Hard clam, continued 



sounds, and estuaries from intertidal zones to a depth 

 of 15 m or more. Although they occur in the open 

 ocean, hard clams appear to prefer relatively shallow 

 waters (Killam et al. 1 992). They are typically found in 

 waters less than 10 m deep (Sims and Stokes 1967, 

 Taylor and Saloman 1 970, Godcharles and Jaap 1 973a, 

 Godcharles and Jaap 1 973b, Killam et al. 1 992). Hard 

 clams have been collected from grass flats on the 

 shoreward side of barrier islands (Christmas and Lan- 

 gley 1973, Craig and Bright 1986), and near oyster 

 reefs (Swingle 1 971 ). In northern latitudes, Mercenaria 

 campechiensis may generally occur in deeper waters 

 with higher salinities (Eversole 1987) than does M. 

 mercenaria. 



Temperature - Juveniles and Adults: Juveniles and 

 adults can tolerate temperature extremes ranging from 

 <0° to greater then 35°C (Eversole 1987). The upper 

 lethal temperature of the hard clam is 45.2°C 

 (Henderson 1 929), but temperatures above 30°C may 

 alterclam behavior and physiology (Savage 1 976, Van 

 Winkle et al. 1 976). Growth is negligible at <1 0°C and 

 increases with rising temperatures to an optimum of 

 about 20° to 23°C (Pratt and Campbell 1956). Opti- 

 mum growth temperatures for Mercenaria 

 campechiensis texana are from 15° to 35°C (Craig et 

 al. 1988). In Florida, growth of M. campechiensis is 

 optimal from 15° to 25°C, but is reduced at tempera- 

 tures above 25°C. 



Substrate : Substrate appears to play an important role 

 in distribution and growth (Wells 1 957, Craig and Bright 

 1 986, Coen and Heck 1 991 ). Late larval stages attach 

 to hard substrates with byssal threads. If no hard 

 substrate is available, they attach to sediment par- 

 ticles. Juvenile and adult clams occur primarily in soft 

 bottom habitats of mud and sand. In one laboratory 

 experiment, settling pediveligers were reported to pre- 

 fer sand particles over mud (Keck et al. 1 974). Highest 

 natural densities of clams occur in sand with coarse 

 shell sediments, which provide spatial refugia so that 

 the juvenile clams are better protected from predation 

 (Wells 1 957, Walker et al. 1 980, Craig and Bright 1 986, 

 Killam et al. 1992). Overall, hard clams can utilize a 

 variety of unconsolidated substrates: firm sand, silty 

 sand, sand/mud, sand/shell, sand/gravel, mud/sand/ 

 gravel, and frequently near seagrasses and algae. 

 Hard clams are rare on fine silt and clay bottoms (Pratt 

 1 953, Saloman and Taylor 1 969, Taylor and Saloman 

 1970, Godcharles and Jaap 1973a, Godcharles and 

 Jaap 1973b, Kunneke and Palik 1984). 



Physical/Chemical Characteristics : 

 Temperature - Eggs and Larvae: Spawning occurs 

 generally from 22° to 30°C, with maximum spawning 

 activity found between 24° to 26°C (Loosanoff 1937c, 

 Carriker 1 961 ). Egg survival is high between 1 8° and 

 28°C (Kennedy et al. 1974, Wright et al. 1983). Egg 

 mortality at low (15°C) and high (33°C) temperatures 

 may be reduced through acclimation (Loosanoff et al. 

 1 951 ). Larvae can tolerate temperatures ranging from 

 approximately 13° to greater than 30°C with growth 

 rates increasing with an increase in temperature 

 (Loosanoff et al. 1951, Davis and Calabrese 1964, 

 Wright et al. 1983). Maximum larval growth generally 

 occurs between 22° and 33°C depending on the salin- 

 ity (Davis and Calabrese 1964, Lough 1975). The 

 range of temperatures tolerated by larvae is reduced 

 as salinity decreases (Eversole 1987). As tempera- 

 tures approach 40°C larval mortality increases (Wright 

 etal. 1983). 



Salinity - Eggs and Larvae: Egg development occurs at 

 salinities of 20 to 33%o (Davis 1958). The optimum 

 salinity for egg development to the straight hinged 

 larval stage is approximately 27 to 28%o with metamor- 

 phosis occurring at a minimum of 1 7.5% (Davis 1 958, 

 Davis and Calabrese 1964, Castagna and Chanley 

 1973). 



Salinity - Juveniles and Adults: Juveniles can tolerate 

 salinities as low as 1 2 to 1 5%o, but death usually occurs 

 at <1 0% o within several weeks (Chanley 1 958, Castagna 

 and Chanley 1 973). The optimum salinity for growth is 

 approximately 24 to 28%o (Chanley 1958). Optimum 

 growth salinities for Mercenaria mercenaria texana are 

 22 to 33%o, probably with no growth occurring below 

 20% o (Craig et al. 1988). In the Indian River, Florida, 

 hard clams are reported to do well in salinities above 

 20%o (Arnold et al. 1991, Arnold et al. 1996). During 

 periods of stress, such as sudden extreme changes in 

 water salinity, hard clams can close their shells tightly 

 and respire anaerobically (Lutz and Rhoads 1977, 

 Eversole 1987). 



Turbidity: Hard clams prefer clear water in Tampa Bay 

 (Kunneke and Palik 1984); secchi disc values range 

 from 0.9 to 3.7 m in one study (Godcharles and Jaap 

 1973b). Reduced survival has been noted at high 

 turbidity (Loosanoff 1962). Eggs and larvae develop 

 normally at silt concentrations of <0.75 g/l, but no egg 

 development occurs at silt concentrations of 3.0 to 4.0 

 g/l. Larval growth is retarded at 1.0 to 2.0 g/l and is 

 negligible at 3.0 to 4.0 g/l (Davis 1 960). Huntington and 

 Miller (1 989) found larval growth decreased only at the 

 highest experimental levels of sediment load (2,200 

 mg/l), but survival remained unaffected. Silt concen- 

 trations can also influence growth of juvenile clams. 

 Juveniles (9 mm) are not affected by sediment concen- 

 trations of 25 mg/l, but experience a 16% reduction in 

 growth at 44 mg/l of silt (Bricelj et al. 1984). Water 

 currents are important to the growth and survival of 

 hard clams by removing silts that would otherwise 

 accumulate and produce undesirable soft sediments 



40 



