Social hierarchy modulates responses of fish exposed to contaminants of emerging concern

Independent and synergistic effects of microplastics and endocrine-disrupting chemicals on the reproductive social behavior of fathead minnows (Pimephales promelas)

Microplastics (MPs) have become an environmental concern in recent years, with most research focused on the physiological effects of exposure. Comparatively little consideration has been given to the potential behavioral impacts of exposure, which may also have fitness consequences for individuals. Moreover, MPs can serve as vectors for endocrine-disrupting chemicals and other locally co-occurring contaminants known to impair behavioral responses. This project aimed to determine whether MPs alone or in association with a common environmental EDC (17-alpha ethinyl estradiol; EE2) alter reproductive behavior and decision-making in fish. Male and female fathead minnows (Pimephales promelas) were exposed to MPs associated with either a low (10 ng/L; MPEE2 10) or high (50 ng/L, MPEE2 50) concentration of EE2, or MPs without EE2 (MPvirgin) for 30 days via a dietary feeding protocol. Behavioral trials were conducted on Day 31 to determine the effects of exposure on male-female social interactions. The expression of male sexually selected traits, including courtship, was unaffected by exposure. However, non-exposed females in all treatment groups trended toward discrimination against exposed males, which reached statistical significance for the MPEE2 50 group. Female fish exposed to MPs, alone or in association with EE2, were equally likely to approach and associate with non-exposed and exposed males. The results from this study suggest that MPs may alter social behavior in fishes and that the behavioral impacts of exposure may be more strongly pronounced in females than males. Such individual-level changes in fitness have the potential to impact population size, with downstream effects on the broader aquatic community.

Characterization of vitellogenin concentration in male fathead minnow mucus compared to plasma, and liver mRNA

The objective of this study was to characterize vitellogenin (VTG) protein in male fathead minnow (Pimephales promelas) mucus compared with more conventional measures in plasma and mRNA isolated from liver. To assess the intensity and duration of changes in mucus VTG concentrations, male fathead minnows were exposed to 17α-ethinylestradiol (EE2) for 7 days with a subsequent depuration period of 14 days. The experiment was conducted in a flow-through system to maintain a consistent concentration of EE2 at a nominal EC₅₀ concentration of 2.5 ng/L and high concentration of 10 ng/L as a positive control. Mucus, plasma and liver were sampled at regular intervals throughout the study. Relative abundance of vtg mRNA increased after 2 days of exposure and returned to control levels after 4 days of depuration. VTG protein concentration displayed similar induction kinetics in both mucus and plasma, however, it was found to be significantly increased after 2 days of exposure using the mucus-based assays and 7 days with the plasma-based assay. Significantly elevated levels of VTG were detected by both assays throughout the 14-day depuration period. The elimination of the laborious plasma collection step in the mucus-based workflow allowed sampling of smaller organisms where blood volume is limiting. It also resulted in significant gains in workflow efficiency, decreasing sampling time without loss of performance.

Do microplastics impair male dominance interactions in fish? A test of the vector hypothesis

Microplastics (MPs) are widespread in aquatic environments and have become a critical environmental issue in recent years due to their adverse impacts on the physiology, reproduction, and survival of aquatic animals. Exposure to MPs also has the potential to induce sub‐lethal behavioral changes that can affect individual fitness, but these effects are understudied. Many plastic additives introduced during the manufacture of MPs are known endocrine‐disrupting chemicals (EDCs) that mimic the action of natural hormones, alter sexual and competitive behavior, and impair reproductive success in fish. In addition, EDCs and other aquatic contaminants may adhere to MPs in the environment, the latter of which may serve as transport vectors for these compounds (i.e., the vector hypothesis). In this study, we staged territorial contests between control males, and males exposed to virgin MP particles or to MPs previously immersed in one of two environmentally relevant concentrations of 17‐alpha ethinyl estradiol (EE2; 5 ng/L and 25 ng/L) to evaluate the independent and synergistic effects of exposure to MPs and a common environmental estrogen on male–male aggression and competitive territory acquisition in a freshwater fish, Pimephales promelas. Short‐term (30 days) dietary exposure to MPs did not impair the ability of males to successfully compete for and obtain a breeding territory. Overall levels of aggression in control and exposed males were also similar across trial series. These results help to fill a critical knowledge gap regarding the direct and indirect (vector‐borne) effects of MPs on the reproductive behavior of aquatic vertebrates in freshwater systems.