River Erosion as a Natural Experiment for Studying Fish Evolution

The diversity of life on Earth is often attributed to the effects of tectonic movements, such as mountain building and continental drift, that create new habitats and isolate populations.

However, a new study of the freshwater greenfin darter fish suggests that river erosion can also be a driver of biodiversity in tectonically inactive regions.

The greenfin darter fish

(Photo : Junko Kimura/Getty Images)

The greenfin darter (Etheostoma chlorobranchium) is a species of freshwater ray-finned fish, belonging to the family Percidae, which also includes perches, ruffes, and pikeperches.

It is endemic to the eastern United States, where it inhabits the Cumberland Plateau, a relatively flat and stable region in the southeastern states of North Carolina, Georgia, and Tennessee.

The fish is known for its striking coloration and variation across different populations.

Some populations have bright green fins and spots, while others have red or yellow fins and stripes. The fish also varies in body size, fin shape, and other morphological traits, as per EOL.

The researchers from MIT and the University of Tennessee wanted to understand the evolutionary history and genetic diversity of the greenfin darter.

They collected samples from 17 populations across the Cumberland Plateau and sequenced their DNA. They also measured various morphological traits, such as body size, fin shape, and color patterns.

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The Role of river erosion

The researchers found that the greenfin darter populations were genetically distinct and diverged from each other about 2 million years ago, during the Pleistocene epoch.

This was surprising because the Cumberland Plateau has not experienced any major tectonic events since the Paleozoic era, more than 250 million years ago.

The researchers hypothesized that river erosion was responsible for creating new habitats and barriers for the greenfin darter.

They used a computer model to simulate how rivers eroded the plateau over time and how this affected the connectivity and isolation of fish populations.

They found that river erosion created steep gorges and waterfalls that separated populations and reduced gene flow. The isolated populations then adapted to their local environments and developed different traits.

The researchers concluded that river erosion can shape fish evolution by creating ecological opportunities and genetic isolation in tectonically inactive regions.

They suggested that river erosion could explain other biodiversity hotspots in similar regions, such as the Appalachian Mountains and the Ozark Plateau, as per Natural Atlas.

The study was published in the journal Proceedings of the National Academy of Sciences.

The greenfin darter is a benthic insectivore, feeding mainly on insect larvae in cold, high-elevation creeks and rivers.

A study by Bryant et al. revealed the diet of the greenfin darter may be the most diverse in the genus, with some individuals' guts containing up to 15 different species of insects.

Its predators are mainly larger freshwater fish, such as smallmouth bass (Micropterus dolomieu) and madtoms (genus Notorus).

Darters in general are important food sources for many fish. Freshwater eels commonly consume adult darters, and suckerfish prey on their eggs, as per Phys.org.

The species is relatively abundant and has been found to hybridize with E. camurum and E. rufilineatum, where ranges overlap.

Like many other darter species, the greenfin has adapted morphologically to reduce drag and allow stationary positioning on the substrate even in the midst of fast-flowing riffles.

Spawning occurs from late May to early August. Females bury themselves in the substrate in areas protected from strong currents and release eggs upon the arrival of a mate.

The male may remain at the spawning site for several hours, guarding the eggs4. Growth is extremely rapid, with juveniles reaching 45 mm in just a year.

The mean length is 62 mm, with some individuals reaching as much as 110 mm. Lifespans can be up to five years. E. chlorobranchium is thought to be the largest species in the subgenus Nothonotus.

E. chlorobranchium has not been evaluated by the IUCN, although it is classified as state-threatened in Georgia.

However, the species is found and protected in the Great Smoky Mountains National Park and the Southern Appalachian Biosphere Reserve.

Greenfin populations are certainly vulnerable to siltation, water impoundment (or any change to water velocity), and agricultural runoff.

A close relative, E. camurum, has seen huge population declines due to anthropogenic environmental disturbances and is listed as critically imperiled in several states.

Hybridization between E. chlorobranchium, E. camurum, and E. rufilineatum does not appear to be a threat to the species.