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By Ben Mayne 21 April 2022 2 min read

Taking a skin biopsy from a captive turtle. Credit: Katia Ballorain, Centre d'Etude et de Découverte des Tortues Marines (CEDTM).

Age is an important characteristic for understanding populations from humans to green sea turtles. This is why we provide the age of all the occupants in our homes during the census.

Age can reveal whether a population is increasing or decreasing. Governments can use age data to work out if a suburb needs more schools or retirement homes. Wildlife managers can use age data to work out if a population is at risk of extinction or set sustainable targets for fishing.

Unfortunately, we can’t ask animals to fill out a census, nor do they carry birth certificates. We need alternatives to determining age.

Ageing green sea turtles

The age of marine turtles is very difficult to determine. We can count growth rings in the cross section of the humerus (upper forelimb bone), like counting growth rings in a tree trunk. But we can only use this method to age deceased turtles. Not surprisingly, this method isn't used for wildlife management. It would also be counterproductive for conservation efforts to use a lethal method to estimate the age range of a population of turtles.

We recently developed a way to age fish using a DNA test as a safe, cheap alternative to counting growth rings in otoliths (ear bones). The test requires only a small piece of tissue, such as a fin clip, which doesn't harm the fish. We've now adapted this test to determine the age of green sea turtles using their DNA.

Details in the DNA

DNA records the genetic code of living things. But DNA also has small chemicals that can bind to it, causing epigenetic modifications. These modifications regulate which genes are turned on or off, without changing the genetic code.

DNA methylation is one kind of epigenetic modification. It adds and removes methyl groups (CH3) at certain sites in DNA. Normally, cells maintain DNA methylation when they replicate. But as animals age, cells accumulate mistakes in DNA methylation and sites may gain or lose methyl groups. We have figured out a way to track these mistakes and use them as a molecular clock to determine the age of animals.

Our ageing test for green sea turtles is the first of its kind for any reptile. The test uses DNA sequencing to measure DNA methylation from a small sample of the foot skin of turtles. We developed it by taking foot skin samples from turtles of known age living in captive populations and from wild turtles caught at known time intervals using mark-recapture methods. This enabled us to relate DNA methylation to age and use it to determine the age.

A green sea turtle swimming in the sea.

Managing wild populations of marine turtles

Our method is a safe way to age green sea turtles. It is also cheap and fast. We can alayse hundreds of samples at a time.

Our next step is to study populations of turtles in wildlife settings. We are also expanding our DNA test to work for other species of marine turtles to help better understand and manage wild populations of these beautiful animals.

The paper entitled Age prediction of green turtles with an epigenetic clock was published in Molecular Ecology Resources. The authors are from CSIRO, Cayman Turtle Conservation and Education Centre, the Centre d'Etude et de Découverte des Tortues Marines La Réunion, Kélonia l’observatoire des tortues marines, Department of Biodiversity, Conservation and Attractions Western Australia, The University of Queensland and the University of Western Australia.

This work is part of our Environomics Future Science Platform.

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