Accumulation of mutagenic activity in bile fluid of river Rhine fish

Abstract

Thyroid hormones (THs) are involved in regulation of many life history traits of fishes. Development, growth, reproduction and shaping the adult phenotypes were found to be regulated by THs. Regulatory evolution is one of the main factors driving morpho-ecological diversification and speciation. We tested a hypothesis whether closely related but ecologically diverged species may have a diverse level of THs. We compared seasonal levels of plasma concentrations of triiodothyronine (T3) in two cyprinid species, benthophagous bream Abramis brama and zooplanktivorous blue bream Ballerus ballerus. Significant differences in T3 concentrations were detected. B. ballerus had substantially reduced T3 levels as compared to most other species of fish. Two hypotheses explaining the thyroid divergence are discussed. Despite differences in absolute T3 concentrations, both species manifest similar seasonal cycling with increased plasma T3 levels in warmer seasons and decreased T3 in colder seasons. Several peaks of plasma T3 level were registered: at pre-spawning periods, at mid-summer and in the early autumn. The pre-spawning peak noted in cyprinids was a novel observation. Significant positive correlations were detected between water temperature and plasma T3 concentrations, as well as between the duration of daylight and T3 plasma concentrations in both species.