Separating oil from water: suspension-feeding goldfish ingest liquid vegetable oil

Abstract

We show that goldfish (Carassius auratus) voluntarily ingest liquid canola oil at the surface of the water and can swallow significant quantities of oil. The ability of fish to separate floating oil from water has not been tested previously, and the mechanisms used to retain oil in the form of suspended droplets, globules, or a surface film are unknown. Chromatograms of fatty acid methyl esters (FAMEs) prepared from gut samples confirmed that goldfish were able to obtain a substantial proportion of their daily lipid intake from canola oil at the surface of laboratory aquaria. Quantification of goldfish suspension-feeding, processing, and spitting behavior suggested that upper jaw protrusion with a closed mouth during processing was important for the handling of different food types, including oil. Crossflow filtration and the generation of vortices could be involved in oil retention by goldfish, as these processes are used industrially to separate oil from water. These results have implications for the uptake of hydrophobic pollutants and dietary lipids at the surface by suspension-feeding fishes.