Age isn't just a number for one of the ocean's top apex predators. It's a mystery University of Georgia researchers are solving.
A new UGA study has determined how to estimate the age of sharks by analyzing changes to individual animal's DNA. Known as epigenetic clocks, these scientific age estimators focus on chemical alterations, known as DNA methylation, to measure wear and tear on the animal's cells, providing a noninvasive, fast and accurate assessment of an animal's age.
The predictive clock model has been successfully tested in dozens of other animals, but this study marks the first time it's been used in sharks.
The tool can help shed new light on not just the biological evolution of shark populations but also the life cycle of other marine creatures with unknown aging data, the researchers said.
"Age structure is one of the most important things conservation biologists can measure," said Benjamin Parrott, corresponding author of the study and an associate professor at UGA's Savannah River Ecology Laboratory and Odum School of Ecology. "If we could figure out how old wild sharks are, we could finally understand the age structure of their populations. That's incredibly important for conservation of marine populations."
Age estimation tool suggests DNA changes slow in adulthood
The researchers took blood samples from more than 50 zebra sharks across southeastern aquariums, including the Georgia Aquarium.
As creatures age, the chemical tags in their DNA form change, but predictable patterns emerge over time.
With just 10 different DNA tags, the researchers quickly estimated the zebra sharks' ages within two years of their chronological ages, all without needing to analyze an entire genetic sequence. The estimates were compared against age records from the aquariums to assess accuracy.
Given a zebra shark's life expectancy of up to 30 years, that would be comparable to mistaking a person for 26 when they are actually 27.
The epigenome is a lot like an old barn. It starts out well built, but over time things slowly fall apart.
Benjamin Parrott, Odum School of Ecology & Savannah River Ecology Laboratory
"The epigenome is a lot like an old barn. It starts out well built, but over time things slowly fall apart," Parrott said. "Our bodies age the same way. Our skin changes, our hair turns gray and DNA patterns drift too. The clocks pick up on that process."
Even when testing an additional dozen wild-caught sharks outside of the aquarium with unknown birth dates, the pinpointed ages were within roughly three to four years when compared to initial measurements made on the sharks when they were caught.
The study also found that zebra sharks may age more dramatically before reaching sexual maturity. Changes in DNA patterns were most striking in young growing sharks before the rate of change slowed down in adulthood.
That trend of slower changes in older animals matches what epigenetic clocks have shown in mammals. This finding could mean aging processes have stayed relatively similar across vertebrates (animals with backbones) for millions of years.
"We knew that this process occurred in lots of mammals and some non-mammalian species, but we didn't know how widespread this phenomenon was," said Samantha Bock, corresponding author of the study and a doctoral graduate of the Odum School. "This finding means we can potentially unlock key insights into the foundation of long life."
Understanding aging can enhance marine life conservation
Traditional shark age-estimation models require killing the shark and counting growth rings on its spine. Researchers can also gauge age by simply looking at sharks, but that isn't always reliable. This new method could further aging research without harming the animals.
Understanding aging trends is key for species health, the researchers said. Age data can contribute to survival rates and reproductive changes, and assess whether populations are growing or declining. If you see mostly older sharks, for example, their population may be in decline.
"Determining the age of an animal is often very difficult," Bock said. "But if we can use epigenetic patterns to measure the ages of sharks, we can use it as a tool to see these trends in vulnerable populations."
The clock can also help analyze how and why stress, disease or environmental pressures speed up molecular aging in sharks.
Following the success of the epigenetic clock with zebra sharks, this study could provide a foundation for exploration on other vulnerable marine creatures, assisting scientists in future decision-making for population management.
"Sharks play really important roles in maintaining balance in ecosystems," Parrott said. "If you remove a predator from an ecosystem, things can get out of balance. The goal is to give conservation biologists another tool to make better management decisions."
This study was published in Molecular Ecology.
