Sea cucumbers as bioindicators in N-W Mediterranean : focus on trace elements and microplastics

The Mediterranean is the recipient of numerous anthropic contaminants, such as trace elements (TE) and microplastics (MP), both sharing an affinity for sediments. As detritus feeders living in close proximity to sediments, sea cucumbers make good bioindicator candidates for their monitoring. However, before developing protocols relying on these animals, it is crucial to ensure that they reflect accurately the contamination status of their habitat.
Specimens of Holothuria tubulosa and Holothuria polii were collected at three periods in three locations of Calvi Bay (fish farm, sewer exit and Punta Revellata) to measure TE concentrations in their organs, in order to study the variability of these concentrations. In the meantime, the MP content of their digestive tract was compared between sites.
The choice of organ for the TE analysis had the biggest influence, with a different sensitivity of every organ to the sources of variability considered (i.e. spatial, temporal and interspecific). The bodywall displayed the lowest TE concentrations, which were homogenous between species at any time and for each site. In contrast, they were the highest in the hæmal system, especially in H. polii, with clear differences between sites. Finally, the muscles and the respiratory trees recorded intermediate concentrations, with respectively weak temporal and interspecific variations. This highlights the inadequacy of bodywalls for TE monitoring, as well as the hæmal system’s potential to detect spatial variations of TE in sediments.
Most of the plastics extracted were MP, with a dominance of filaments/fibers. Individuals near the fish farm ingested more items, whereas those living at the sewer exit stood out from the ingestion of rubbers and fragments but also from a higher mean length of the plastics. This ingestion of MP by holothuroids in a way that reflects the anthropization of their habitat confirms their potential as bioindicators of plastic pollution in sediments.