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Abstract

The fathead minnow, Pimephales promelas, a small cyprinid from North America, is widely used as laboratory test fish. In the present study, rearing and breeding conditions for the fat-head minnow were optimized in our laboratory to produce eggs on a regular basis for testing. Firstly, these eggs were observed and a timeline for normal development of the embryo was established. Results showed only minor differences to already existing studies (Devlin et al. 1996, Manner and Dewese 1974). Based on fathead minnow-specific developmental data, the embryo toxicity test was adapted and refined. Using the fathead minnow embryo test, LC50 data were generated for a heavy metal (cadmium chloride), a substituted aniline (3,4-dichloroaniline), which is currently used as the positive control in the fish embryo toxicity test with the zebrafish (ZFET) and a substi-tuted phenol (2,4-dichlorophenol). Fathead minnow embryos proved to be slightly less sensi-tive for cadmium chloride and clearly less sensitive for 3,4-dichloroaniline than adult fathead minnow (OECD TG 203), whereas for 2,4-dichlorophenol no difference in sensitivity could be detected. However, the comparison with the ZFET showed that zebrafish embryos are sig-nificantly more sensitive. For all three substances, toxicity increased after hatching (≥ 96 and 120 hpf). Differences in toxicity between the fathead minnow and the zebrafish embryo tests might be due to differences in the chorion. Examinations of the chorion with light, confocal laser scanning and transmission electron microscopy revealed significant differences to the zebrafish chorion. The fathead minnow chorion was thicker and had less and smaller pores on the outside of the chorion. Further-more, the TEM images showed a layer composition different to the zebrafish chorion. Histo-logical stains and a test with the fluorescent dye 2,7-dichlorofluorescein supported the as-sumption that at least the outer layer of the fathead minnow chorion is composed of a material different to the zebrafish. A preliminary attempt with the fathead minnow embryo instead of the zebrafish embryo in the newly developed in vivo EROD assay (Kais) was performed successfully. However, first sig-nals of EROD activity were detected at least 24 hours later than in the zebrafish, and the sig-nal intensity was weaker than in zebrafish.