The Key To Shark Survival Lies In Their Genetic Stability

Sharks have been navigating our oceans for million years (they appear before trees, to put it into perspective). And during this timespan, Earth and its inhabitants have experienced monumental transformations, but sharks have remained remarkably consistent in both their biology and body shape. This unchanging nature has long puzzled scientists, but a groundbreaking international research effort, uniting experts from Germany, Australia, Sweden, and the United States, has unveiled the extraordinary secret behind the shark’s enduring legacy. Led and coordinated by the research team under the guidance of Senior Professor Manfred Schartl at the Department of Developmental Biochemistry of the Julius-Maximilians-Universität Würzburg (JMU), this groundbreaking study, now published in the esteemed journal Nature Communications, shines light on the sharks’ genetic stability and its far-reaching implications.

For this scientific endeavor, the researchers ventured to the northeast coast of Australia to capture epaulette sharks. The epaulette shark (Hemiscyllium ocellatum) is a small shark species belonging to the Hemiscylliidae family. Notably known as the ‘walking shark,’ these creatures exhibit a unique locomotion style reminiscent of walking on the ocean floor, thus their name. Commonly found in the western Pacific Ocean, particularly in the waters off the coasts of Australia and Papua New Guinea, inhabiting shallow coral reefs, sandy, and rocky seabeds. Here they are primarily nocturnal, foraging for small invertebrates like crustaceans and small fish during the night and retreating to crevices and burrows within the coral reefs during the day.

Once in Australia, the team established a breeding station at the Australian Regenerative Medicine Institute (ARMI) at Monash University. A group of breeding epaulette sharks was kept in a closed, recirculating marine system here, originally obtained as mature individuals from various locations in Queensland. Blood samples were collected, and a specific breeding pair was housed in a custom-built tank, enabling the collection of eggs with known parentage. These eggs were tagged, transferred to separate aquaria, and raised to late pre-hatching stages before being preserved for DNA extraction. The research team first constructed a high-quality reference genome and then meticulously sequenced the complete genomes of the shark parents. This innovative setup allowed scientists to, for the first time, genetically evaluate the mutation rate within a shark family tree.

The findings were staggering. With an estimated mutation rate of 7×10-10 per base pair per generation, the lowest ever recorded in vertebrates. This mutation rate is ten to twenty times lower than that found in mammals. On the surface, this incredibly low mutation rate might seem like good news, particularly concerning cancer rates in sharks. “The low mutation rate could play a decisive role in this,” explains Schartl. However, this genetic stasis comes with a double-edged sword. Mutations play a pivotal role in enhancing genetic diversity within populations, enabling adaptation to changing environments and driving evolutionary progress. The snail-paced evolution of sharks puts them at risk of struggling to cope with ecological pressures like overfishing and habitat loss. And that is exactly what is happening. Shark populations worldwide are facing severe declines, with many of the approximately 530 known species now on the brink of extinction. As sharks hold a crucial position in marine ecosystems, their protection and the preservation of their genetic diversity become paramount.

The low mutation rate observed in epaulette sharks may be attributed to various factors. One potential reason is their poikilothermic nature, which is characteristic of cold-blooded animals with a low metabolic rate. This, in combination with effective purifying selection within a species boasting a substantial long-term population size, acts to remove slightly harmful mutations. This selection process may involve favoring genes responsible for DNA repair efficiency, as genes maintaining genome stability have shown signs of positive selection in elasmobranchs (which includes sharks). “Extrapolating our findings to other shark species that lack the population size stability evident in epaulette sharks suggests a similar low mutation rate may result in long-term negative effects of population bottlenecks in already endangered and overfished species,” the authors state. “Our study, therefore, provides compelling evidence for the need to prioritze preservation of the remaining genetic diversity of global shark populations.”

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