Neonoemacheilus labeosus. Image credit: Šlechtová et al. (CC BY 4.0)
Stone loaches, also known as Nemacheilidae, are a large family of fish commonly found in the streams and rivers of Europe and Asia, with a small number of species also inhabiting certain Ethiopian lakes.
How these fish, which originated in Asia about 50 million years ago, made their way into European and African waters remains poorly understood. Major geological and climate changes took place throughout this period, from the formation of the Himalayas and other mountain ranges in eastern and western Asia to large drops in temperatures or rainfalls in certain regions.
Šlechtová et al. studied the influence of these events on the spread and evolution of stone loaches. The team used a large dataset of 471 samples obtained from more than 250 species to reconstruct the evolutionary tree of the Nemacheilidae. The analysis uncovers six major groups (or clades) within the family, all stemming from a common ancestor living 48 million years ago in Indochina (current mainland Southeast Asia). Each clade has separate yet sometimes overlapping geographical distributions. They followed distinct routes to spread across Asia and Europe, which Šlechtová et al. were able to examine in the light of geological and climate changes.
For instance, a major aridification event taking place in Central Asia between 34 to 23 million years ago created a geographical divide within an ancestral stone loach group, splitting it into two parts that evolved separately to form two of the six current clades. While the Himalayas also acted as a strong barrier, growing highlands in eastern and western Asia expanded the range of suitable habitats for the fish, allowing them to colonize central and northern Asia and, from there, Europe.
Other major geological events played a strong role in the propagation of the Nemacheilidae. When a small tectonic plate known as West Burma Terrane first contacted Southeast Asia 33 million years ago and later northeast India around 30 million years ago, the ancestral fish family used the plate like a ferry boat to spread to these new territories, and from there, expand into the Near East, Southeast Europe and Northeast Africa.
These findings build on prior work investigating how geological and climate events have shaped evolution. However, they are the first case study to show the complete evolution of an animal group over such a large area and long period. It is the first detailed example of its type and could be precious to inform future work on evolution.
