By collecting ocean water where whale sharks had been spotted, researchers from the University of Copenhagen were able to able to retrieve and analyze DNA from that shark population, the Los Angeles Times reports.
The team led by Copenhagen's Philip Francis Thomsen collected 20 seawater samples from near the Al Shaheen oil field that's located offshore of Qatar. Whale sharks had recently been seen aggregating there seasonally. As the researchers report in Nature Ecology & Evolution this week, they sequenced mitochondrial DNA obtained from these samples and compared them to tissue samples that had been previously collected from 61 sharks at that same location. The mtDNA haplogroups found in the environmental DNA samples included the ones from the tissue samples, plus a few additional ones.
Through a principal components analysis of these haplogroups, Thomsen and his colleagues found that this population of whale sharks was more closely related to populations in the Indian and Pacific Oceans than ones in the Atlantic Ocean.
Along with the whale shark DNA, the researchers also picked up mackerel tuna DNA. As the LA Times notes, the two seemed correlated: when the seawater samples had high levels of shark DNA, it also had high levels of mackerel tuna DNA, and samples with low levels of shark DNA also had low mackerel tuna DNA levels. "We argue that this result most probably reflects the predator–prey relationship observed between the two species," Thomsen and his colleagues write.