Sediments from Piles Creek (PC) have elevated levels of many metals and organic contaminants that were derived from industrial sources. In the 1980s, Hg levels in the sediments were 10-20 ppm, but they have declined in recent years. Fiddler crabs Uca pugnax from this site are more tolerant to methylmercury (meHg) than conspecifics from reference sites. Because tolerance could not be induced by preexposure of reference site crabs, it appears to have a genetic basis (Callahan and Weis, 1983). Likewise, grass shrimp Palaemonetes pugio larvae from PC were more tolerant than those from reference sites to meHg, but not more tolerant to inorganic Hg. Adult shrimp from PC, however, were more tolerant to both inorganic and organic forms of Hg (Kraus, Weis, and Weis, 1988; Kraus and Weis, 1988). Metallothioneins were present in greater amounts in the adult PC than the reference shrimp, and these may have been responsible for their tolerance to inorganic Hg, which was not present in larvae. These data suggested that tolerance to inorganic Hg was acquired after exposure (acclimation), whereas the tolerance to meHg was genetic and present in the larvae as well as adults.
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