Research highlight

Seawater shares shark secrets

Nature Ecology & Evolution

November 22, 2016

The genetic diversity of the world’s largest fish, the whale shark, is characterized by analysing a small amount of seawater. Described in a paper published online in Nature Ecology & Evolution this week, the technique, known as environmental DNA (eDNA) sequencing, extracts trace amounts of an organism’s DNA from water, from which useful information on the health of whale shark populations can be inferred.

Conservation biologists routinely use information on the genetic diversity of wild populations to inform decisions about how best to manage them, but getting this data relies on taking invasive tissue samples from the study organism. It also necessitates taking enough samples to adequately represent the population as a whole, which can be particularly difficult to do for marine animals living in large expanses of ocean.

Philip Thomsen and colleagues overcome these limitations by demonstrating that useful information on the genetics of whale sharks can be gained by analysing less than 30 litres of seawater from where the sharks seasonally gather to feed in the Arabian Gulf. From this, they were able to make an estimate of the population size of both locally and across the entire Indo-Pacific, and show that these populations, including the Gulf, are genetically distinct from whale sharks found in the Atlantic Ocean.

Sequencing eDNA from water has previously been used to detect the presence of aquatic organisms, but this is the first time it has been used to gather information on the genetics of whole populations. Whale sharks are currently listed by the International Union for Conservation of Nature (IUCN) as endangered, and the authors suggest that their more efficient monitoring technique can complement existing tools for the conservation of this, and many other, species.

doi: 10.1038/s41559-016-0004

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