Exploratory behaviour divergence between surface populations, cave colonists and a cave population in the water louse, Asellus aquaticus

Behaviour is considered among the most important factors in colonising new habitats. While population divergence in behaviour is well-documented, intraspecific variation in exploratory behaviour in species with populations successfully colonising and adapting to extreme (compared to the ‘typical’) habitats is less understood. Here, by studying surface- vs. cave-adapted populations of water louse ( Asellus aquaticus ), we tested whether (i) adaptation to the special, ecologically isolated cave habitat includes a decrease in explorativeness and (ii) recent, surface-type cave colonists are more explorative than their surface conspecifics from the source population. We repeatedly tested dispersal related novel area exploration and dispersal speed in both the presence and absence of light. We found that surface populations showed higher behavioural activity in dark than in light, and they were more explorative and dispersed faster than their cave conspecifics. Recent colonists showed a trend of higher dispersal speed compared to their source surface population. We suggest that extreme and isolated habitats like caves might work as ‘dispersal traps’ following successful colonisation, because adaptation to these habitats includes the reduction of explorativeness. Furthermore, we suggest that individuals with higher explorativeness are likely to colonise markedly new environments. Finally, we provide experimental evidence about surface A. aquaticus moving more in dark than in light. Significance statement Environmental conditions in caves are differing drastically from those of the surface. Consequently, animals colonising subterranean habitats are subject to different selective forces than those experienced by the ancestral surface-living population. Behaviour is believed to be a key factor in successful colonisation to novel habitats; however, intraspecific behavioural variation in species with both surface- and cave-adapted populations is less known. Here, we compared dispersal related novel area exploration and dispersal speed across surface and cave-adapted populations of the freshwater crustacean Asellus aquaticus . Our results show that cave-adapted A. aquaticus are significantly less explorative and disperse slower than surface-type populations, indicating that caves may act as ‘dispersal traps’, where adaptation includes the loss of explorativeness. Also, recent cave colonists show a trend to be faster dispersers than peers from the surface source population, suggesting that individuals with higher explorativeness are likely to colonise markedly different new environments.