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.
 

Abstract

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.
 

Introduction

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.

Results – Hormone

• Estrogen levels in treated females were significantly lower than in controls during November, December and February.
• 11-Keto testosterone levels in treated males were significantly lower than control levels in December and February.

Results – E/T Ratios

• Controls:
• Female E/T ratios at the start of the test ranged between 1.2 - 5.6 (mean 2.6). Mean ratios ranged between 2.7 - 3.3 for the remainder of the test except for October when they dropped to 1.7 when the heaters failed in the tanks.
• Male ratios were 0.5 at the start of the test and ranged between 0.6 - 1.0 for the remainder of the test.
• High Dose:
• E/T ratios in the high dose females declined to 0.7  after one month exposure then increased to 2.1 in the second month and thereafter ranged between 1.4 - 1.7.
• E/T ratios in males rose the first month to 1.2 decreased to 0.92 in the second month and thereafter ranged between 1.1 - 2.7.
• Low Dose:
• E/T ratios in low dose females declined to 0.9 in the first month, rose to 1.5 then remained between 1.1 - 3.3.
• E/T ratios in males rose to 1.2 the first month, decreased to 1.0 the second month and ranged between 0.9 - 1.3 for the remainder of the test.

Results – Histology

• Atretic follicles were more common in females exposed to Hg than in control females.
   
• Atrophied seminiferous tubules were found in exposed males but not in control males.
   
• Dilated or cystic seminiferous tubules were more common in males exposed to Hg.

Discussion

• Estrogen production in females dosed with methyl mercury decreased relative to the controls.
• 11-Keto testosterone production in treated males decreased relative to the controls.
• E/T ratios in treated fish differed significantly from the controls at the end of the test.
• Evidence of abnormal gonadal development was more common in fish dosed with methyl mercury.

Conclusions

• Liver mercury concentrations > 1.9 mg/kg dry wt ( 0.49 mg/kg wet wt) prevent the hormonal surge of estrogen in female and 11-keto testosterone in male tilapia.
• Liver mercury concentrations > 0.5 < 1.9 mg/kg dry wt (0.13 mg/kg wet wt) reduce the production of estrogen in females and 11-keto testosterone in male tilapia.
• Fish with liver mercury concentrations of > 0.5 mg/kg dry wt were more likely to have histological damage in the gonads.