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This study investigated the progressive effects of HgCl2 in the testis and sperm of the tropical fish tuvira Gymnotus carapo L. exposed to increasing Hg concentrations (5-30 μm) and increasing exposure times (24-96 h). Histopathology and metal concentrations in the testis were observed. Hg concentrations in the testis reached 5.1 and 5.2 μg g(-1) in fish exposed to 20 and 30 μm of Hg, respectively. Hg effects on testicular tissue were observed even at low Hg concentrations, with no alterations in gonadosomatic index. However, the quantitative analysis of the induced alterations (lesion index) demonstrated that the Hg effects in testis became more severe after exposure to higher concentrations (20 and 30 μm) and during longer exposure (72 and 96 h), probably leading to partial or total loss of the organ function. Hg exposure (20 μm) also affected sperm count and altered sperm morphology. This study showed that HgCl2 caused progressive damage to testicular tissue, reduced sperm count and altered sperm morphology. These results are important in establishing a direct correlation between Hg accumulation and severity of lesions.

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Abstract. Hg is toxic metal mostly due to adverse effects on structure and function of tissues and organs in humans and animals. Male reproductive systems of fish species are also sensitive to Hg action. However, the histological alterations in tropical fish testis are less known and little information is available concerning the underlying mechanisms of metal pathogenesis in reproductive functions. Further investigations dealing with Hg direct effects on tissue and organs of tropical species are a need. The present study investigated HgCl 2 toxic effects in testes and sperms of tropical fish Gymnotus carapo. The histopathology, germ cell structure and number were analysed to elucidate the pathological process during exposure to increasing metal concentrations (1 µM -30 µM). Fishes exposed to 20µM and 30µM reached testicular Hg concentrations of 5.1 µg.g -1 and 5.2 µg.g -1 , respectively. No significant alterations in gonadosomatic index (GSI) occurred between control, and Hg exposed fishes. Untreated fishes showed characteristic organization of testicular tissue, with germ epithelium organized in cysts where spermatogenesis occurs. Germ cells and spermatozoa are seen an inner the cysts. HgCl 2 induced severe damages characterized by complete disorganization of seminiferous lobules, proliferation of interstitial tissue, congestion of blood vessels, reduction of germ cells and sperm aggregation. Exposed fishes showed a decrease in the sperm's number. Initial reduction of a sperm's number (36,8%) was observed after 20 µM/24h treatment and subsequent decrease (48,7%) was obse rved after 20µM/96h. Hg (20µM) also altered sperm morphology in 24h and 96h where sperm head abnormalities were present. In conclusion, the present study showed HgCl 2 progressive damages in testicular tissue, sperm count and morphology of tropical species Gymnotus carapo. The effects in testicular tissues were observed since low Hg concentrations. These results are important to establish a direct correlation between the mercury accumulation and severity of lesions.