Bay scallop, continued 



the decline in summer temperatures, August to Octo- 

 ber (Sastry 1962, Barber and Blake 1983). Scallops 

 can be conditioned in the laboratory to spawn out of 

 season by raising the temperature to 30°C followed by 

 gradual cooling to 28-26°C (Castagna and Duggan 

 1 971 , Castagna 1 975). Gametogenesis is triggered by 

 food and temperature (Sastry 1975, Hall 1984). With 

 adequate food supplies, a minimum temperature of 1 5- 

 20°C is necessary for its initiation (Sastry 1968, Sastry 

 and Blake 1971), with slightly higher temperatures 

 required for complete maturation of gametes and spawn- 

 ing (Sastry 1 966, Sastry 1 968). As the gonads mature, 

 nutrients stored during the nonreproductive period are 

 diverted to their development (Sastry 1975). Few 

 studies have investigated salinity as a factor in spawn- 

 ing. 



Fecundity : Kraeuter et al. (1 982) reported a fecundity 

 estimate of 100,000 to 1,000,000 eggs per female. 

 Bricelj et al. (1987) reported fecundities ten to twenty 

 times greater. Some scallops may survive to spawn a 

 second time, but most do not (Robert 1978). 



Growth and Development 



Egg Size and Embryonic Development : The unfertil- 

 ized mature oocyte is 62-63 (j.m in diameter (Sastry 

 1965, Sastry 1966). After fertilization, the first polar 

 body occurs in 35 minutes with the second cleavage 

 stage occurring in 105 minutes. By 5 hours and 15 

 minutes the blastula has formed and rapidly develops 

 to the ciliated gastrula stage by 9 to 10 hours and 

 reaches the trochophore stage by about 24 hours 

 (Gutsell 1930, Sastry 1965). 



Age and Size of Larvae : Larval development in bay 

 scallops proceeds rapidly. The transition from tro- 

 chophore to straight-hinged larval stage occurs in 

 about 24 hours (Gutsell 1930, Sastry 1965, Rhodes 

 1991). In laboratory studies at 24° C the veliger 

 (shelled) larval stage develops within 48 hours at a size 

 of approximately 101 u.m (Sastry 1965). By the tenth 

 day of the veliger phase, the pediveliger begins to 

 develop and is complete by day 12, beginning the 

 settlement process at a size of approximately 184 |im 

 (Sastry 1 965, Castagna and Duggan 1 971 , Hall 1 984). 

 Attachment with byssal threads occurs between the 

 10th and 19th day of the veliger stage with the devel- 

 opment of the prodissoconch (=1 90 u.m) and metamor- 

 phosis into the juvenile stage commences. The juve- 

 nile stage is reached about 29 days from fertilization 

 when larval development is complete (Sastry 1965). 

 Loosanoff and Davis (1 963) reported larval growth rate 

 to be greater than 10 urn/day. 



Juvenile Size Range : By day 35 the young scallop 

 resembles the adult and is approximately 1.175 mm in 

 length (Sastry 1965). Juveniles remain attached by 



byssal threads until 20-30 mm in size, but retain the 

 ability to attach throughout their lives (Hall 1 984, Garcia- 

 Esquivel and Bricelj 1993). Growth is dependent on 

 temperature and food availability (Sastry 1 965). Growth 

 rates are rapid during the warm months, and a market- 

 able size of 50 mm is reportedly reached within 1 2 to 1 3 

 months on the U.S. east coast (Castagna and Duggan 

 1 971 , Spitsbergen 1 979, Rhodes 1 991 ), or within 6 to 

 8 months in Florida (Arnold pers. comm.). Little growth 

 occurs during winter, especially in the northern part of 

 the bay scallop's range. When growth resumes in the 

 spring, a raised shell check or color change occurs in 

 the shells of these individuals. Growth rates of 3.8 to 

 8.0 mm/month (umbo to ventral margin) have been 

 determined. Optimal growth occurs in currents <1cm/ 

 s and no growth occurs in currents >12 cm/second 

 (Kirby-Smith 1972). 



Age and Size of Adults : Maturity is reached by the end 

 of the first year, and is a function of age and not size 

 (Gutsell 1 930, Sastry 1 963). Adult sizes range from 60 

 to 70 mm with a reported maximum of 90 mm. Life 

 expectancy is 12-30 months, and is usually less than 

 two years (Belding 1910, Gutsell 1 930, Robert 1 978). 



Food and Feeding 



Trophic Mode : The bay scallop filter feeds at all 

 development stages (Castagna 1 975). Veliger feed by 

 means of cilia on their velum (Hall 1984). Chipman 

 (1 954) determined that young scallops filter at a rate of 

 3 l/hour, which increases as they grow reaching an 

 average of 15 l/hour, and a maximum of 25.4 l/hour. 

 Intensity of feeding increases with temperature. 



Food Items : The bay scallop feeds primarily on phy- 

 toplankton, but it also consumes zooplankton, sus- 

 pended benthic particles, bacteria, detritus, organic 

 matter, gametes from other species and algae spores. 

 In the laboratory larvae grow and develop well on a diet 

 of unicellular algae and naked dinoflagellates (Castagna 

 1975), although some algal species have low nutritive 

 value and can result in poor growth and survival 

 (Nelson and Siddall 1 988). Juveniles and adults ingest 

 phytoplankton and detritus as well as benthic diatoms 

 (Gutsell 1930, Davis and Marshall 1961, Broom 1976, 

 Fay et al. 1 983), but what is actually assimilated has not 

 been determined. 



Biological Interactions 



Predation : Known and suspected predators of the bay 

 scallop include various gulls and wading birds, starfish, 

 cow-nosed rays, pinfish, boxfish, toadfish, whelks, and 

 various crabs (Thayer and Stuart 1974, Broom 1976, 

 Peterson et al. 1989, Prescott 1990). Scallops in 

 intertidal and/or bare bottom areas appear to be more 

 vulnerable to predation than individuals in seagrass 

 beds or covered by 1 -3 cm of water or more (Peterson 



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