Background: Evaluating methylmercury and associated pathophysiology in bottlenose dolphins (Tursiops truncatus) provides insight regarding the possible adverse effects and mechanisms of tissue distribution of mercury. Skin biopsies have been collected from the Sarasota Bay, Florida, bottlenose dolphin population since 1997 as part of ongoing health monitoring of the population. Samples are collected during summer and occasionally during winter to document seasonal changes. Prior to this study, histopathological examination of the biopsies had not been performed. Further, it remains uncertain whether there is a relationship between skin morphology and mercury concentrations that might impact animal health.

Questions: Our objectives were to identify histopathological changes in skin biopsies and determine if correlations exist between skin morphology and mercury concentrations.

Methods: Formalin-fixed skin biopsies (n= 35 animals) were processed and examined via light microscopy. Skin indices assessed included mitotic count (basal layer), heights of strata (externum, intermedium, spinosum, and basale), and collagen fiber diameter (via transmission electron microscopy). We performed ANCOVA to test for the effect of season on mercury concentrations while controlling for possible age effects. We used linear least-squares regression to quantify the relationship between mercury concentrations and skin morphology variables.

Results/Conclusions

Results: As anticipated, mercury was positively correlated with age in the ANCOVA, but no seasonal effect in mercury concentration was detected. However, the heights of the strata externum and intermedia, as well as mitotic index were greater in winter. Mercury concentration was negatively related to the height of the stratum spinosum only. Perivascular collagen degeneration was noted. Electron microscopic examination revealed collagen fiber widths that varied from 38 - 160 nm, with no relationship detected with age or mercury concentrations.

Conclusions: The negative relationship between mercury concentration and height of the stratum spinosum may suggest a negative effect on keratin production or keratin changes alter Hg concentrations. Similarly, increased mitotic rate (i.e., in winter) would result in increased sulfhydryl production, necessary for keratin synthesis. Thus, it is possible that there is either greater depuration or greater binding of Hg in the skin during winter. These preliminary findings warrant further investigation to confirm the suspected link between Hg concentration and keratin production, and determine the fate of Hg in the skin during winter. Understanding these processes and the associated effects on health can be used to guide conservation strategies for many taxa (i.e., those with keratinized skin) and can be applied to our understanding of Hg effects on human health.