Abstract
Marine microplastic pollution is of growing concern, thus research to understand the effects of microplastic ingestion has been studied in a variety of organisms. Previous research on the effects of both microplastic ingestion and exposure to plastic associated chemicals (e.g., plasticisers) have occurred in isolation of one another. Therefore, the aim of the present work was to assess the effect of exposure to the plasticiser di-(2-ethylhexyl)-phthalate (DEHP) via ingestion of dosed microplastics on the reproductive output, behaviour, and biochemistry of the mottled triplefin Forsterygion capito. Here, 60 fish (30 breeding pairs) were split into three treatment groups, control, virgin polystyrene (PS) and DEHP-dosed polystyrene (DEHP) and were exposed to a given treatment over a five-week period. In Chapter Two, five-weeks exposure had no impact on the reproductive output, gonad condition and vitellogenin mRNA expression. Despite this, in Chapter Four, all markers of both antioxidant defence (enzyme activity) and oxidative damage (protein carbonyls and lipid peroxides) were significantly increased in the white muscle tissue of fish from Chapter Two. DEHP exposure therefore may incur an energetic cost, the consequences of which could manifest over time. In addition to this experiment, reproductive behaviour of male F. capito was assessed following the exposure of 90 fish (30 per group) to one of the same treatments (control, PS, and DEHP) over a ten-week period (Chapter Three). Though no change to aggressive or attractive behaviours was observed, ten weeks exposure did cause significant increased dopamine concentrations as well as decreased serotonin concentrations in the brain. This experiment also saw the ability to display nuptial colouration was decreased in DEHP-exposed fish compared to that of the control. This could indicate a change in the reproductive condition of exposed fish. DEHP exposure via microplastic ingestion did not have the same impacts as those seen in studies of constant aqueous exposure, this highlights the importance of environmentally relevant protocols as the delivery method will impact the degree of change observed.
Overall, it was found that, in terms of reproductive effects, the variable exposure resulting from the present treatment method led to results vastly different from those observed in studies of constant exposure to aqueous DEHP. Though this exposure method did not cause reproductive dysfunction or change to reproductive behaviours, it was clear that the fish were responding to the chemical as a result of ingesting dosed microplastics. This is evidenced by the increased oxidative stress markers as well as changed neurotransmitter levels and presentation of reproductive signals. The work in the present study therefore highlights the importance of exposure vector on observed results as well as the importance of environmental relevance of experiments. A comprehensive understanding as to the effects of microplastics and their additives on marine taxa is crucial in our assessment of the impact anthropogenic activity has on the marine environment. As such, research conducted must be environmentally relevant to allow the clearest picture possible.