hard clams pH should not fall below 7.00, and reproduction is not successful 

 when pH is appreciably above 9.00. Larvae can survive severe physical 

 disturbance, such as vigorous disturbance prior to taking quantitative 

 samples. But they can be killed by low concentrations of certain toxicants 

 and in the laboratory are subject to disease. Larvae grow more rapidly in 

 certain insecticides and other toxins, but others have marked adverse effects 

 on growth, and at higher concentrations kill. If larvae are growing normally 

 but mortality is high, disease is suspected. If growth is poor and mortality 

 is high, toxins are suspected. Some materials toxic to bivalves are listed. 

 Hard clam larvae can use Chlorella at 25 to 30°C, but not at 15°C, but they 

 can grow at 15°C if fed naked flagellates. Salinity tolerance varies with 

 temp and with stage of larval development. It has been assumed that 

 tolerance to any factor will increase with age, but this is not invariably 

 true. - J.L.M. 



239 



Calabrese, Anthony, and David A. Nelson. 1974. 



Inhibition of embryonic development of the hard clam, Meroenar-ia meveenaria , 

 by heavy metals. Bull. Environm. Contain. Toxicol. 11(1): 92-97. 



Experiments were conducted in synthetic seawater in which background 

 concentrations were: Kg 0.00003, Ag 0.003, Zn 0.029, Ni 0.0065, and Pb 0.007. 

 Tests were terminated after 42-48 hrs . Clam embryos were much more sensitive 

 than oyster embryos to Pb and Ni, somewhat more sensitive to Zn, as sensitive 

 to Hg, and less sensitive to Ag. Hg (as HgCl2) was 100% lethal to clams at 

 0.0075 ppm and Ag (as AgNC>3) at 0.045.ppm. LC50 values for Hg and Ag were 

 0.0048 ppm and 0.021 ppm, respectively. Zn (as ZnCl2) and Ni (as NiCl2.6H 2 0) 

 were 100% lethal to clams at 0.25 and 0.60 ppm, respectively , and LC5Q were 

 0.166 and 0.31 ppm. Pb (as Pb(N03) 2 ) was least toxic of metals tested, lethal 

 at 1.2 ppm. Results might have been different in natural seawater, which 

 contains ligands which might increase or decrease toxic levels. Natural sea- 

 water was not used because it might be variable in trace metal content, and 

 in dissolved organic or particulate matter. - J.L.M. 



240 



Calabrese, Anthony, Frederick P. Thurberg, and Edith Gould. 1977. 



Effects of cadmium, mercury, and silver on marine animals. Mar. Fish. Rev. 

 39(4): 5-11. 



Embryos and larvae of Mercenaria mere en aria were exposed to Hg and Ag. 

 Levels that caused 50% mortality in 48 hr (LC 50 ) were respectively 4.8 and 

 21.0 ppb for embryos and 14.7 and 32.4 ppb for larvae. Clam embryos and 

 larvae were as sensitive to Hg as oyster embryos and larvae, but less sen- 

 sitive to Ag. Sublethal effects of Ag on adult hard clam were tested at 

 various salinities for 96 hr. Respiration was significantly higher than 

 controls after exposure to Ag levels as low as 100 ppb. This sensitivity 

 varied with salinity, and certain Ag-salinity combinations were lethal. In 

 long-term exposures (30-90 days) to 10 ppb Ag, oxygen consumption rates were 

 also elevated. Clams placed in "clean" water for 30 days after 30 days in 

 Ag-contaminated water still had elevated respiration. The most obvious 

 generalization was that order and degree of metal toxicity vary, not only 

 with salinity and metal salt form, but also with life stage and species. 

 Early life stages of mollusks appear to be more sensitive to Hg and Ag than 

 to Cd. Juveniles and adults take up more Hg and Ag in their tissues than 

 Cd. In older animals, however, Cd produces more severe effects than Hg or 

 Ag. Two general and basic effects of sublethal metal challenge were 

 observed: 1) induction of enzymes that are either directly attacked or 

 involved in mobilization of energy (glycolysis) or in production of 

 metabolites for biosynthesis (pentose shunt) . Chronic demand for enzyme 

 production is costly in metabolic energy; and 2) loss of ligand sensitivity, 

 by which enzyme reaction rates are regulated, which is perhaps the more 

 serious effect. - J.L.M. 



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