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The potential effects of exposure of fish to reproductive endocrine disruptors (REDs) is of major concern. This study reports on the effects of sewage effluent exposure on morphology of male mosquitofish (Gambusia a. holbrooki) in a tributary of the Hawksbury-Nepean River system in New South Wales, Australia. The growth and development of the modified anal fin (the gonopodium, GP) is a secondary sexual characteristic in males, forms under the influence of testosterone, and is critical for sperm transfer. The GP was reduced in length in males sampled downstream from a sewage treatment plant discharge point compared to GP fin length in males upstream or from other comparison sites. The reduction in size of this androgen-dependent structure suggests the presence of RED substances in the water. The presence or absence of spermatozeugmata (sperm packet) was not related to a reduction in GP length, which suggests spermatogenesis may not be reduced, but other measures of fertility remain to be evaluated. These results are discussed in the context of RED contaminants associated with sewage effluent.

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