Hormonelike effects of humic substances on fish, amphibians, and invertebrates.

Humic substances comprise the majority of organic matter in freshwater ecosystems and were thought to be inert or refractory, except for photolytic degradation. However, evidence is increasing that humic substances interact with aquatic organisms similarly to weak anthropogenic chemicals with nonspecific and specific effects. One specific effect is a hormonelike effect, namely, modulation of the number of offspring, which was first described with the nematode Caenorhabditis elegans. Yet a hormonelike effect is not restricted to only the nematode. With the ornamental swordtail fish, Xiphophorus helleri, and the South African clawed frog, Xenopus laevis, we present phenomenological evidence that slight feminization occurred when these vertebrate species were exposed to a synthetic humic substance, a condensation product of polyphenols. The slight feminization was dose dependent.