Tiger sharks (Galeocerdo cuvier) are highly mobile, widely distributed sharks that feed on a range of vertebrate prey. They live in diverse habitats in tropical and subtropical oceans throughout the world and are expected to mix and interbreed freely due to their ability to travel widely.
However, a recent analysis of the genomes of 242 tiger sharks from ten localities around the world has revealed that there are two distinct populations of tiger sharks in the world’s oceans. Conducted by Dr. Andrea Bernard from Nova Southeastern University, the research identifies an Atlantic and an Indo-Pacific population that are significantly different from one another.
“These results mean that Atlantic and Indo-Pacific tiger sharks, despite their ability to travel vast distances, have not intermingled to reproduce for a long time,” explained study co-author Professor Mahmood Shivji, the director of the Save Our Seas Shark Research Centre (SOSF-SRC).
“This long-term separation between Atlantic and Indo-Pacific tiger sharks has resulted in them developing into separate populations, each with its own unique genetic diversity.”
The study also found that tiger sharks from Hawaii, one of the world’s most geographically isolated archipelagos, are genetically different from those found in the Indian Ocean.
The scientists made use of genome-scale analysis at the sites of 2,000 different single nucleotide polymorphisms (SNPs), which allowed a very detailed look at the differences between tiger shark populations.
“SNPs repeat across the entire genome,” explained Professor Shivji. “This increases the amount of information for geneticists to work with relative to previous surveys that were limited to small subsections of genetic information.”
The SOSF-SRC uses genetics to gain knowledge and understanding about sharks and rays in the world’s oceans. Its aim is then to apply this knowledge to improve the protection of these species.
“A guiding principle for our work is to focus primarily on shark species that are of high conservation concern,” said Professor Shivji. “We spread our work across both discovery research questions (for instance, understanding how sharks function) and applied conservation research (for instance, understanding interactions between shark migration patterns and fisheries or investigating trade in shark parts). The important point is that all of this information advances our knowledge of sharks and informs conservation.”
Tiger sharks are apex predators and are therefore important in the ecology of ocean systems. Even though they are globally distributed, their populations are threatened by fishing and their ecosystems are impacted by climate change, pollution and habitat destruction.
“Overfishing poses a clear danger to sharks. The genetic diversity in overfished species will inadvertently be diminished and they won’t adapt to the fast pace of environmental change,” said Professor Shivji.
“Compared to other fish species, tiger sharks occupy an unusually wide variety of habitats. Given their influential ecological role and their widespread movements that expose them to a variety of fisheries, science-informed fisheries management of tiger sharks is important to achieve global conservation goals.”
Considering the results of the study, scientists will need to manage the two distinct populations of tiger sharks in specific ways in order to preserve their genetic diversity. This will be particularly important as climate change affects ocean ecosystems and tiger sharks require all their genetic resources in order to adapt rapidly. The study recommends that a targeted conservation plan is devised for Hawaiian tiger sharks as well.
“Tiger sharks are part of not only the ocean’s overall health, but also its beauty and wonder. Our ongoing mission at the SOSF is to know more in order to do better by these species and their ocean home. Improving our insights into tiger sharks is ultimately linked to our health and well-being,” said His Excellency Abdulmohsen Abdulmalik Al-Sheikh, the founder of the SOSF.
If our understanding of tiger sharks had remained that populations are similar wherever they are found across the oceans, we might have stood to lose their true diversity and undermined their resilience as a powerful and ancient species.
The study is published in the Journal of Heredity,