Subterranean animals are known for their highly evolved phenotypes. They are eyeless, depigmented and possess elongated appendages compared to their surface relatives. Increased antenna and leg length of cave species has traditionally been explained as a consequence of selection for non‐visual senses and increased food‐finding ability in an environment low in energy. Variation in appendage length between cave species is usually thought to result from differences in time since the colonization of the subterranean habitat. In this study, we analyzed appendage length variation in the Dinaric amphipod speciesNiphargus croaticus. Relative length of appendages varied substantially among populations. Using multilocus phylogenetic analysis, we showed that the species is nested within highly specializedN. steuerispecies complex and rejected the time hypothesis. Next, we explored the effects and strength of two environmental factors, water flow and presence of a competing species,N. subtypicus. Populations in caves with flowing water had shorter appendages than populations in cave lakes. Presence of the competing sister species did lead to longer appendages in stagnant water, but had no effect in flowing water. Abiotic factors had a stronger effect than biotic factors, but their relative strength differed among appendage pairs. High variation in appendage length between adjacent population shows that the morphology of cave arthropods is changing quickly and therefore cannot be used to predict species age. Rather than being a general adaptation to cave life, long appendages seem to be associated with the absence of water flow as well as character displacement when in sympatry with ecologically similar competing species.

In comparison to biodiversity on Earth's surface, subterranean biodiversity has largely remained concealed. The olm (Proteus anguinus) is one of the most enigmatic extant cave inhabitants, and until now little was known regarding its genetic structure and evolutionary history. Olms inhabit subterranean waters throughout the Dinaric Karst of the western Balkans, with a seemingly uniform phenotypic appearance of cave‐specialized traits: an elongate body, snout and limbs, degenerated eyes and loss of pigmentation (“white olm”). Only a single small region in southeastern Slovenia harbours olms with a phenotype typical of surface animals: pigmented skin, eyes, a blunt snout and short limbs (“black olm”). We used a combination of mitochondrial DNA and genome‐wide single nucleotide polymorphism data to investigate the molecular diversity, evolutionary history and biogeography of olms along the Dinaric Karst. We found nine deeply divergent species‐level lineages that separated between 17 and 4 million years ago, while molecular diversity within lineages was low. We detected no signal of recent admixture between lineages and only limited historical gene flow. Biogeographically, the contemporaneous distribution of lineages mostly mirrors hydrologically separated subterranean environments, while the historical separation of olm lineages follows microtectonic and climatic changes in the area. The reconstructed phylogeny suggests at least four independent transitions to the cave phenotype. Two of the species‐level lineages have miniscule ranges and may represent Europe's rarest amphibians. Their rarity and the decline in other lineages call for protection of their subterranean habitats.