EFFECTS of LOW LEVELS of METHYL MERCURY on the DEVELOPMENT of GONADS in the NILE TILAPIA (OREOCHROMIS NILOTICUS)
Arnold, B.S.1,3, Jagoe, C.H.2, Gross, T.S.3, Howerth, E.W.1, and Reinert, R.E.1
1University of Georgia, Athens, GA
2Savannah River Ecology Laboratory, University of Georgia, Aiken, SC
3USGS - Florida Integrated Science Center, Gainesville, Florida
Presented at the 20th annual meeting of the Society of Environmental Toxicology and Chemistry
(SETAC) - Philadelphia, PA Nov 1999, and printed for the Center Review in September 2000.
Mercury contamination is a major concern in the southeastern United States, but little is known about effects of low level exposure on health and reproduction in fish. To examine possible reproductive effects, tilapia were dosed using intraperitoneally implanted capsules containing either 0.1 (low dose) or 1.0 (high dose) mg of CH3HgCl2 designed to dissolve over 180 days; placebos contained only the carrier matrix. After 6 months, blood was collected for hormone analysis, and other tissues for histopathology and Hg analysis. Mean liver concentrations were 0.3, 0.9, 5.4 mg/kg dry wt in females and 0.3, 0.8, 3.0 mg/kg dry wt (control, low dose, high dose respectively) in males. Mean muscle concentrations in females were 0.3, 1.5, 6.5 mg/kg dry wt and 0.2, 0.4, 2.7 mg/kg dry wt in males (control, low dose, high dose respectively). 11-Keto testosterone levels in males and estrogen levels in females were negatively correlated with tissue Hg. Plasma estrogen/testosterone ratios in treated fish differed significantly from pretreatment means, and from control fish. There were no differences among treatments for liver-somatic index (LSI) or gonad-somatic index (GSI). However, histological examination showed that female fish from Hg treatments had increased atretic follicles, ruptured follicles, and granulomatous inflammation in the interstitium. Male fish exposed to Hg tended to have decreased spermatogenesis, atrophy of the seminiferous tubules, and dilated or cystic seminiferous tubules. These results indicate that environmentally realistic methyl mercury concentrations can disrupt endocrine function and potentially impair reproduction in fish.
Mercury has been shown in laboratory studies to affect the reproductive development of rats, mice, and hamsters. It has also been shown to reduce gonadal growth in fish. Most studies in aquatic systems however, have focused on bioaccumulation of mercury by fish and few have examined effects of realistic Hg burdens on the fish themselves.
In order to better understand the reproductive effect of methyl mercury on fish, the variations of estrogen and 11-keto testosterone were measured relative to blood mercury concentrations over eight months and gonads were examined for histological changes which could be attributed to mercury concentration.
Materials and Methods
- 100 Tilapia were randomly mixed by dose and sex and housed in 3 replicate 200 gallon fiberglass tanks supplied with a semi-closed aquatic system.
- Fish were dosed with methyl mercury chloride (CH3HgCl2) with intraperitoneally implanted capsules designed to dissolve over 180 days.
- –High Dose - 1.0 mg
- –Low Dose - 0.1 mg
- –Control - carrier matrix
- Blood samples were taken prior to dosing and during the test, then analyzed for mercury concentrations and hormone levels.
- At necropsy, gonads were measured and sectioned for histological examination.
Results - Blood Mercury
- High Dose - At one month post-dose, mean Hg levels in the blood were 0.45 mg/kg (wet wt.). Concentrations decreased by the second month to 0.28 mg/kg and remained > 0.16 for the length of the test.
- Low Dose - At one month post-dose, mean Hg levels were 0.03 mg/kg. Concentrations fell in the second month to 0.02 mg/kg and remained between 0.02 - 0.04 for the length of the test.
- Control - Blood concentrations of Hg stayed between 0.0043 - 0.0087 mg/kg.