 in species occurrence and abundance (e.g., 

 Whitlatch 1977; Dobbs 1981). Large fluc- 

 tuations in population size are attribut- 

 able to the short life span of most infau- 

 nal species (probably 1 to 3 years), sea- 

 sonal reproductive cycles, predation by 

 vertebrates and benthic invertebrates, and 

 large-scale habitat heterogeneity. Sea- 

 sonal patterns of population and community 

 change are reflected as sudden rises in 

 the densities of certain species or groups 

 of species followed by declining densities 

 over a period of weeks to months. Specific 

 patterns of seasonal change in New England 

 are tied to latitude, and increased infau- 

 nal abundance may be a response of benthic 

 organisms to seasonally-induced variations 

 in food supplies. Natural selection favors 

 individuals that reproduce at about the 

 time that food for juveniles (e.g., plank- 

 tonic plants and animals) is increasing in 

 abundance. The result of such a response 

 is temporal acceleration of birth rates in 

 response to seasonally-induced increases 

 in the availability of prey and/or nutri- 

 ents. Seasonal reduction in abundance of 

 tidal flat benthos begins about July in 

 Massachusetts (Green and Hobson 1970; 

 Whitlatch 1977) and slightly later in 

 Maine (L. Watling; University of Maine, 

 Walpole; February 1981; personal communi- 

 cation) and Nova Scotia (Levings 1976). 

 Seasonal decreases in benthic organism 



32 



