246 EISLER 



Connor, 1971); sex (Vernberg, DeCoursey, and O'Hara, 1974; 

 Vernberg and Vernberg, 1972); general health of animal (Portmann, 

 1968; Shealy and Sandifer, 1975); previous exposure to organic 

 compounds (Boney and Corner, 1959); or to mercury (Corner and 

 Sparrow, 1956; Foster and Olson, 1951; Gillespie, 1972; Green et al., 

 1976); moulting stage of crustaceans (Wilson and Connor, 1971); and 

 presence of parasites (Clemens and Sneed, 1958). 



It was not possible to document a specific trend for any 

 individual modifier. Some studies showed relatively short survival 

 times at comparatively high temperatures, and others demonstrated 

 unchanged survival times for some species at a At of 10° C or more. 

 Most studies, however, showed that previous exposure to mercury 

 salts was in some way associated with increased sensitivity to 

 mercury and other stressors; shrimp were an exception. Pre-exposure 

 of shrimp for 57 days to 1.0 /jg Hg/liter did not affect LC^o values 

 (Green et al., 1976). There was general agreement that organo- 

 mercury compounds were more toxic than inorganic compounds; 

 that chelating agents in combination with mercury salts produced 

 less-than-additive toxicity but other compounds, especially salts of 

 copper, lead, or zinc act synergistically to increase toxicity; and 

 that salinity stress, especially abnormally low salinities, reduced 

 significantly the survival time of mercury-exposed organisms. The 

 evidence suggests that species adapted to a fluctuating estuarine 

 environment could be more vulnerable to the added stresses of heavy 

 metal pollution, including mercury, than species inhabiting more 

 uniformly stable environments (Jones, 1973). 



Sublethal and Latent Effects 



Mercury adversely affects various metabolic processes essential to 

 normal growth, development, reproduction, and general well-being of 

 marine and estuarine biota. Typically, metabolic effects are mani- 

 fested at mercury concentrations that are orders of magnitude below 

 those producing death. Morphological variations, for example, occur 

 in dinoflagellates at 1.0 /ig Hg/liter (Kayser, 1976), and teratogenic 

 effects, such as the abnormal growth of multiple eyes and tentacles, 

 are found in marine gastropods exposed only as embryos for 3 to 24 

 days to 10.0 /ug Hg/liter (Reinhart and Myers, 1975). Exposure of 

 shrimp larvae for 48 hr to high sublethal mercury levels is associated 

 with reduced survival at post larval stage, delayed molting, extended 

 development time, and morphological deformities (Shealy and 

 Sandifer, 1975). At 1/lOOth of the lethal concentration, mercury 

 disrupts avoidance-learning responses in fish (Weir and Hine, 1970), 

 and, in fact, mercury is more effective in reducing these conditioned 



