Cold-water octocoral interactions with microplastics under laboratory conditions

Microplastic pollution is ubiquitous in the oceans, threatening the health of marine ecosystems. The deep sea is recognized as a sink for microplastics, but there is a paucity of information on how deep-sea organisms are being affected by this stressor. Considering their vulnerability to disturbance, this information is crucial to fully understand the need for conservation actions. Here, we develop a novel methodology to provide a detailed characterisation of the behavioural responses of the cold-water octocoral Viminella flagellum to microplastic exposure under laboratory conditions. Coral fragments were individually exposed to a concentration of 1500 items/L of fluorescent green polyethylene microspheres biofouled for three weeks, for a period of 24 h, and carefully monitored for the entire exposure period using high resolution time-lapse video. After exposure, each fragment was transferred to another tank, free of microplastics, and monitored for further 24 h. The coral fragments were dissected at the end of the experimental period to assess the number of microplastics that remained in the digestive tract of each polyp. Our results showed that during this short-term exposure period, V. flagellum was ingesting microspheres, but most importantly it demonstrated the capacity of egesting all particles within 24 h. These results are especially important when quantifying microplastic contamination in cold-water corals in their natural habitat, as only recently ingested microplastics may be detected, leading to potential underestimations of their exposure. Additionally, our results indicated that microplastics adhered to the coral tissue surface could be discarded through periodic shedding of the mucus. These observations suggest that cold-water octocorals can handle microplastics as they do with other foreign particles, although the cleaning mechanisms may require significant energy expenditures.