Scientists have described a new species of eyeless cave fish that lives in underground waters

The study is published in the journal Integrative Organismal Biology.

At first glance, such animals seem like an evolutionary dead end: they have lost their eyes, live in isolated caves and depend on very narrow environments. But the new work shows otherwise. The ancestors of these fish were already underground dwellers, and then their descendants split into several species. So evolution continued even underground.

The main idea behind the study is simple: for us, caves look like dead ends, but for these fish, underground water could have been hidden pathways. Aquifers helped them disperse, isolate themselves, and evolve into different species over time.

Details

The authors studied southern cave fishes of the genus Typhlichthys. Two species, Typhlichthys subterraneus and Typhlichthys eigenmanni, were previously known. New work has added a third, Typhlichthys styx. Genetic analyses showed that these three lineages descended from a common ancestor that was already adapted to life underground.

The name styx refers to the river Styx from Greek mythology - the border of the underworld. For a cave fish living in darkness and underground waters, this name looks quite symbolic.

To confirm that in front of them a separate species, the researchers used not only DNA, but also the structure of the body. CT scans revealed a feature of the new fish's skull: a remnant of eye socket bone, which is not present in the other two species of southern cavefish. All of these fish have lost their eyes, but their bones still hold traces of their evolutionary past.

Geology proved particularly important. The distribution of the different populations did not match the usual rivers and streams on the surface. But it did correspond well with the boundaries of aquifers - underground layers where water moves. Scientists believe it was these underground systems that helped the fish disperse and split into new species.

Why it matters

For a long time, cave animals were often seen as "remnants of ancient life": it's as if they went underground, adapted to the darkness, and barely changed from there. The new study shows a more lively picture. Even in closed, dark and resource-poor underground ecosystems, new species can emerge.

This is important for conservation, too. Cave fish depend on clean underground water. If the water is polluted, over-pumped or its movement altered by construction, agriculture and dams, such species can disappear very quickly.

There is also a practical conclusion: it is impossible to protect a species if science doesn't already know it exists. The description of Typhlichthys styx shows that even in well-studied countries, there may be individual life lines lurking underground that were previously thought to be part of already known species.

Background

Cave fish are one of the classic examples of evolution in extreme environments. In total darkness, vision becomes useless, so many such fish lose their eyes and colouration over time. They may, however, develop other ways of orientation, such as sensitivity to water movement, odours and chemical signals.

Southern cavefish live in the underground waters of the eastern United States. Studying them helps us understand not only how the animals lose their eyes, but also how they settle in hidden aquatic systems where humans have little or no access.

The new work also continues a broader theme: cave species can look similar because of living in similar environments, but genetically have complex and branching histories. So appearance alone is not enough to understand such animals - we need DNA, geology and groundwater data.

Source

Chase D. Brownstein et al, "Aquifer-mediated speciation in cave-adapted fishes", Integrative Organismal Biology, 2026.