The impact of microplastics on larvae of the sea urchin Pseudechinus huttoni

Microplastic pollution in the marine environment is of increasing global concern and, therefore, determining the effect of its presence on marine organisms should be a research priority. Although ingestion and adverse effects of microplastics have been confirmed for multiple marine taxa, limited studies have assessed ingestion of microplastics by marine invertebrate larvae and the related effects. Determining the effects of microplastic ingestion by marine organisms, especially during the sensitive larval stages, is an important step in understanding wider ecosystem responses. Thus, in the present study 7- and 14-day post-fertilization (dpf) planktotrophic pluteus larvae of the sea urchin Pseudechinus huttoni were used as model organisms to investigate ingestion and retention of 1–5 μm fluorescent green spherical microplastics (10, 100, 1000, 10,000 mL-1). In addition, the lethal and sub-lethal responses of 7 dpf larvae to a short-term, 10-day exposure of 1–4 μm spherical polyethylene microplastics at varying concentrations (10, 100, 1000 mL-1) were evaluated. Survival, development, and oxidative damage were used as measures to identify adverse effects. Development was assessed by measuring nine different morphometric dimensions over the experimental period, while the difference in post-oral arm length was used as a measure for abnormal development. To identify the oxidative stress defence mechanisms activated in response to microplastic exposure, antioxidant enzymes, lipid peroxidation, and protein carbonylation in the whole- body tissues of P. huttoni larvae were assessed. Microplastics were ingested by larvae in a dose-dependent manner and were retained for up to 90 (7 dpf larvae) and 150 min (14 dpf larvae) within the gut, before egestion. Survival, as measured by larval density, over the experimental period was not significantly affected by microplastic exposure at any of the concentrations tested. In contrast, following exposure to microplastics, a teratogenic response in terms of delayed development, resulted in an increase of larval arm asymmetry. Furthermore, detailed measures indicated body length and post-oral arm gap were significantly affected by microplastic exposure. More strikingly, short-term microplastic exposure resulted in oxidative lipid and protein damage in larval body tissue despite a significant upregulation of antioxidant defences, indicating the potential for oxidative stress biomarkers to be useful in future microplastic exposure research. Results of the present study indicate the necessity to further investigate the effects of microplastic exposure on the early life stages of marine invertebrates, as well as broadening the experimental duration to include processes such as settlement and metamorphosis. Microplastic-induced adverse effects in these sensitive stages of their life cycle may have consequences on recruitment and ecosystem dynamics in the marine environment, however due to many knowledge gaps, particularly on the abundance of microplastics in the size range used in the present study, it is difficult to conclude to what degree microplastics are a threat. Moreover, the multiple stressors marine invertebrate larvae face in the marine environment need to be considered when interpreting results of microplastic exposure studies