Comparison of zebrafish (Danio rerio) and fathead minnow (Pimephales promelas) as test species in the Fish Sexual Development Test (FSDT)

Results are presented from a validation (with 5 laboratories) of the Fish Sexual Development Test (FSDT) developed to detect endocrine disrupters (EDs) and included in the OECD (Organisation for Economic Co-operation and Development) working program. The aromatase-inhibiting fungicide prochloraz was tested in zebrafish (Danio rerio) and fathead minnow (Pimephales promelas). The fish were exposed during sexual differentiation and development from 0 to 60 days post hatch (dph). After exposure, the vitellogenin (VTG) concentrations were quantified in head/tail homogenate and the sex ratio was determined (defined as female, male, intersex or undifferentiated). NOEC/LOEC and EC(x) designs were compared to optimize the test approach. Results show that both species are highly sensitive to prochloraz during sexual development. They respond by skewing of the sex ratio towards male phenotype and by a VTG decline in females. The NOEC/LOEC approach is preferred because sex ratio is difficult to analyze with a regression model. The mean NOEC/LOEC for prochloraz on the sex ratio was 43.3/134 μg/L and 101/293 μg/L for zebrafish and fathead minnow, respectively. The mean NOEC/LOEC on the decline in female VTG concentration was 65/110 μg/L and ~30/68 μg/L respectively. In conclusion, zebrafish and fathead minnow are suitable species in the FSDT and their sexual differentiation is equally labile to EDs.

Pleistophora infestation in fathead minnows, Pimephales promelas (Rafinesque)

Pleistophora infestation was observed in adult fathead minnows, Pimephales promelas, held under laboratory conditions. Fish were clinically healthy, and presented no gross findings at necropsy. Histopathology revealed parasitic stages only in the ovaries. Spores within sporophorous vesicles were mainly encountered in late vitellogenic oocytes and were ultrastructurally identified as a microsporidian parasite. Heavily parasitized oocytes underwent degeneration followed by the release of spores into the ovarian interstitium. Degenerating oocytes and interstitial spores caused ovarian inflammation. Male fish showed no parasites in the testes. Parasitic infestation was compared with body length, body weight, gonadal weight, gonadosomatic index and plasma vitellogenin levels, and revealed no statistically significant differences between non-parasitized and parasitized females. The isolated holding conditions of the fish and the presence of parasitic stages in the ovaries suggested that an infestation with Pleistophora ovariaeSummerfelt, 1964 was more probable than that with Pleistophora mirandellae (Vaney & Conte, 1901).