CHAPTER 3 

 BENTHIC INVERTEBRATES 



3.1 INTRODUCTION 



Living in close association with 

 tidal flat substrata are a variety of 

 benthic invertebrates. These organisms 

 may be extremely abundant and play major 

 roles in the tidal flat habitat. The 

 benthos are, for instance, a major link in 

 the coastal detritus-based food web. Many 

 species feed on detrital materials and 

 associated microorganisms and, by doing 

 so, accelerate the decomposition of 

 organic materials deposited on the sedi- 

 ment surface (see Chapter 2). Many of 

 these same species then serve as food for 

 bottom-dwelling fishes (Chapter 4), birds 

 (Chapter 5), as well as commercially 

 important species of crabs. In addition, 

 as the mobile benthos forage on or burrow 

 through the sediment, they promote sedi- 

 ment mixing. Biologically-induced sediment 

 mixing (bioturbation) has the potential of 

 greatly modifying the biological, physi- 

 cal, and chemical properties of the sedi- 

 ments. Such activities alter sediment 

 stability, vertical profiles of sedimen- 

 tary materials, movements of organic and 

 inorganic materials across the sediment- 

 water interface, and the distribution and 

 abundance patterns of other benthic spe- 

 cies. In a recent review, Zeitzschel 

 (1980) estimated that between 30% to 100% 

 of the nutrient requirem.ents of shallow 

 water phytoplankton populations were 

 derived from sediments with the benthos 

 playing a major role in promoting regen- 

 eration and recycling of inorganic nu- 

 trients from the sediments to the water 

 column. And last, several benthic inver- 

 tebrate species are commercially and 

 recreational ly important in New England 

 (Chapter 6). 



By convention, benthic invertebrates 

 have been divided into generalized groups 

 based upon life mode. Organisms living on 

 the surface of the sediment are termed 



epifauna and most are actively mobile mem- 

 bers of the phyla, Arthropoda and Mol- 

 lusca. The infauna consist of organisms 

 that live in the sediments. These species 

 include a taxonomical ly broader group of 

 organisms ranging from small nematodes and 

 ostracods to larger annelids, crustaceans, 

 and molluscs. Categorization of benthic 

 organisms as "infaunal" and "epifaunal" 

 remains somewhat arbitrary. Many infaunal 

 species spend certain portions of time 

 foraging and reproducing on the sediment 

 surface or have been found swimming in the 

 water column in inshore areas (e.g., 

 Thomas and Jelley 1972; Dean 1978a, b; 

 Dauer et al. 1980). While the latter 

 behavior may be related to reproductive 

 and feeding activities or environmental 

 cues (e.g., changes in salinity, tempera- 

 ture, and light), much of the migrational 

 activity into and out of the sediments 

 remains unexplained and may result from 

 overcrowding or habitat degradation. 



3.2 BENTHIC EPIFAUNA 



Because of its general lack of suit- 

 able substrate for settlement of larvae, 

 there are few permanently attached organ- 

 isms living on tidal flats. Unlike some 

 areas along the Atlantic coast (Bahr and 

 Lanier 1981), extensive intertidal oyster 

 ( Crassostrea virginica ) reefs do not occur 

 in New England. Overexploitation coupled 

 with pronounced environmental variability 

 in New England probably control the upper 

 limit of intertidal distribution of the 

 oyster. The only significant populations 

 of this bivalve are found in subtidal, 

 commercially maintained areas. Blue mussel 

 ( Mytilus edulis ) beds, however, are found 

 throughout New England tidal flats (espe- 

 cially in Maine) and occur in the lower 

 elevations of the intertidal zone in dense 

 concentrations. Along some parts of the 

 Maine coast, mussel densities are high 



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