Colonization and Expansion of Burrowing Isopod Sphaeroma on Styrofoam Floats as New Habitats in Mariculture

Plastic debris is generally recognized as a vector for marine organisms. However, the impact of macroplastic debris at the ecological level remains poorly understood. Sphaeroma, a mangrove-damaging burrowing isopod, has been demonstrated to inhabit floating Styrofoam and can serve as a good example for researching such impact. In this study, a large scale of field investigations was conducted for the population of burrowing isopods (S. tetrebrans and S. retrolaeve) on Styrofoam floats and mangroves along the coastal waters of China during 2021-2024. The colonization of burrowing isopods Sphaeroma on Styrofoam floats was simulated in situ in mariculture areas for two years, and their behaviors were observed in the laboratory. Our field investigation suggested that S. tetrebrans had more widespread distribution, higher population densities, a more female-biased sex ratio, and larger body size on floats (16 sites with 0.22 ind./cm2) compared to the conspecifics in adjacent mangrove habitats (12 sites with 0.15 ind./cm2). Burrowing isopods Sphaeroma could colonize on fresh Styrofoam floats within 1 month, and the population quickly increased in the following two years on Styrofoam floats. In the laboratory, S. tetrebrans tended to adhere to the substrate (i.e., Styrofoam floats) after a short while (e.g., 4.4 s) of backstroke swimming and could bore a complete burrow in foam blocks as fast as within 4 h. Our study indicated that the burrowing isopods Sphaeroma showed great fitness on expanded polystyrene and rapid colonization as well as expansion on foam substrates in mariculture areas, which might be collectively driven by the environmental factors of mariculture areas, the physical ones of expanded polystyrene, and the biological ones of Sphaeroma.