a description of these species; also atlas map 4). In addition, flats are a 

 major feeding ground for several species of fishes, waterbirds, and waterfowl. 



Because flats have a low grade the bands of infaunal animals are broad. Since 

 the grade is so slight, often irregular, and the sediment is fine, some water 

 remains in the sediment between tides. Since desiccation is probably a major 

 factor affecting the distribution of invertebrate species and most 

 invertebrates present are infaunal, precise delineation of specific zones in a 

 flat is difficult. Sagadahoc Bay (region 2) is a good example of a low grade 

 sand flat. Here tides with an amplitude of about 3 m cover a flat that is 

 almost 4000 m (over 2 miles; 1.2 km) long (Bradley and Cooke 1959). 



In addition to tidal height a number of other factors determine species 

 distribution. Animals in these low grade areas respond strongly to the type 

 and stability of sediments, currents, wave action, salinity, and availability 

 of food (Odum et al. 1974; Sanders et al. 1962). Some of these factors are 

 related in varying degrees to tidal heights. The habitat requirements of some 

 species (e.g., amphipods and polychaetes) are so specific that the species can 

 only live on sand "waves" (Sanders et al. 1962). 



On flats some animal species are scattered throughout (exclusion), whereas 

 others tend to congregate into groups or patches (clumping). Exclusion means 

 that individuals are separated spatially, whereas clumping means the 

 individuals are grouped together spatially, often quite densely (Little and 

 Boyden 1976). The Baltic clam is an example of a species that exhibits 

 exclusion (Segerstrale 1965). Since the Baltic clam is a deposit feeder, it 

 excludes other species by eating almost anything organic (including clam 

 larvae) that comes within range of its siphon. 



Clumping is exhibited by a number of species. Some larvae select the type of 

 substratum on which to settle and/or settle near animals of their species. 

 Other animals brood their young, which accounts for their presence in the 

 vicinity of others of the same species. 



Migration of adults may play an important role in species distribution. Dauer 

 and Simon (1976) found that adult polychaetes accounted for over 90% of the 

 intertidal populations in an area that had been essentially devoid of fauna. 

 Dean (1978 a and b) has noted mass migrations of bloodworms and sandworms in 

 the Damariscotta River estuary. These migrations were in winter and not 

 related to the reported reproductive periods of these species. Yeo and Risk 

 (1979) report that adult baltic clams can migrate laterally to repopulate ice- 

 scoured or storm-damaged areas. 



Almost all of the infaunal species that live in the flats, with the exception 

 of burrowing forms such as large soft-shelled clams and adult sandworms, are 

 present in the upper 6 inches (15 cm) of sediment. Stromgren and coworkers 

 (1973) found on a Norwegianf lat that within the upper 6 inches (15 cm), most 

 of the species were present in the top few centimeters. Many were located 

 near the surface because the availability of food was greater on the diatom- 

 detritus rich surface. Some were known to migrate to the sediment surface 

 when the tide was high. The larger forms were either mobile (sandworms) or 

 had long siphons that reach to the surface (soft-shelled clams). 



5-99 



10-80 



