Plastic type influences which marine organisms colonize it and their dispersal potential

Plastic debris that reaches the sea not only cause pollution but can also become mobile habitats for numerous organisms. A recent study resulting from a collaboration between the Institute of Marine Sciences (ICM-CSIC), the University of Barcelona (UB), and the University of Catania shows that not all plastics play the same role in this process: their physical characteristics influence which organisms colonize them and may affect their ability to disperse across the Mediterranean.

The study, recently published in the journal Journal of Hazardous Materials Advances, examines how different types of plastic materials collected along the Barcelona coast are colonized by different organisms. The results show that the characteristics of the debris play an important role in determining which communities become established on them. Rigid plastics, for instance, exhibited higher diversity of organisms than plastic films.

One of the most notable associations was observed on foamed plastics, such as porexpan, which is commonly used in transport boxes and industrial packaging. Specifically, the research team found that spirorbids—tiny polychaete worms that build calcareous tubes attached to surfaces—occurred more frequently on these materials with great buoyancy.

“Because they can remain at the surface for extended periods, they act as mobile habitats and can transport organisms over long distances. This is especially important for species with a short larval stage and very limited dispersal capacity,” explains Gabriel Sonoda, predoctoral researcher at ICM-CSIC and lead author of the study.

Sonoda also notes that spirorbids are very difficult to distinguish with the naked eye, which is why they are often not identified to species level in studies of marine debris. To overcome these limitations and gain a better understanding of which species colonize plastics, the team is combining morphological and molecular analyses. This ongoing line of research will allow their taxonomic identification to be refined.

Plastics as dispersal vehicles

This phenomenon, known as rafting, or dispersal on floating objects, can enable organisms with limited mobility to potentially reach distant areas. This process is particularly relevant in the case of non-native species, as it may help them overcome barriers they would be unlikely to cross naturally.

‘We have identified, for the first time, three species of bryozoans that had not previously been documented as colonising plastic waste. We have also detected three non-native species, including the bryozoan Watersipora subtorquata, which is considered an invasive species and is associated with ecological and economic impacts in various temperate and tropical regions,’ states the ICM-CSIC researcher and study author Blanca Figuerola.

This species forms dense colonies that can cover both natural substrates and man-made structures and compete with native invertebrates for space. Furthermore, it can grow even on surfaces treated with anti-fouling paint designed to prevent organisms from attaching to ship hulls, which may give it an advantage over other species. Finally, its colonies can also act as a settlement surface for other organisms, including non-native species.

All of this highlights the importance of identifying organisms down to species level, as some non-native species might go unnoticed if monitoring is limited to broader taxonomic groups.

According to Figuerola, an expert in bryozoans, the presence of a non-native species on a fragment of plastic does not necessarily mean an ecological invasion will occur. However, transport on this debris may facilitate the organism's arrival in a new area and, if suitable conditions are encountered, its establishment and reproduction. This does not mean that the species will necessarily spread or cause ecological impacts, but it may increase opportunities for the introduction and establishment of non-native species. This risk may be particularly relevant in highly altered coastal areas, where plastic debris is abundant and can facilitate the dispersal of organisms into new habitats.

Rethinking how marine litter is monitored

The findings show that assessing the ecological impact of plastic pollution requires more than simply knowing how much litter is in the sea. It is also necessary to understand the characteristics of the debris and to identify the organisms colonizing it more precisely.

“Environmental monitoring programmes need to integrate analyses of the physical properties of debris, such as texture, polymer type, or buoyancy, as these characteristics can help determine its potential to act as a biological vector,” notes Anna Sanchez-Vidal, professor and researcher at the University of Barcelona and co-author of the study.

This approach would make it possible to identify which types of debirs pose a greater potential risk for dispersal of organisms and to refine strategies for preventing and managing marine pollution.

Overall, this research builds on a line of work initiated by Figuerola and Sánchez-Vidal to understand the extent to which plastics may alter connectivity among marine communities. In a previous study, the team had already warned that plastic debris can act as a potential vector for dispersal of marine organisms. The new study goes one step further, showing that this potential is not the same for all types of debris but instead depends on characteristics such as plastic type, shape, and buoyancy.

The team is currently conducting controlled experiments in port environments, as these areas are often act as entry points for non-native species and contain numerous artificial substrates on which organisms can become established. In parallel, the researchers are applying population genetics tools to determine the origin of these travelling species and to assess more precisely the dispersal risk associated with different types of plastic.