Interactive Threats: Do Microplastics Mediate Response to Thermal Stress in a Marine Invertebrate?
By: Oliver Coyle
Department: Biology
Faculty Advisors: Dr. Jonathon Stillman, Dr. Vance Vredenburg
Aquatic organisms face multifarious environmental conditions that act simultaneously and can have interactive effects on aquatic organisms. For example, increasing average temperatures and heat waves coincide with exposure to microplastic pollution, the effects of which are poorly understood. Furthermore, these abiotic stressors co-occur with sources of biotic stress, such as parasites, that can also mediate host responses to environmental change. We investigated whether microplastic ingestion and parasite burden by spiny headed worms, Acanthocephala profilicollis, influence responses to thermal stress response in the pacific mole crab, Emerita analoga. We hypothesized greater microplastic ingestion and parasite burdens would be associated with lower body condition and thermal tolerance.
Body condition was determined using Le Crens relative condition factor (Kn) by dividing recorded wet weight (WW) with the expected weight derived from a fitted growth equation relating WW to Carapace Length (CL) such that WW = a CLb. Thermal tolerance was indexed as the critical thermal maxima, CTmax, by the loss of escape behavior during thermal ramps. Parasite burden was determined by counting parasites during dissections of crab guts for microplastic analysis. Microplastic load was determined using Nile red staining and fluorescence microscopy of KOH digested crab guts.
Initial results show that crabs with higher microplastic loads had higher CTmax values, but that this effect varies by sex. Females with higher microplastic loads show a significant higher CTmax in contrast with males where no significant relation between microplastic load and CTmax was observed. The observed increase in heat tolerance in individuals with higher microplastic burdens was contrary to our hypothesis but could reflect metabolic depression induced by microplastic accumulation in the gut or reallocation of energetic reserves (e.g., reproductive energy budget) to cellular stress response systems (e.g., heat shock proteins and antioxidant enzymes) that increase thermal tolerance at the expense of growth and reproduction.