Whale sharks are the world's largest known fish, growing 40 feet or more in length. Nicknamed "gentle giants," they are slow-moving filter feeders that pose no threat to humans. One of the sharks' favorite spots is the Arabian Gulf, also known as the Persian Gulf, where they seasonally gather to feed. The location is in waters off the country of Qatar. A researcher from Maersk Oil of Qatar, Steffen Sanvig Bach, was a co-author.
Sigsgaard, Bach and their team collected a few small seawater samples totaling less than 8 gallons from the Gulf. They then sequenced trace amounts of the whale shark's DNA contained in the samples, and estimated the DNA's mutation rate by comparing it with genetic data previously obtained from other whale sharks at various locations. Mutation rates are inherently tied to breeding rates, which are an indicator of an animal's population size.
"The genetic diversity we find in the water samples represents an effective population size - number of reproductively active individuals - of around 70,000 females, based on our calculations," Sigsgaard said. "As we find that the Qatar aggregation belongs to the Indo-Pacific population, we expect this estimate to reflect the entire Indo-Pacific population."
That covers an enormous territory, as the Indo-Pacific comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two in the general area of Indonesia.
Many tuna were also detected to be living in the Gulf waters with the sharks, strongly suggesting that whale sharks follow tuna, which they do not even eat. Caviar is what they are after, according to the researchers.
Thomsen explained, "The whale sharks feed on the tuna eggs, not the tuna themselves."
DNA in water has already been used to estimate the populations of many other animals. In freshwater, it has suggested numbers of fish, amphibians, birds, insects and crustaceans in different specific locations. In seawater, it has been analyzed to estimate fish, seals, whales and birds.
Its application to sharks holds great potential since these marine predators are challenging to study due to their wide ranges, location of some species in very deep water and other factors.
Analyzing eDNA in water poses its own challenges, since water is not a stationary substance.
"DNA in water has an approximate degradation time of a couple of days or weeks," senior author Philip Francis Thomsen, an assistant professor at the University of Copenhagen's Natural History Museum of Denmark Center for GeoGenetics, told Seeker. "This makes it possible that non-local DNA can be transported with ocean currents. But we expect this to be a minor or even non-existing input compared to the local source of DNA."
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Ryan Kelly, an assistant professor at the University of Washington's School of Marine and Environmental Affairs, has also conducted studies using eDNA. He told Seeker that Sigsgaard and her colleagues "have certainly pushed the boundaries of what is possible to do with environmental DNA."
He added that the research "represents a conceptual advance, and will be a basis for conversation and thought among those in the field, particularly among those - and I count myself among them - who are interested in moving genetics-based monitoring out of the lab and into the real world."
The applications are seemingly limitless. They include studying other populations of endangered marine species, detecting invasive species on the move, monitoring the effects of climate change on populations and much more, Thomsen said.
They might even solve some long-standing mysteries, such as what exactly lurks in Scotland's infamous Loch Ness.
Thomsen said that "if it is a real animal living there, then it could potentially be detected" via eDNA. "But it would not mean much unless one actually captures the species or photographs it authentically - not just someone seeing something strange."
For now, he and his team are sticking to studies in waters off of Qatar. They are now trying to detect whales, sea snakes, turtles, fish and other sharks using eDNA from seawater samples.
Photo: Whale shark at Al Shaheen, Arabian Gulf. Credit: Steffen Sanvig Bach, Maersk Oil Research and Technology Center, Qatar