Cave depth and subterranean connectivity are drivers of intraspecific trait variability in two subterranean congeneric beetles

Subterranean habitats host organisms with convergent morphological and physiological adaptations—known as troglomorphies—including reduced eye and pigmentation, elongated appendages and enhanced sensory systems. These traits confer selective advantages in dark, nutrient-poor environments. While interspecific variability in troglomorphic traits is documented, links between species abundance, trait expression and environmental gradients remain unclear. We investigated how cave depth and habitat connectivity influence local abundance, troglomorphic trait expression, and intraspecific morphological variability in two congeneric subterranean beetles, Duvalius carantii and Duvalius hartigi, living in contrasting geological contexts: a deeper, isolated alpine system and a more connected, shallow volcanic network. Both species display intraspecific variation in troglomorphic traits, including elytra, leg and antenna length, and pigmentation. We tested four hypotheses: (i) species abundance increases with cave depth; (ii) troglomorphic traits intensify with depth; (iii) morphological variability is lower in the more isolated D. carantii; and (iv) increased connectivity in the volcanic system promotes population mixing and greater intraspecific variability in D. hartigi. Our findings support these hypotheses: abundance increases with depth in both species, with deeper peaks in the volcanic system. Troglomorphic traits intensify with depth only in the alpine species. Morphological variation is lower and more homogeneous in D. carantii, while D. hartigi shows higher variability across sites, likely reflecting enhanced subterranean connectivity. Overall, geological context and habitat connectivity jointly shape trait expression and population structure in cave beetles. These results highlight the interaction between habitat connectivity and environmental gradients in shaping trait expression and population structure, outlining a framework applicable beyond subterranean ecosystems.