A new Yale study identifies a distinct species of eyeless cavefish, a discovery that challenges long-held conventional wisdom that caves and other subterranean ecosystems are evolutionary dead ends.
The study shows that three species of Southern cavefish, including the newly discovered species, Typhlichthys styx, evolved from a common ancestor that had adapted to life underground and dispersed through aquifers in soluble subterranean rock formations in the southeastern United States. It provides the best evidence to date that speciation — when a single species splits into two or more species — can occur in species adapted to only survive in subterranean ecosystems.
“We show that, in terms of evolution, what’s happening underground matters,” said Chase Brownstein, a graduate student in ecology and evolutionary biology in Yale’s Graduate School of Arts and Sciences, and the study’s lead author. “In this case, our analysis found that three species diverged from each other after their ancestor had invaded caves. It also found that underground geology was key to facilitating this speciation.”
The findings are published in the journal Integrative Organismal Biology. Thomas Near, professor of ecology and evolutionary biology in Yale’s Faculty of Arts and Sciences (FAS), is the study’s senior author.
The process of speciation in underground ecosystems is poorly understood, the researchers explained.
In his pathbreaking 1859 book, “On the Origin of Species,” Charles Darwin described cave-dwelling organisms as “wrecks of ancient life” — the remnants of old lineages that had survived by adapting to isolated environments while their related species went extinct. A widely held hypothesis developed that underground ecosystems are “evolutionary dead ends” because species that have adapted to them seem to stop diversifying into new species.
This new study examines three species of Southern cavefish that diverged from one another after their ancestors invaded subterranean ecosystems: two previously recognized, Typhlichthys subterraneus and Typhlichthys eigenmanni, and the newly discovered third species, Typhlichthys styx.
For the study, the researchers analyzed genetic data to create a time-calibrated evolutionary tree of Southern cavefish, which inhabit cave systems from the Appalachians to the Ozarks in the southeastern United States. The genetic analysis revealed a geographically distinct third lineage of Southern cavefish, in addition to the two previously known species, with populations in Tennessee, Alabama, and Georgia. It also showed that the lineages last shared a common ancestor 8 million years ago.
The researchers conducted CT scanning of specimens from the various lineages showed differences in the skeletal structure of this new lineage compared to the other two lineages. While all Southern cavefish species lost their eyes as they evolved to life in permanent darkness, the study showed that the skulls of the new species retain remnants of interorbital bones that once formed parts of its eye sockets. The same bone is not present in the other two species.
“The combination of genetic and anatomical data makes it clear that this is a distinct species,” Brownstein said.
The distribution of various Southern cavefish populations along their ranges do not map onto the region’s surface rivers and streams, which complicated attempts to understand how the species dispersed, researchers said.
Instead, they looked underground for a potential dispersal mechanism. They found that the primary divergences among the populations and species correspond to the boundaries of the region’s aquifers — subterranean geological structures that hold groundwater. The aquifers created openings and channels in the region’s karst landscapes — terrain formed when water dissolves limestone and other soluble rock — that facilitated the cavefishes wide dispersal.
“The aquifers act as underground rivers of dispersal, which allowed the cavefishes to speciate within the cave systems,” Brownstein said.
Many populations of Typhlichthys styx and other Southern cavefish species are threatened by human activity that reduce and contaminate groundwater, including the construction of dams, the overuse of water, and the discharge of industrial and agricultural waste, the researchers said.
The study is an example of the cutting-edge biodiversity research happening across Yale, said Near, who is also the Bingham Oceanographic Curator of Ichthyology at the Yale Peabody Museum.
“Species discovery is an important to addressing the biodiversity crisis that is rapidly reducing the variety of life on our planet,” he said. “For one thing, you can’t protect a species if you don’t know that it exists. Our work on cavefish joins a wide range of research happening on campus to better understanding our biodiversity and develop ways to protect it.”
The study’s coauthors are Gregory J. Watkins-Colwell of the Yale Peabody Museum; Maxime Policarpo of the University of Basel and Max Planck Institute for Biological Intelligence; Richard C. Harrington of the South Carolina Department of Natural Resources; Eva A. Hoffman of the American Museum of Natural History; and Didier Casane of Paris-Saclay University.