The surface film of sheltered water often supports a population of diatoms, bacteria etc which is termed the ‘neuston’. Hydrobia 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, and the Hydrobia on the left is holding a grain between the lobes of its bifid snout while it grazes off the micro-organisms. Specimens from Aust, Severn Estuary, in water at salinity 22.7p.p.th. Floating is reported not to occur at very low salinity (Green, 1968); perhaps it is not buoyant enough.
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