How blind are they? Phototactic responses in stygobiont diving beetles (Coleoptera: Dytiscidae) from calcrete aquifers of Western Australia

Abstract

AbstractSubterranean water beetles endemic to groundwater calcretes of Western Australia exhibit convergent traits typical of troglomorphic arthropods, including loss of eyes, pigmentation and wings. As these dytiscid species are estimated to have been isolated underground in permanent darkness for over three million years, it is predicted that they will completely lack phototactic responses. We tested this hypothesis by analysing the behaviour of six subterranean beetle species within an observational arena with dark and light hemispheres. Scan samples at 1 min intervals and total time spent on each hemisphere were recorded over a 20 min period, testing at least 15 individuals per species. We quantified behaviour as an index (dark ratio) so that individual species in this, and future, studies can be consistently compared. Results analysed as both categorical and absolute proportion of time spent in each hemisphere suggest negative phototaxis in Paroster macrosturtensis. The remaining five species did not display any preference for either light or dark hemispheres. These results raise the possibility that some ancestral Paroster species may have exhibited negative phototactic behaviour prior to subterranean colonization. The retention of such a behavioural trait in lightless environments could represent the maintenance for some unknown pleiotropic function. Alternatively, it is possible that insufficient time has passed for neutral processes to render photoreception genes and phototactic behaviours non‐functional. Our study adds to a growing body of evidence that implies highly troglomorphic animals may have evolved from ancestral species that exhibited negative phototaxis as a preadaptation to living in permanent darkness.