Tropical coral reefs are colourful, beautiful - and rich in species. The fish diversity is particularly large. Researchers estimate that up to 8,000 species of fish can be found in coral reefs worldwide.
However, as the coral reefs are declining extremely fast, it is still unclear how reef fish are distributed while their total diversity is still not accurately quantified.
Among other things, this has to do with the fact that many fish species lead very secretive lives, are very similar to each other or live partly in the open sea and are therefore difficult to detect. In order to record the presence of fish in an area, biodiversity research has mostly depended on visual observations by divers (or catching fish).
Now, a new method is making its way into ecology that circumvents such difficulties: environmental DNA (eDNA). The idea of this new approach is that living beings leave their genetic material or parts of it in the environment.
The researchers then only have to take water samples at one location, isolate the DNA (fragments) contained in them and sequence them, i.e. determine the sequence of DNA building blocks. Finally, they can compare the determined sequences with reference DNA sequences that come from reliably identified specimens - and the biodiversity researchers can know whether a species occurs at the location in question.
This is precisely the method that an international research team led by researchers from the University of Montpellier (F) and ETH Zurich has now used to study the occurrence of reef fish.
The researchers collected 226 water samples at 26 sites in five tropical marine regions in 2017 and 2019 and analysed the DNA isolated from them, which they then assigned to the corresponding species or families.
One-sixth more diversity detected
Using eDNA, the researchers found a 16 per cent higher diversity of reef fishes than through conventional survey methods such as visual observations during dives. "Thanks to the eDNA method, we can detect many fish species and families much faster than with observations alone," emphasises Loïc Pellissier, Professor of Ecosystems and Landscape Evolution at ETH Zurich, and one of the two lead authors of a study. This has just been published in the scientific journal "The Proceedings of the Royal Society". The DNA analyses were completed after only two years, but the visual observations that went into the study came from countless observers and cover 13 years of observation activity.
