western Maine, apparently account for the differences. The factors that 

 trigger spawning have not been clearly defined, although spawning has been 

 induced in culture by cyclic fluctuation in water temperature (Stickney 

 1964a). 



Approximately 3 million eggs a year can be produced by a clam that is 2.4 to 

 3.2 inches (60 to 80 mm) in length. Gametes are released into the water 

 through the exhalant siphon. Larvae are pelagic for 12 days in laboratory 

 conditions (Stickney 1964a) and perhaps longer in natural conditions. In 

 nature the larvae are subjected to the biotic and abiotic stresses of the 

 pelagic environment. They are also carried by water currents, which 

 ultimately determine their distribution. 



After about 2 weeks the larvae undergo metamorphosis and attach to the 

 sediment surface by byssal threads. Upon attachment the animals are 

 considered juveniles. Growth in the first summer ranges between 0.2 and 0.4 

 inches (5 and 10 mm) in coastal Maine (Stickney 1964b). Growth in winter is 

 slowed by a decrease in food supply as well as lower temperatures. In 

 Massachusetts, natural mortality rates for dense populations after settlement 

 are estimated to be 70 to 80% per year (TRIGOM 1974). 



The burrowed clam obtains its food and oxygen by flushing water through 

 siphons, which are extended above the sediment surface. This action also rids 

 the clam of body wastes. The animal may also adjust its siphon and take in 

 bottom sediments for food, thereby feeding for longer than the period when it 

 is covered with water. 



Habitat Preferences 



Clams of commercial size are most abundant in the lower one-third of the 

 intertidal zone; however, they are less abundant at the mean low water line 

 (personal communication from W. R. Welch, Maine Department of Marine 

 Resources, Augusta, ME; November, 1979). Optimal growth rates of soft-shell 

 clams in coastal Maine occur in salinities of 15 to 32 ppt. 



Pelagic larvae live in the water column of the estuarine and nearshore marine 

 systems. Juveniles live in small patches of sediment found in almost every 

 type of coastal aquatic habitat, whereas most adults are found in intertidal 

 unconsolidated sediments. Adults have been found to live subtidally in upper 

 reaches of estuaries, where temperature and salinity regimes may be 

 unfavorable to their predators (Larsen and Doggett 1978b). Most of the 

 commercial production comes from intertidal mud and sand flats. Adult clams 

 are present in low abundances in dense clay which is found under the silt-clay 

 surface of most mud flats, in sediment pockets on rocky shores, and among the 

 roots of marsh grasses ( Spartina alterniflora ) in emergent wetlands. It is 

 more difficult for predators to attack clams in these areas (TRIGOM 1974) and 

 therefore, these clam populations are potentially a source of larvae that may 

 replenish the flats. 



Factors of Abundance 



A number of natural factors contribute to fluctuations in soft-shell clam 

 abundance. Among natural factors, predation is the most readily recognized. 

 Diving ducks, bottom feeding fish, horseshoe crabs, boring gastropods, 



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