Microhabitat structure predicts eye presence in bryophyte-dwelling tardigrade communities

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 vision are frequently linked to environmental conditions, we investigated which factors influence the occurrence of eyes 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 in alpine sites, and the presence of eyes in tardigrades found in those samples was analyzed at the individual level using mixed-effects models that incorporated bryophyte light preference and its morphological complexity. Eyeless tardigrades were relatively more frequent in morphologically complex bryophytes and, more weakly, in bryophytes associated with high-light environments. Together, these patterns suggest that eye reduction may be associated with low light availability within bryophyte interiors rather than with external habitat exposure. Therefore, our study indicates the role of fine-scale environmental filtering in structuring sensory trait variation in microscopic animals.