Microhabitat structure predicts eye presence in bryophyte-dwelling tardigrade communities

Yelyzaveta Matsko, Krzysztof Miler , Izabela Poprawa, Daniel Stec

Microscopic invertebrates often exhibit variation in eye presence within closely related taxa, yet the ecological correlates of this variation remain poorly understood. In tardigrades, eyes may occur or be absent even among species inhabiting similar environments. Because structures that enable seeing are frequently linked to environmental conditions, we investigated which factors influence eye occurrence in bryophyte-dwelling tardigrade communities. We tested whether eye presence is primarily associated with broad-scale light conditions or with fine-scale microhabitat characteristics related to bryophyte structure. Bryophyte samples were collected from shaded and sun-exposed alpine sites, and eye presence was analyzed at the individual level using mixed-effects models incorporating bryophyte light preference and its morphological complexity. The probability of eye presence decreased significantly with increasing structural complexity of bryophyte mats, whereas direct effects of site exposition were not supported. Association with bryophyte light preference was weaker but directionally consistent. These results indicate that microhabitat structure, rather than coarse environmental light availability, is a factor associated with vision in limno-terrestrial tardigrade communities. Therefore, our study indicates the role of fine-scale environmental filtering in structuring sensory trait variation in microscopic animals.