Microplastics (>5 mm) are increasing in marine ecosystems and may pose threats to marine organisms, especially when ingested. Mnemiopsis leidyi, a species of ctenophore, is a key component of the marine food chain and a vital source of nutrition for larger organisms. We investigated the effect of microplastic beads (250-300 μm) on the clearance rate—the volume of water cleared of particles per unit time—by ctenophores. We set up three aquaria, each with 15L of filtered seawater, with the following: 1) control tank with prey only, 2) control tank with ctenophores (n=10) and prey, and 3) experimental tank with ctenophores (n=10), prey, and microplastic beads. This experiment was replicated four times. Prey (300 zooplankton/L) and microplastic bead (30 MpB/L) densities were both at ecologically relevant levels. We calculated the difference in clearance rates (by ctenophore volume) by comparing the control tank with ctenophores and prey to the experimental tank with ctenophores, prey, and microplastic beads. A Student’s t-test was used to analyze the data. We also observed ctenophores ingesting and egesting microplastic beads throughout the experiment.
Results/Conclusions
The mean clearance rate of ctenophores exposed to prey and microplastic beads (38.4 mL [seawater] mL (ctenophore)-1 hr-1 ± 11.83 SD) was significantly lower compared to those exposed to prey only (71.5 mL [seawater] mL [ctenophore]-1 hr-1± 14.95 standard deviations) (t(6) = 3.466, p = 0.013). The mean clearance rate of ctenophores exposed to prey and microplastic beads was 46% lower than those exposed only to prey. The results of this study demonstrate the potential for microplastics to negatively impact the total intake of food by ctenophores, thereby potentially decreasing nutrient intake. As important primary predators, ctenophores occupy a space towards the bottom of many marine food webs. Decreased food intake may reduce their nutritional value to consumers, resulting in increased levels of ctenophore consumption or decreased nutrient availability. Additionally, ctenophores that are consumed with ingested microplastic beads could contaminate larger marine organisms through bioaccumulation.