enough to be conrercially harvested. The 

 initial formation of these beds on tidal 

 flats is dependent upon the existence of a 

 hard substrate such as stones, mollusc 

 shells, or other debris. After establish- 

 ment, other mussels settle and the bed 

 spreads laterally forming a copiplex mat of 

 sediment, shell debris, and animals. The 

 mussel beds provide a stable substrate 

 upon which other sessile epifauna attach 

 as well as serving as protection for 

 mobile epifauna and infauna. Lee (1975) 

 found many species of annelids, molluscs, 

 and crustaceans associated with mussel 

 beds in Long Island Sound. New England 

 tidal flat mussel beds have not been well- 

 studied and in some areas may be ephemeral 

 features of the habitat. Field (1923) 

 indicated that many beds in Long Island 

 Sound only last two to three years. Be- 

 cause of the limited availability of firm 

 substrate for attachment, physical dis- 

 turbance such as ice, storm waves, and 

 accreting sediment contribute to the tem- 

 poral instability of mussel beds. 



The mobile invertebrate epifauna com- 

 prise two taxonomic groups — arthropods and 

 molluscs (Table 5). Both groups exhibit 

 low habitat specificity although predatory 

 gastropods are found in sandy areas where 

 their preferred prey items (bivalve mol- 

 luscs) reside. Distribution and activity 

 patterns of these epifauna are affected by 

 seasonal changes in water temperature. As 

 water temperature declines in the fall, 

 all the crustacean species migrate into 

 deeper water where many burrow into the 

 subtidal sediment and become semi-torpid. 

 The gastropods are apparently less sensi- 

 tive than arthropods to low temperatures 

 and tend to remain on tidal flats until 

 the beginning of ice formation. In rela- 

 tively mild winters, some species do not 

 migrate into deeper water. 



The receding tide may reveal large 

 populations of gastropods on New England 

 tidal flats. In high intertidal areas, 

 concentrations of common (Littorina lit- 

 torea ) and rough ( Littorina saxatilis ) 

 periwinkles are often found. These gas- 

 tropods are herbivorous and are often seen 

 scraping the sediment surface for micro- 

 algae or grazing on pieces of Ulva and 

 Enteromorpha . Another species found in 

 this area is Hydrobia totteni . This minute 

 gastropod browses upon sediment particles 



consuming microalgae and associated micro- 

 organisms. Although abundant on many tidal 

 flats, it is often overlooked because of 

 its small (2 to 4 mm) size. 



Extremely large and often dense 

 aggregations of the mudsnail, Ilyanassa 

 obsoleta , frequent New England tidal 

 flats. This species displays catholic 

 feeding behavior ranging from strict her- 

 bivory to carnivory (Brovm 1969; Connor 

 1980). Aside from the snail's impact on 

 the benthic microalgal community (Chap- 

 ter 2), several authors have documented 

 the effects of its feeding and sediment 

 disruption upon the benthic infauna. Move- 

 ments by Ilyanassa reduce the abundance of 

 nematodes (Nichols and Robertson 1979) and 

 the infauna associated with amphipod tubes 

 (Grant 1965). Snail enclosure experiments 

 conducted at Barnstable Harbor, Massachu- 

 setts, resulted in pronounced decreases in 

 the infauna particularly newly settled 

 juveniles of near-surface dwelling poly- 

 chaetes (Whitlatch unpublished data). 

 Boyer (1980) has shown that the mudsnail 

 decreases stability of the sediment-water 

 interface. Ilyanassa migrates into deeper 

 waters during the winter and reappears 

 each spring. Brenchley (1980) feels that 

 this migratory pattern may be altered by 

 the presence of Littorina littorea which 

 may also interfere with the reproductive 

 activities of Ilyanassa . 



Several species of mollusc-eating 

 gastropods are common in southern New Eng- 

 land. The most abundant is the moon snail, 

 Polinices duplicatus ; this active predator 

 leaves distinctive circular bore holes in 

 the shells of its victims. Edwards and 

 Huebner (1977) concluded that Pol in ices 

 eats only living prey items and prefers 

 the soft-shelled clam, Mya arcnaria. 



Wiltse (1980) demonstrated the influence 

 of the snail's foraging activities on the 

 infauna using caging experiments in the 

 field. When snails were excluded from 

 cages, increased numbers and diversity of 

 both prey (molluscs) and non-prey (anne- 

 lids, sipunculids) species were found 

 inside the cages. The snail's influence 

 was both through direct consumption of 

 prey items and indirect disruption of the 

 upper few millimeters of the sediment sur- 

 face as it plowed along in search of food. 

 Boyer (1980) found that the foraging 

 behavior of Polinices destroyed blue-green 



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