Behavioural innovation is a key process for successful colonisation of new habitat types. However, it is costly due to the necessary cognitive and neural demands and typically connected to ecological generalism. Therefore, loss of behavioural innovativeness is predicted following colonisation of new, simple and invariable environments. We tested this prediction by studying foraging innovativeness in the freshwater isopod Asellus aquaticus. We sampled its populations along the route of colonising a thermokarstic water-filled cave (simple, stable habitat with only bacterial mats as food) from surface habitats (variable environment, wide variety of food). The studied cave population separated from the surface populations at least 60 000 years ago. Animals were tested both with familiar and novel food types (cave food: bacterial mats; surface food: decaying leaves). Irrespective of food type, cave individuals were more likely to feed than surface individuals. Further, animals from all populations fed longer on leaves than on bacteria, even though leaves were novel for the cave animals. Our results support that cave A. aquaticus did not lose the ability to use the ancestral (surface) food type after adapting to a simple, stable and highly specialised habitat.