The topmouth gudgeon’s diet is quite varied, but usually consists of small insects, crustaceans and other aquatic invertebrates, as well as occasional fish eggs and larvae and some plant material (1) (7) (8) (10). This species sometimes takes food from the water’s surface, where its feeding activity is accompanied by an audible clicking sound that gives the topmouth gudgeon its alternative name of ‘clicker barb’ (7).
In its native range, the topmouth gudgeon typically breeds between May and August, but it may start breeding slightly earlier in parts of Europe (2) (3) (4). During the breeding season, male topmouth gudgeons establish territories which are aggressively defended. Larger males are likely to be more successful at defending a territory and attracting females (7).
In most members of the Cyprinidae family, females only spawn once a year, scattering large numbers of eggs which are left unguarded and vulnerable to predators (7). However, the female topmouth gudgeon lays several batches of eggs over the breeding season, and the eggs are guarded by the male until they hatch. Each female can lay from a few hundred to a few thousand eggs and may spawn with several different males, laying small batches of a few dozen eggs at a time (2) (3) (4) (7).
The eggs of the topmouth gudgeon are yellowish and very sticky (4) (7), and are laid on a range of substrates, including plants, stones, shells, sand and even boats and plastic pipes (2) (3) (4) (7) (8). The female topmouth gudgeon cleans the substrate before the eggs are laid (2) (3) (4). At temperatures of 20 degrees Celsius, the eggs take about 7 days to hatch. The young topmouth gudgeons initially hide beneath and between stones, only later moving into open water (7). The topmouth gudgeon reaches sexual maturity much faster than most cyprinid species (7), being able to breed at just a year old (2) (3) (4) (8).
The lifespan of the topmouth gudgeon is only around three to four years (4) (7) (8). However, by guarding its eggs this species increases their hatching success, and laying batches of eggs over an extended period makes the young fish less vulnerable if environmental conditions change. Combined with its early sexual maturity, this means that the topmouth gudgeon can reproduce rapidly, which is likely to have contributed to its success as an invasive species (7) (8).