Four P. ulvae under mucus raft feeding on neuston. The surface film of sheltered water often supports a population of diatoms, bacteria etc. which is termed the ‘neuston’. P. ulvae can feed under the water surface with the support of a mucus raft which traps the neuston. In this picture several grains of sand are trapped in the mucus. The P. ulvae can hold a grain between the lobes of their bifid snouts (enlarged inset) while grazing off the micro-organisms. Floating is reported not to occur at very low salinity (Green, 1968); perhaps it is not buoyant enough.
Aust, Severn Estuary, salinity 22.7p.p.th. Aug. 2011.