Hard clam, continued 



bringing the egg diameter up to approximately 1 25 \im. 

 The gelatinous envelope imbibes water causing the 

 egg to swell, providing buoyancy to the egg and further 

 increasing the diameter to 270 u.m (Carriker 1961). 

 Lipids stored in the egg provide energy and nutrients to 

 the embryo, and are important to the embryo's devel- 

 opment and survival (Lee and Heffernan 1991). Egg 

 cleavage begins within 30 minutes of fertilization at 

 27°-30°C and after 10 hours a ciliated gastrula has 

 developed. The ciliated blastula emerges from the 

 gelatinous egg and becomes a trochophore larva 

 (Carriker 1961). 



Age and Size of Larvae : The first two larval stages, the 

 trochophore and early veliger stages (85-90 u.m), are 

 non-shelled and possess a ciliated velum for propul- 

 sion (Carriker 1961 , Eversole 1987). By day 1 the first 

 shelled stage, the straight hinged veliger, develops 

 ranging in size from 90-140 urn. By day 3 the second 

 shelled stage, the umboed veliger, develops. The 

 umboed veliger stage may last 3 to 20 days, depending 

 on water temperature and food availability, and ranges 

 in size from 140 to 220 urn in length. The pediveliger 

 stage follows lasting 6 to 20 days with a size range of 

 170 to 220 |im. The pediveliger possesses a strong 

 ciliated velum and foot that allow the larvae to swim and 

 crawl in search of a suitable settlement site. At 200-230 

 |im the velum is lost, and the newly settled plantigrades 

 are referred to as spat. The spat use byssal threads to 

 attach and detach from various substrates. For ap- 

 proximately 2 weeks the spat alternate between crawl- 

 ing and attaching to substrates. By 7-9 mm the byssal 

 gland is lost and the juvenile plantigrade settles perma- 

 nently to its benthic existence (Carriker 1 961 , Eversole 

 1987). 



Juvenile Size Range : Juvenile growth is influenced by 

 temperature, food availability, siphon nipping, and type 

 of substrate (Pratt 1953, Pratt and Campbell 1956, 

 Loosanoff and Davis 1963, Coen and Heck 1991, 

 Coen et al. 1 994). Growth is more rapid in smaller hard 

 clams, and most of it occurs during the initial several 

 years of life, particularly the first year (Eversole et al. 

 1986, Jones et al. 1990). Thereafter, the growth rate 

 declines progressively with age (Gustafson 1955). 

 Growth may be affected by substrate and current 

 regime more than increased exposure time at low tide 

 (Walker 1989). In Florida, Menzel (1961) found that 

 Mercenaria campechiensisgrevj most during the spring 

 through fall months with little growth occurring during 

 winter. In contrast, M. mercenaria grew in spring and 

 fall with very little growth in summer or winter, which 

 agreeswith later work by Peterson etal. 1983, Peterson 

 et al. 1985, and Jones et al. 1990. Growth rates of M. 

 mercenaria imported into Texas remained different 

 from native M. campechiensis texana which showed 

 little growth occurring during summer (Craig et al. 



1988). Growth rates in M. campechiensis exceed 

 those of M. mercenaria and their hybrids. Taylor and 

 Saloman (1968) reported average growth of Tampa 

 Bay hard clams over a four year period as age I - 50 

 mm, age II - 73 mm, age 111-81 mm, and age IV - 90 mm. 

 Growth is rapid and variable through the first three 

 years and clams generally reach 50% of adult maxi- 

 mum size. M. campechiensis reaches a commercially 

 marketable size of 45 mm within 1 .5 to 2 years (Peterson 

 et al. 1983, Kunneke and Palik 1984, Eversole et al. 

 1986, Eversole 1987). Juvenile M. mercenaria were 

 found to reach marketable size faster at lower stocking 

 densities than those stocked at higher densities (Rice 

 et al. 1989, Eversole et al. 1990). Those planted in 

 subtidal areas also grew faster than clams in intertidal 

 areas. By five years M. campechiensis reach 70% of 

 their maximum size (Taylor and Saloman 1969). Hy- 

 brid clams exhibit a growth rate greater than northern 

 hard clams (Chestnut et al. 1 956, Haven and Andrews 

 1 957, Menzel 1 964, Loosanoff and Davis 1 963, Taylor 

 and Saloman 1969). Overall growth rates of southern 

 populations of hard clams are more rapid than those of 

 northern populations; however, populations in the south 

 do not appear to live as long (Jones etal. 1990). Size 

 appears to determine sexual maturity more than age 

 does (Quayle and Bourne 1972, Eversole 1987). 

 Maturity is achieved at approximately 30-40 mm in 

 length at an age of 1 to 2 years depending on environ- 

 mental conditions (Eversole et al. 1980, Bricelj and 

 Malouf 1980). 



Age and Size of Adults : Hard clams in the Gulf of 

 Mexico can live up to 28 years and maximum size can 

 exceed 170 mm (Taylor and Saloman 1969, Kunneke 

 and Palik 1984, Jones et al. 1990). On the Atlantic 

 coast, two hard clams used in a growth experiment 

 reached estimated ages of 33 and 36 years (Eversole 

 1987). The annual mortality for clams raised under 

 laboratory conditions is about 4% (Eversole et al. 

 1986). The growth rate of hard clams decreases with 

 increasing size and age (Eversole et al. 1986). 

 Peterson's (1985) growth equation [length (in cm) = 

 3.176 + 1.819 In (number of annual bands)] becomes 

 a very poor predictor of age based on size after 4.5 

 years. Growth rates for the hard clam also vary with 

 geographical area (Jones et al. 1990). Growth in 

 Florida Gulf of Mexico sites is most rapid in the spring. 



Food and Feeding 



Trophic Mode : Hard clams are selective, omnivorous 

 filter-feeders, utilizing a siphon system to take in sus- 

 pended particles and dissolved organics carried along 

 in bottom currents (Eversole 1987). 



Food Items : Food is obtained from suspended par- 

 ticles entering through the ventral inhalant siphon and 

 passed to the gills. The particles are sorted in the gills, 



42 



