Microplastics may induce food dilution and endocrine disrupting effects in fathead minnows (Pimephales promelas), and decrease offspring quality.

Microplastics are a complex environmental contaminant that have been reported to cause a variety of impacts, although the mechanism of these impacts remains unclear. Many studies have investigated either sub-organismal or apical endpoints, while very few have attempted to integrate and link endpoints seen at multiple levels of organization. Here, we exposed fathead minnows to microplastics for their entire lifecycle, from the egg stage through to reproduction, and raised a subset of the offspring in clean water. We show that both preconsumer and environmentally sourced microplastics impact adult growth, lipid storage, and external colouration, suggesting a potential food dilution effect. Environmentally sourced microplastics, but not preconsumer microplastics, had further endocrine disrupting impacts on the parental generation and their offspring in the low concentration treatments such that egg production began later, eggs were less viable, and the offspring had higher rates of malformation. Low dose effects are a typical dose-response for endocrine disrupting contaminants. These results suggest that microplastic exposure, at concentrations relevant to what is being found in the environment, has potential implications for forage fish populations. Our findings also highlight the importance of using an integrative approach to understanding the mechanisms behind how and why microplastics impact organisms.

No evidence of spherical microplastics (10-300 µm) translocation in adult rainbow trout (Oncorhynchus mykiss) after a two-week dietary exposure.

The consumption of fish contaminated with microplastics is often cited as a pathway for human exposure. However, because their guts are generally removed before consumption, exposure may be low compared to other routes such as shellfish, drinking water and dust. Still, microplastics have been found to translocate from the gut to other tissues, making exposure from eating fish fillets or other seafood products a potential concern. To better understand fish as an exposure route for microplastics in humans, we tested hypotheses about whether translocation occurs and if efficiency of translocation is dependent on particle size. We investigated the amount and distribution of fluorescent polyethylene microspheres (10-300 µm) in the gut, liver, fillets and gonads of adult rainbow trout after a two-week dietary exposure. Fish were fed food pellets dosed with up to ~9,800 microspheres per gram of food. Total exposures over the entire experiment ranged from ~80,000-850,000 microspheres per fish. We did not find any particles in the fillets, liver, or gonads of any fish, suggesting that translocation of spherical microplastics of this size range does not occur in adult rainbow trout. The quantity of microplastics found in the gut was also low or absent after a 24-hour depuration period, indicating effective excretion in this laboratory population. This research suggests that the consumption of fish fillets may not be a significant exposure pathway for microspheres >10 µm in size to contaminate humans. Future studies should test for different sizes, morphologies and species to further our understanding.

Testing for local adaptation in adult male and female fitness among populations evolved under different mate competition regimes.

Mating/fertilization success and fecundity are influenced by sexual interactions among individuals, the nature and frequency of which can vary among different environments. The extent of local adaptation for such adult fitness components is poorly understood. We allowed 63 populations of Drosophila melanogaster to independently evolve in one of three mating environments that alter sexual interactions: one involved enforced monogamy, while the other two permitted polygamy in either structurally simple standard fly vials or in larger "cages" with added complexity. Adult male and female reproductive fitness were measured after 16 and 28 generations, respectively, via full reciprocal transplants. In males, reciprocal local adaptation was observed between the monogamy and simple polygamy treatments, consistent with the evolution of reproductively competitive males under polygamy that perform poorly under monogamy because they harm their only mate. However, males evolved in the complex polygamy treatment performed similarly or better than all other males in all mating environments, consistent with previous results showing higher genetic quality in this treatment. Differences in female fitness were more muted, suggesting selection on females was less divergent across the mating treatments and echoing a common pattern of greater phenotypic and expression divergence in males than females.