Resistance to Hg in the Piles Creek (PC) killifish population is much more complicated. It was initially found that the embryos from females from reference sites were highly variable in their response to meHg, that is, some females produced eggs that were very resistant to teratogenic effects, others produced eggs that were susceptible, and others produced eggs of intermediate susceptibility (Weis, Weis, and Heber, 1982a).
In contrast, killifish from PC produced almost all resistant embryos (Weis et al., 1981); eggs and sperm also tolerate meHg (Khan and Weis, 1987). Part of the resistance was due to reduced uptake of Hg through the chorion, a trait associated with the female. Batches of eggs that were tolerant of meHg were not necessarily tolerant of other toxicants. In fact, PC embryos were more susceptible to inorganic Hg than embryos from reference populations (Weis et al., 1982). Therefore general chorionic impermeability is not the mechanism of resistance to Hg. Another potential mechanism of tolerance is more rapid development, which would allow the embryos to pass through sensitive critical early life stages more quickly (Toppin et al., 1987). Newly fertilized eggs do not express the protein (Weis, 1984).
PC eggs do not fertilize successfully in full strength seawater, whereas eggs and sperm from reference populations fertilized successfully in a widevarietyofsalinities (BushandWeis, 1983).This reduced salinity tolerance may be a "cost" associated with Hg tolerance in the PC population. The high variability of susceptibility within reference populations may allow them to withstand episodic influxes of contamination.
Despite resistance in PC gametes and embryos, itwas not seen in larvae or adults (Weis etal., 1985; 1987). If chorionic permeability and developmental time are the major contributors to the embryonic tolerance, these are irrelevant after hatching. Sensitivities of larvae to acute exposures were comparable in PC and reference populations. Subsequent studies on sublethal effects revealed that PC larvae were less able to depurate Hg than larvae from the reference population and were less resistant to behavioral effects of meHg (Zhou and Weis, 1998). Larval exposure increased susceptibility to predation to a greater degree in PC larvae than in reference larvae (Zhou and Weis, 1998).
PC adults showed increased signs of stress, slower growth, shorter life span, and decreased tolerance than reference adults (Toppin et al., 1987). However, they became reproductive at a smaller size and younger age similar to metal-adaptedpop-ulations of the isopod Porcellio scaber (Donker, Zonneveld, and van Stralen, 1993). Subsequent studies revealed that the PC fish had abnormal behavior - they were sluggish and poor at capturing prey and avoiding predation (Smith and Weis, 1997). Field-collected specimens had a diet comprised largely of detritus, rather than live food. Detritus provides little food value for killifish, which may account for their slower growth and reduced lifespan. While the abnormal behavior was correlated with brain Hg, it may be a result of overall contaminant loads rather than any specific chemical. The abnormal behavior is associated with reduced serotonin in the brains of the adults (Smith et al., 1995) as well as altered thyroid status (Zhou etal., 1999). Clearly, PC fish demonstrate a number of pollution-related problems and abnormalities, as well as the life stage-specific tolerance.
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