Food preference and food type innovation of surface- vs. cave-dwelling waterlouse (Asellus aquaticus) after 60 000 years of isolation

Abstract

Abstract Behavioural innovativeness is important for colonising new habitats; however, it is also costly. Along the colonisation event of a simple, stable and isolated habitat offering only new food sources, one could hypothesize that the colonising individuals are more innovative than the average in their source population, showing preference to the new resource, while after colonisation, the adapted population will lose its innovativeness and become specialised to the new resource. To test this hypothesis, we compared food preference and food type innovation of a cave-dwelling waterlouse (Asellus aquaticus) population (genetically isolated for at least 60 000 years) to three surface-dwelling populations, also sampling individuals that have recently entered the cave (‘colonists’). In the cave, the only food sources are endogenous bacterial mats, while surface populations feed on various living and dead plant material together with their fungal and bacterial overgrow. We assayed all populations with the familiar and unfamiliar food types from the natural habitats and two novel food types not occurring in the natural habitats of the species. We found that all populations preferred surface to cave food and consumed the unnatural novel food types. Surface populations avoided cave food and colonists spent the most time with feeding on surface food. We conclude that the cave population maintained its preference for surface food and did not lose its food type innovativeness. We suggest that adapting to the special cave food was a major challenge in colonising the cave. Significance statement Behavioural innovativeness is a key trait for adapting to environmental changes or to colonise new habitats. However, it has developmental and maintenance costs due to the high energy need of the necessary sensory and neural organs. Therefore, we asked whether behavioural innovativeness decreases after colonising an isolated, stable and highly specialised habitat. By comparing food type innovativeness of surface-dwelling populations of waterlouse (Asellus aquaticus) to a population that has colonised a cave at least 60 000 years ago, we found that the high innovativeness towards unnatural food was retained in the cave population. Further, all populations preferred surface food (decaying leaves), with surface populations almost completely avoiding cave food (endogenous bacteria mats). We suggest that (i) food type innovativeness is evolutionary rigid in our system and (ii) the cave food was rather an obstacle against than a trigger of cave colonisation.