Ingestion and defecation of dispersed oil droplets by pelagic tunicates

Subsurface plumes of small, stable dispersed oil droplets are a feature of oil spills treated with dispersants. The doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea), a zooplankton species abundant in the Gulf of Mexico, was observed to ingest dispersed oil droplets (1–30 µm in diameter), produced by vigorous mixing of a dispersant with crude oil. Oil droplets were first observed in the doliolid stomach, followed by appearance in the fecal pellets formed within the doliolid. Released fecal pellets had numerous oil droplets. Concentrations of ingested oil droplets in doliolids, exposed to high droplet concentrations (17 000 droplets/mL), increased from 800 to 5300 droplets/doliolid after 4 and 24 h, respectively. At a lower concentration (1200 droplets/mL), the ingested droplet concentration after 12 h was 450 droplets/doliolid. Fecal pellets were an important route for the elimination of oil droplets. Oil droplet concentrations in fecal pellets were 85 droplets/fecal pellet and 10 droplets/fecal pellet at high and low concentrations of dispersed oil, respectively. A calculation of the amount of oil in doliolid fecal pellets, based on doliolid concentrations, their fecal production rates and oil concentration in the fecal pellets, indicated that 200 µg oil/m3-day could be carried to the benthos via fecal pellets.