First evidence of ingestion and retention of microplastics in seahorses (Hippocampus reidi) using copepods (Acartia tonsa) as transfer vectors

Microplastics (MPs) are a major concern for marine ecosystems since they can be ingested by a wide range of marine species and transmitted through the food web. However, the potential hazardous impact of MPs in fishes, especially in early developing stages, is relatively unknown. In the present study, we assessed for the first time the ingestion and retention of MPs in early developing seahorses Hippocampus reidi. Seahorses are vulnerable species that may also be affected by both the direct ingestion of MPs through their preys and the accidental ingestion of MPs particles present in the water (i.e., seahorses ingest the prey by suction). We used copepods as both preys for seahorse juveniles and transfer vectors of MPs. Fed or starved copepods previously exposed to polyethylene microspheres (1-5 μm in diameter; 10 and 100 μg L-1) for 60 min at 26 °C showed fast evacuation of microspheres. The presence of MPs in copepods was significantly higher in previously fasted copepods compared to fed copepods. Seahorse juveniles fed on copepods pre-exposed to MPs, accumulated MPs in the gut proportionally to the concentration of MPs in copepods. A lower concentration of MPs in seahorses was observed at the longer exposure time (60 min), especially in fish fed with fasted copepods. However, after longer exposure, MPs were mainly accumulated close to the anus both individually or forming aggregates. Further studies should be performed to assess secondary effects of MPs ingestion in seahorses since they are considered a flagship species for marine conservation.