Maternal Diet During Oogenesis Is the Major Source of Methylmercury in Fish Embryos CHAD R. HAMMERSCHMIDT † AND MARK B. SANDHEINRICH* ,‡ Department of Marine Sciences, University of Connecticut, Groton, Connecticut 06340, and Department of Biology, River Studies Center, University of WisconsinsLa Crosse, La Crosse, Wisconsin 54601 Development of the early life stages of fishes can be affected adversely by methylmercury (MeHg) transferred from the maternal parent to the developing egg. We examined maternal transfer of MeHg to eggs of fathead minnows Pimephales promelas and evaluated the role of maternal MeHg burden versus that in the maternal diet during oogenesis on egg concentrations. Juvenile fish were fed one of four diets until sexual maturity (phase 1): A control diet (0.06 µg of Hg g -1 dry weight) and three that were contaminated with MeHg at 0.88 (low), 4.11 (medium), and 8.46 µg of Hg g -1 (high). At sexual maturity, female fish were paired with a male, again fed one of the four diets, and allowed to reproduce (phase 2). To assess the significance of female dietary exposure during oogenesis on MeHg in eggs, some fish were fed diets during phase 2 that differed from those during phase 1. Mean concentrations and burdens of MeHg in eggs from fish fed the same diet throughout the experiment varied with MeHg content of the maternal diet and were related positively to levels in the carcass of the maternal fish. However, MeHg in eggs was not proportional to that in carcasses among dietary treatments; MeHg in eggs from adults fed the control, low, medium, and high MeHg diets averaged 14%, 25%, 32%, and 35% of that in adults. For fish fed the control diet as juveniles and MeHg-contaminated diets after reaching sexual maturity, MeHg in eggs increased rapidly with duration of maternal dietary exposure prior to spawning. Moreover, concentrations of MeHg in eggs from fish fed the same contaminated diet as both juveniles and sexually mature adults were not related to the duration of adult exposure, and they were not appreciably greater than those from fish fed contaminated diets only just prior to spawning. These results indicate that the diet of the maternal adult during oogenesis, and not adult body burden, is the principal source of MeHg in fish eggs. Accordingly, the exposure of embryonic wild fishes to MeHg depends on levels of the contaminant in prey of the adult during oogenesis, which can vary intra- and interannually. Introduction The toxicological significance of methylmercury (MeHg) to populations of fish is poorly understood (1, 2). Recent research, however, has shown that environmentally realistic levels of MeHg in the diet of experimental fish can suppress concentrations of sex hormones (3), alter their gonadal development (4, 5), and ultimately reduce overall reproduc- tive success (5). Additionally, MeHg may affect the survival and development of embryolarval stages. For example, increased mortality of rainbow trout Onchorhynchus mykiss embryos was associated with egg mercury (Hg) levels as low as 0.07-0.10 µgg -1 wet weight (6). These concentrations are only six to nine times those measured in eggs of rainbow trout from Lake Ontario (7) and are within the range measured in eggs of yellow perch Perca flavescens from semi-remote lakes in northern Wisconsin (8). Latif et al. (9) examined the effects of both maternally transferred and waterborne MeHg on embryos and larvae of walleye Stizostedion vitreum from industrially polluted Clay Lake and two remote lakes in Manitoba. In their study, environmentally relevant exposures to waterborne MeHg deleteriously affected the hatching success of eggs and the heart rate of embryos; however, MeHg transferred from the maternal parent did not affect either of these parameters, and the growth of larval walleye was not affected by MeHg exposure from either pathway. Exposure of grayling Thymallus thymallus to waterborne MeHg for 10 days during embryogenesis impaired feeding efficiency and reduced competitive ability of the fish as 3-year-old adults (10). Clearly, much additional work is needed to determine the relative sensitivities of different fish species and life stages to embryolarval MeHg exposure. Diet is the principal source of MeHg in wild adult fish (11, 12), and transfer of MeHg from the maternal parent is the major pathway of exposure for fish embryos (7, 8, 13). Uptake of inorganic Hg and MeHg from surrounding water is inhibited by the chorion (13, 14). It is unclear, however, if MeHg in fish eggs is remobilized from parental somatic tissue or if it is from the maternal diet during oogensis. Here we show that maternal diet during oogenesis is the principal source of MeHg in fish eggs. We fed fathead minnows Pimephales promelas diets containing concentrations of MeHg present in some aquatic food webs, maintained the fish through sexual maturity, and evaluated the relative roles of maternal MeHg burden and MeHg in the maternal diet during oogenesis on levels of the contaminant in fish eggs. Experimental Section Study Design. The design of this study has been reported previously in our investigation of the effects of dietary MeHg on reproduction of fathead minnows (5). About 1400 juveniles were randomly placed into each of four 500-L flow-through tanks receiving well water, and they were fed ad libitum one of four phase 1 diets, three of which were contaminated with methylmercuric chloride (phase 1). Phase 1 of dietary exposure to MeHg was defined as the period from 90 days posthatch to the onset of sexual dimorphism (about 240 days posthatch; ref 5). Mean concentrations (µgg -1 dry weight; (1 SE) of total Hg in the diets were 0.060 ( 0.003 (control), 0.88 ( 0.02 (low), 4.11 ( 0.08 (medium), and 8.46 ( 0.17 (high). Levels of MeHg in the test diets spanned those in zooplankton, benthic invertebrates, and small forage fish from low-alkalinity lakes in North America (Table 1 in ref 5) and thus reflect potential dietary exposures of fish in many fresh waters with MeHg-contaminated food webs. After fathead minnows became sexually dimorphic (about 240 days posthatch), mature male and female fish were paired randomly, assigned to quadrants in one of fifteen 50-L flow- through breeding aquaria receiving well water, and allowed to reproduce (phase 2). Phase 2 was defined as the period * Corresponding author phone: 608-785-8261; fax: 608-785-6959; e-mail: sandhein.mark@uwlax.edu. † University of Connecticut. ‡ University of WisconsinsLa Crosse. Environ. Sci. Technol. 2005, 39, 3580-3584 3580 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 10, 2005 10.1021/es0486263 CCC: $30.25  2005 American Chemical Society Published on Web 03/24/2005