Whale sharks may be the biggest fish in the ocean, but surprisingly little is known about their habits. This means researchers studying the gentle giants have had to come up with some novel ways to uncover more about their secretive lifestyles. One group, however, decided to use a relatively new and simple methd: They took seawater samples and tested them for whale shark DNA.
Their results, published in Nature Ecology & Evolution, are impressive. The team not only detected genetic markers for sharks that had recently been in the area, but they were even able to extrapolate on that data and estimate the overall number of mature female whale sharks cruising the Indo-Pacific waters. What’s more, the figures calculated were consistent with those numbers derived from more conventional (and also more difficult) DNA testing.
Known to migrate vast distances, whale sharks have been found to congregate in large numbers in certain regions. One of the most well-known aggregation sites is off the east coast of Mexico, but another major site was recently discovered in the Persian Gulf. It was here that the scientists decided to test whether or not they could detect evidence of the sharks from samples of seawater. To do so, they looked for what is known as environmental DNA, shortened to eDNA.
The team hoped that the water would contain detectable traces of skin cells shed from the sharks, as well as fecal and urine samples. Extracting mitochondrial DNA from these samples and running them through sequencing machines, the researchers were able to detect the fish. By concentrating on a few specific regions, they were then able to assess the genetic diversity of those sharks living in the Persian Gulf, and compare them to those that live in the Atlantic, Indian, and Pacific oceans.
By looking at the number of mutations found within the DNA samples, the researchers were able to calculate an estimate for the total number of female sharks that are swimming in the Indo-Pacific region, as there is little genetic sub-division between shark populations within the area. Based on their data, they think there are around 71,600 females cruising the oceans, compared to previous estimates of roughly 138,000 females (calculated using tissue samples). While the researchers acknowledge that this is about half the number, the degrees of error for the figures did overlap, with the authors noting that their data could still be used as an approximate.
Using samples of seawater to figure out what lives there has been used before in Greenland, but the practice is still in its infancy. If it can be refined, it could offer an incredible non-invasive way to determine exactly what is out there.