Microplastics (plastic < 5 mm) are ubiquitous pollutants in aquatic ecosystems. Although there have been numerous studies examining microplastic ingestion by organisms, other interactions between organisms and microplastics have only rarely been investigated. So far, it is known that marine tube-building polychaetes incorporate microplastics into their housings and that bees use plastic for brood cell construction. In freshwater ecosystems, the larvae of many caddisfly species build cases using sand and plant material. Those cases aid in the larva’s respiration, camouflage and physical protection from predators. Although freshwater ecosystems are polluted with plastic debris, caddisfly cases have only recently been analyzed for microplastics by our research group. We collected case-bearing Lepidostoma basale caddisfly larvae from the field, chemically deconstructed their cases and analyzed them for microplastics using digital microscopy and state-of-the-art micro-Fourier-transform infrared spectroscopy for polymer identification. Similarly, we analyzed sediment and water from the collection site for microplastics. Furthermore, we conducted a laboratory experiment to investigate effects of microplastics in caddisfly cases. Their stability (i.e., resistance to pressure) strongly depends on the type of case material. Hence, we offered PVC and PET microplastics together with sand to individual larvae and analyzed the cases’ stability using a customized 'caddisfly case cracker'.
Results/Conclusions
By analyzing the field-collected cases we found that caddisfly larvae incorporate microplastics of different shapes (fibers, fragments, films, spheres), colors, sizes and polymer types (such as polypropylene, polyethylene, polyester) into their cases in the field. Most microplastics in the cases were films and fragments which likely derived from degrading larger plastic items. Microplastic loads in caddisfly cases were higher than in the surrounding water and sediment, suggesting that caddisfly cases can be microplastic sinks in freshwater habitats. As some fish species, such as trout, regularly ingest caddisfly larvae together with their cases, microplastics in caddisfly cases could be transferred to predatory fish. In fish, microplastics can have negative effects by e.g. causing inflammations. During the laboratory experiment, the larvae used the microplastics and the sand for case building as soon as they were removed from their original cases. When breaking the cases with the 'caddisfly case cracker', we found that with an increasing percentage of PVC and PET particles in caddisfly cases, case stability was reduced. As caddisfly cases act as a physical protection from case-crushing predators such as juvenile dragonflies and fish, microplastics in caddisfly cases may be detrimental for the survival of caddisfly larvae in the field.