Interactive Marine Threats: Do Microplastics Mediate Responses to Thermal Stress in a Marine Invertebrate?

Background/Question/Methods

: Aquatic organisms face a multitude changing 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 pollutants, the effects of which are poorly understood. We investigated whether microplastic ingestion influences responses to thermal stress in the pacific mole crab, Emerita analoga. We hypothesized that E. analoga micro plastic ingestion would be associated with lower body condition, elevated metabolic rate, and lower 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. Higher Kn values reflect higher body condition. Thermal tolerance was indexed as the critical thermal maxima, CTmax, by the loss of escape behavior during thermal ramps. Microplastic load was determined using Nilered staining and fluorescence microscopy of crab gut contents.

Results/Conclusions

: Initial results showed that decreased microplastic load was significantly correlated with decreased CTmax as escape behavior ceased at lower temperatures in individuals with higher microplastic loads (R2 = 0.18, p= 0.05)). Higher microplastic load was also significantly correlated with deceased width (R2= 0.216, p= 0.029), length (R2 = 0.21, p= 0.03), and weight (R2 = 0.231, p= 0.02) but not significantly related with body condition. Our results suggest microplastics may be more readily ingested by larger, older individuals and impact CTmax by sublethal mechanisms not exhibited in body condition. Metabolic rates, determined using intermittent flow respirometry, may help explain this observed effect if, as hypothesized, microplastics increase metabolic rates and susceptibility to increasing temperatures. Alternatively, higher microplastic loads lead to decreased metabolic rates (metabolic depression) that decreases the available energy budget for thermal stress responses. Our results thus far suggest microplastic pollution will decrease the escape capacity of Pacific Mole crabs in the face of elevated ocean temperatures leading to increased predation, higher Mole crab mortality, and the transfer of accumulated microplastics to higher trophic levels. Our results also suggest that microplastic pollution has asublethal, physiological impact on E. analoga that could have interactive effects with other environmental stressors.