DNA analysis reveals that the colour of the water may be driving species diversity in the weakly electric barred knife fish from the Amazon Basin. Our finding helps us understand why there are so many fish in the Amazon!

In 2008, together with my PhD supervisors Prof Luciano Beheregaray and Prof Ning Chao, I headed off into the Amazon to collect specimens of a very unusual fish. I was looking for a particular species of weakly electric fish known as the barred knife fish, Steatogenys elegans.

South American weakly electric fish are nocturnal with very poor vision. They inhabit deep, dark river channels and floating floodplain meadows. What makes these fish special is that they have evolved elaborate electrosensory systems capable of emitting and decoding electrical discharges. They use this electrosensory system as a means of electrolocation and communication. In fact, despite the enormous diversity of these fish, each species has its own unique rhythmic electric organ discharge.

In an Amazonian fishing boat, we travelled over 1000 km along the Amazon River and its two largest tributaries, the Madeira and the Negro Rivers, stopping at nine sites along the way to sample the barred knife fish. We sampled in these rivers because at the junction of the Negro and Amazon Rivers (“the meeting of waters”) two vastly different water types meet. These differences are best illustrated in terms of ‘water colour’. Black water (i.e., Negro River), although translucent, is stained dark by tannins and humic acids leached from vegetation, has an acidic pH (~5 or lower) and is nutrient-poor. White water (i.e., Amazon and Madeira Rivers) by contrast, has an Andean origin, is very turbid with high amounts of dissolved solids and nutrients and has a neutral pH (~7).

What makes these different water colours so interesting in terms of electric fish, is that the chemical differences of two water types could potentially effect the electrical signals emitted by the barred knife fish depending on if it is in white water or black water. So I wanted to find out whether or not water colour was affecting electric fish diversity and speciation in any way.

Using a small trawl net off the back of a motorized canoe, we were able to sample barred knife fish from the Amazon, Madeira and Negro Rivers and from each of these I took a small tissue sample for DNA analysis. Back in Australia, I extracted the DNA and analysed it.

To my surprise, DNA revealed that the barred knife fish was not a single species, but two cryptic ones! Cryptic species are species that basically look the same, but are actually different species, and often DNA is one of the only ways to tell them apart. What was remarkable about these two cryptic species was that they were found in the same locations without interbreeding with each other and had been separate species for at least 6 million years without changing their appearance! This made sense considering that barred knife fish have very poor vision and use their electric organ discharges, not their looks, to tell each other apart.

Using a DNA analysis known as a “Genome Scan” I then investigated each of the cryptic species’ relationship to water colour. Excitingly, I found that there appeared to be strong natural selection taking place between populations from the black and the white water. In other words, water colour appears to be driving species diversity between river systems in both species.

Since water chemistry can affect the emission of electric signals between fish, what my result suggests is that fish from the black water have difficulty communicating with fish from the white water. Could water colour be a reason for so many species of Amazonian electric fish?

Results from this study help us understand how different environments influence the distribution and evolution of biodiversity in this complex and species rich ecosystem. This is important, especially considering that Amazonian biodiversity is increasingly threatened by deforestation and climate change.

Dr Georgina Cooke
Chadwick Biodiversity Fellow

More Information:
Cooke, G.M., Landguth, E.L., & Beheregaray, L.B. (2014). Riverscape genetics identifies replicated ecological divergence across and Amazonian ecotone. Evolution. Early View.

This research was a collaboration between Australian Museum Research Institute scientist Dr Georgina Cooke, Dr Erin Landguth (University of Montanna) and Prof Luciano Beheregaray (Flinders University).