Life history traits of an invasive population of bighead goby Neogobius kesslerei (Günther, 1861) from the middle Danube, including absolute and relative fecundity, egg size, number of spawning batches and size at first maturation, were examined and evaluated within an epigenetic context. Ripe bighead goby females attained 42.8–142.5 mm LS, with absolute fecundities ranging from 669 to 5646 eggs (mean 2109 eggs), and relative fecundities of 61.6–174.0 eggs g−1 body weight (mean 119.6 eggs). Egg diameters varied between 0.04 mm and 1.70 mm (mean = 0.57 mm). In the pre-spawning period there was no clear size distinction in eggs (0.12–1.45 mm; mean = 0.52 mm) in 34.1% of females; whereas in 65.9% of females, two egg size groups were distinguished: group I diameters of 0.06–0.85 mm (mean = 0.43 mm), and group II diameters of 0.55–1.70 mm (mean = 1.17 mm). Females with size-group II eggs at the beginning of the reproductive season were assumed to be ready to spawn and the others to be subsequent spawners. Bighead goby appears to be altricial compared to the round goby, although in both species a shift from highly precocial towards a less precocial life history was observed. These differences, affected by epigenetic mechanisms and resulting in alternative ontogenies, may have important implications for a species’ potential success in novel environments, favouring the round goby over short time periods (several years) and bighead goby over longer periods of time (decades and longer).