Defecation in preparation for ecdysis drives microplastic clearance in cricket nymphs

Abstract

Plastic pollution is widespread in both terrestrial and marine environments, creating significant ecological concerns. Animals that occupy lower trophic levels, like many small insects, can ingest and retain plastics for extended periods before eliminating them. Ecdysis, or moulting, occurs in arthropods during development and facilitates growth, but its role in microplastic (MP) clearance, and whether it is impacted by plastics, are largely unexplored. We used the cricket Gryllodes sigillatus to examine how moulting influences MP clearance in a hemimetabolous species known to ingest and tolerate MPs throughout development. We tested two competing hypotheses of how moulting influences plastic clearance: (1) that moulting of the gut lining removes MPs, or (2) that cessation of feeding and purging frass before ecdysis occurs removes MPs. In doing so we also provide new evidence that cricket nymphs exhibit a cyclical (cosinor) pattern of frass production associated with ecdysis. MPs were eliminated from the crickets gut before ecdysis, driven by the cessation of feeding and cyclical frass production that clears the gut in preparation for ecdysis. We also observed that the timing of ecdysis and overall frass production remained unaffected by continuous MP ingestion, whereas switching between MP and non-MP diets caused modest changes in frass timing within an instar. Crickets are known to biofragment MPs into NPs, which, with gut-clearing, likely allows them to deposit plastic-laden frass rapidly and reduces the likelihood of upward trophic transfer by reducing plastic retention.