As a semi-enclosed sea affected by a growing coastal urbanization, the Mediterranean is the recipient of numerous anthropic pollutants. These include trace elements (TE) and microplastics (MP), two different types of contaminants who share a common affinity for sediments. Both of them can enter and accumulate in marine organisms, causing metabolic disorders when present in excessive amounts. The potential impact of these two types of contaminants combined with their association with sediments highlight the importance of developing accurate tools of surveillance for this compartment of the marine environment, such as bioindicator species. Due to their proximity with sediments and to their feeding mode relying on detritus, sea cucumbers have been raising the interest of scientists for the monitoring of marine sediments. However, to be able to make use of the bioindicator qualities of these animal, it is crucial to better understand their relationship with the two contaminants mentioned, in order to ascertain that they reflect accurately the contamination status of their habitat. Therefore, several specimens of Holothuria tubulosa and Holothuria polii were collected during three periods of the year at three locations in the Bay of Calvi to measure the concentration in TE of their organs, with the aim of identifying the factors influencing these concentrations. In the meantime, the microplastic stomachal content of the sea cucumbers collected on the different sites was compared. This work confirms that the organ chosen for the TE analysis is the main factor influencing their concentrations. Moreover, every organ displays a different sensitivity to the sources of variability considered. Thus, the bodywall of sea cucumbers displays the lowest TE concentrations, which are homogenous between species at any time of the year and for each sampling site. On the opposite, these concentrations are the highest in the haemal system, especially in H. polii, particularly at the fish farm compared to the sewage system exit or to the STARESO station. Muscles mixed with the water vascular system record periodic variations in TE concentrations, with the highest values in summer. Finally, the concentrations measured in the respiratory trees only differ according to species, with the highest values in H. polii. In future studies using these animals as bioindicators, it will be crucial to select the organ whose sensitivity reflects best the difference in contamination that has to be highlighted, either from a temporal or from a spatial perspective. The majority of the numerous plastic debris extracted from sea cucumbers guts belonged to the class of MP, with a large dominance of fibers and filaments. Individuals collected near the fish farm ingest more plastic items, whereas those living near the exit of the sewage systems stand out from the ingestion of rubbers and fragments, as well as from a higher mean length of the ingested items. These results confirm the tendency of sea cucumbers to ingest numerous plastic debris in a way that reflects the degree of human pressure on their habitat as well as the potential of sea cucumbers as bioindicators of plastic pollution in sediments. |
