Genetics

Shark Sleuthing: How scientists can identify which species of sharks attack

Paper: Oury, Nicolas, et al. “Forensic genetic identification of sharks involved in human attacks.” Forensic Science International: Genetics (2021): 102558. DOI: https://doi.org/10.1016/j.fsigen.2021.102558

Shark Attack Stats

Shark attacks have captured human attention since they were first documented, and countless books, movies, and even songs have been made about these events around the world. However, shark attacks are not at all common – with only 57 unprovoked attacks documented worldwide in 2020. In addition, most attacks typically do not result in fatalities, with only 13 shark related deaths occurring in 2020. This number is much lower than the amount of people killed by lions, hippos, and even snails each year.

While sharks may not be the mindless, human-hunting machines that horror movies portray them to be, this does not mean that shark attacks are not serious, especially for those unlucky few who are bitten. When attacks occur, it is important for local officials to try to determine what type of shark caused the attack. This is so accurate records can be kept, and patterns, if any, can be discovered. If a particular species of shark, or a particular individual shark, is responsible for many attacks in an area, then risk management strategies can be put into place to prevent people from coming into contact with those species or individuals.

Currently, the way to identify which shark species is responsible for an attack is to examine the bite wounds of victims, and take eyewitness accounts of the attack. This method can be inaccurate if eyewitness accounts are missing detail, or if bite wounds do not have clear tooth and jaw patterns. To avoid these problems, researchers at the Université de La Réunion in France set out to try two new methods of identifying the sharks responsible for attacks using DNA.

A schematic about how DNA barcoding works. First, a sample is taken from the organism (or wound) that needs to be examined. All of the DNA in the sample is then extracted from all other materials present (ex. parts of tissue, hair, or muscle). Then, only the DNA of interest (shark DNA in this case) is isolated and amplified so there are many copies of it. The DNA sequences of these many copies are then analyzed to determine the specific species the DNA is from. Original image by David CasMor via Wikimedia Commons. Image altered by Francesca Giammona.

DNA Detective Work

Researchers first collected DNA from two shark bites that occurred near Reunion Island in the southwestern Indian Ocean. They then used two different, genetics-based methods to discover the shark species responsible for the attacks: DNA barcoding and microsatellite genotyping. DNA barcoding is generally used to identify what species a particular sample of DNA may be from. It relies on “barcoding regions” – areas of the DNA where the sequence is known for many species. These barcoding regions are similar enough between species that they can be easily identified as being from the barcoding region, but different enough that each species has several unique DNA barcoding segments.

In analyzing shark bite DNA, researchers first isolated shark DNA from victim bite wounds, and then looked at the barcoding regions of their samples. By comparing the barcoding regions of the samples with DNA from barcoding regions of known shark species, they were able to match the unknown shark samples to species, and identify the types of sharks responsible for the attacks.

An illustration detailing DNA microsatellites. A DNA strand will contain several segments that repeat, known as microsatellites (denoted in red). Depending on the species, or even the individual, the same DNA strand may have different numbers of these microsatellite repeats. By comparing microsatellites in unknown DNA samples to microsatellites in known samples, scientists can identify the species of unknown DNA samples. Image created in BioRender by Francesca Giammona.

Microsatellite genotyping involves examining short sections of DNA that repeat over and over on particular, known locations on DNA strands. These short, repeating DNA segments are called microsatellites. Every living thing has these microsatellites, but the location and specific DNA microsatellite sequence will vary between species, and even individual organisms.

With this method, researchers first narrowed down the list of potential shark species they believed lived near the area of the attacks, as obtaining this information allowed them to lessen the amount of data needed to collect and compare. Then, they looked for the microsatellites present in their shark bite wound samples. They then matched the microsatellites from their samples to microsatellite samples from known shark species that typically live in the area of the attacks. Researchers determined the sharks that caused the attacks by observing which microsatellites matched between the bite wound samples and known shark species.

The bull shark (Carcharhinus leucas). These sharks are one of the top three sharks most commonly responsible for attacks on humans. In the two shark attack bites examined in this study, DNA barcoding and microsatellite genotyping both concluded that two different bull sharks were responsible. Image by Albert Kok via Wikimedia Commons.

Microsatellite genotyping is a little more specific than DNA barcoding because it can also be used to compare the DNA of samples to see if they are from the same individual. As a final step here, researchers compared the microsatellites between the two shark attack samples to see if they were from the same individual shark. Using the two DNA comparative methods, researchers identified the sharks responsible for two attacks as bull sharks. Using microsatellite genotyping, they were also able to tell that the two attacks were not caused by the same exact shark.

Why does shark identification matter?

How does the ability to accurately identify the sharks responsible for attacks help humans? When researchers receive accurate information from bite wounds and compare it to data on known sharks, they can gain a clearer picture of not only which species tend to be responsible for shark attacks, but also the typical age, sex, and size of attacking sharks. If young, male sharks tend to attack humans more often than other types of sharks, then measures to better protect people can be put into place when those sharks are around. In addition, by learning (through microsatellite genotyping) if particular individual sharks are responsible for multiple attacks, it can aid local governments near attack sites in potential capture programs. While most humans thankfully have no need to worry about shark attacks, the use of DNA sequencing methods to gain valuable knowledge about shark bites can be incredibly important for those few who are directly affected by them.

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