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BiologyWith a mature female at 18 mm SL, this is the smallest European freshwater fish; 2.1 mm at hatching its larvae are the smallest free-living European vertebrates. Found in areas covered by aquatic vegetation, in reed stands; in sheltered bays down to 15 m (Ref. 59043); at the mouth of tributary streams and has been observed in small shoals near the surface, over open areas of gravel or stones (Ref. 26100). Feeds on invertebrates, mainly on cladocerans and larvae of mussel Dreissena polymorpha during the spawning season. Spawns after first winter, from February to May, with eggs (0.7 x 0.6 mm) deposited in a cavity (usually inside standing, broken reeds); these are cleaned and guarded by the male. A single nest may occupy up to 2000 eggs spawned by several females; with females spawning at least twice during the season. Eggs hatch in about a day, the larvae living on the surface layer, near shore, and after yolk absorption, live in deeper layers or near bottom, moving upward at night. Maximum known age is 21 months. Land reclamation, lake level fluctuations resulting to water abstraction and pollution threaten survival of this species (Ref. 59043).