Hard clam, continued 



(Killam et al. 1992). In addition, currents are also 

 important for providing food, maintaining acceptable 

 water quality, removing biodeposits, and transporting 

 eggs and larvae. 



Dissolved oxygen (DO): One hundred percent egg 

 mortality occurs at oxygen concentrations of 0.2 part 

 per million (ppm). Embryos from Long Island Sound, 

 New York develop normally at 0.5 ppm and above, and 

 larval growth is lower at 2.4 ppm than at 4.2 ppm 

 (Morrison 1971). However, larvae from Indian River 

 Bay showed no significant differences in growth and 

 survival when exposed to hypoxic conditions, but a 

 decrease of growth was observed in larvae subjected 

 to hyperoxic conditons (13.7 ppm) (Huntington and 

 Miller 1 989). In Tampa Bay hard clams were found in 

 oxygen saturation conditions, while from Charlotte 

 Harbor they are taken at 4.6 to 9.6 parts ppm (mean = 

 6.6 ppm), and at 4.0 to 7.8 ppm (mean = 5.8 ppm) from 

 the Ten Thousand Island area (Taylor and Saloman 

 1970, Godcharles and Jaap 1973b). 



pH: Normal development of embryos occurs between 

 a pH of 7.00 and 8.50. Optimum larval growth occurs 

 between pH 7.50 and 8.00 with a minimum of 6.25 and 

 a maximum of 8.75. The pH must be greater than 7.0 

 for successful recruitment of juveniles to occur 

 (Calabrese and Davis 1966, Calabrese 1972). 



Migrations and Movements : Egg and larval stages are 

 subject to tidal action and currents. Larvae are capable 

 of migrating vertically throughout the water column to 

 retain themselves in the estuary. Pediveliger larval 

 stages crawl and swim in search of a settlement site. 

 Juveniles and adults exhibit limited horizontal and 

 vertical movement through the sediment, but do not 

 migrate extensive distances (Eversole 1987). Upon 

 removal from the sediment in Narragansett Bay, hard 

 clams less than 83 mm in valve length (VL) are able to 

 reburrow within a week (Rice et al. 1 989). Hard clams 

 exceeding 83 mm VL demonstrate the least capability 

 of reburrowing. 



Reproduction 



Mode : Hard clams are protandrous hermaphrodites 

 which release their gametes into the water column for 

 external fertilization. Mercenaria mercenaria exhibit 

 consecutive hermaphroditism, passing through a pre- 

 adult sexual phase at around 6-7 mm shell length. 

 Individuals usually function as males during the pri- 

 mary sexual phase, but their gonads have both male 

 and female sex cells. The primary sex phase lasts 

 throughout the first year. Following the primary sex 

 phase, the clams experience a permanent sex change 

 after which the male-female ratio changes to 50:50, 

 and they will function primarily as male or female 

 (Loosanoff 1937a, Merrill and Tubiash 1970, Walker 



and Stevens 1989). Subsequent reproductive efforts 

 are sexual with separate male and female sexes 

 (gonochoristic), with rare instances of hermaphrodit- 

 ism. Mercenaria campechiensis also tends to be 

 protandric in its development (Dalton and Menzel 

 1983). Clams in the 60 mm size class have been 

 reported as the most reproductively active (Belding 

 1912), but there appears to be no evidence of repro- 

 ductive senescence in larger, older clams (Peterson 

 1983). 



Spawning : Spawning occurs generally from 20° to 

 30°C, with maximum spawning activity found between 

 24° to 26°C (Loosanoff 1937c, Carriker 1961, 

 Hesselman et al. 1989), in the marine and estuarine 

 subtidal seawater zone (Dalton and Menzel 1983). 

 Spawning activity has bimodal annual peaks in the 

 more southern portion of the hard clam's range, such 

 as the Gulf of Mexico (Eversole 1 987). In Florida, these 

 peaks occur in the spring (February-June) and fall 

 (September-December) with spawning beginning in 

 February-March and ending in October (Dalton and 

 Menzel 1983). In the Tampa Bay area, spawning 

 occurs during April and continues to August (Belding 

 1912, Kunneke and Palik 1984, Hesselman et al. 

 1989). Temperature influences gonadal development 

 (Loosanoff 1937b, Porter 1964), and spawning may 

 occasionally occur all year in warmer parts of the hard 

 clam's range such as Florida (Dalton and Menzel 1 983, 

 Hesselman et al. 1 989). When the water temperature 

 averages >30°C gametogenesis is inhibited and spawn- 

 ing ceases (Hesselman et al. 1989). In addition to 

 climatic influences, spawning frequency may also be 

 differently influenced by genetic factors in different 

 populations of hard clam (Knaub and Eversole 1988). 

 Spawning appears to coincide with high algal concen- 

 trations during spring, fall and winter, allowing ample 

 food resources for larval stages (Heffernan et al. 1 989). 

 Gametes are broadcast into the water column, and 

 fertilization is external (Belding 1912, Loosanoff 1 937b, 

 Kunneke and Palik 1984, Eversole 1987). 



Fecundity : Egg production estimates range from 2-3 

 million all the way up to 39.5 million per individual for an 

 entire spawning season (Davis and Chanley 1956, 

 Ansell 1967, Bricelj and Malouf 1980) with up to 24.3 

 million eggs reported in a single spawn (Davis and 

 Chanley 1956). Fecundity is directly related to clam 

 size (Bricelj and Malouf 1980, Peterson 1983), and 

 reported differences may be due to clam size and 

 condition at time of spawning. 



Growth and Development 



Egg Size and Embryonic Development : Hard clam 

 eggs develop oviparously. Unfertilized eggs range 50- 

 97 urn in diameter (Carriker 1961, Bricelj and Malouf 

 1980). A gelatinous envelope surrounds the egg 



41 



