Impacts of barite and bentonite sedimentation on the behaviour, mucus production and cnidocytes of the deep-sea coral Flabellum (Ulocyathus) alabastrum

As oil and gas exploration expands in deep-sea environments, understanding the impacts of wastes produced by this industry is urgent. One of the main wastes of offshore drilling is the discharge of drill muds, commonly including barite and bentonite clay. Here, we investigated the effects of these two byproducts on the expansion behaviour, mucus secretion and cnidocyte density of the solitary cup coral Flabellum (Ulocyathus) alabastrum. A total of 96 live corals were exposed over 10 days to one of three treatments (barite, bentonite, or barite and bentonite combined) or assigned as an untreated control. Daily waste additions aimed to reach a total sedimentation depth of 6.3 mm (common ‘probable no-effects threshold’). Photography was used to monitor behavioural changes and mucus production, and histology was used to assess changes in tentacle cnidocytes. Overall, responses of F. alabastrum in the form of excessive polyp expansion (swelling) and mucus production were more severe under the barite and bentonite combined treatment, followed by barite alone, bentonite alone and lowest/absent in the control. Moreover, elevated nematocyte densities were measured under exposure to the mix of barite and bentonite. However, F. alabastrum returned to baseline metrics during a 10-day recovery period. The results suggest that sedimentation of common drill mud ingredients to <6.3 mm depth can cause observable and non-lethal responses in F. alabastrum, but remain conservative relative to what may be experienced in situ. As the most adverse effects were observed under the barite and bentonite combination treatment (tested together for the first time here), potential synergistic effects of the two should be considered in future research. Understanding long-term consequences, as well as impacts of more abrasive drilling waste (drill cuttings) on F. alabastrum requires further study.