The green sea turtle (Chelonia mydas) is a marine reptile found in the coastal waters and nesting beaches of over 140 countries. As the largest of the hard-shelled sea turtles, it plays a significant role in maintaining the health of marine ecosystems, primarily as a unique herbivore in its adult life stage. This ancient lineage undertakes extensive migrations across entire ocean basins, connecting distant foraging and nesting habitats. The species’ long life cycle and complex movements make understanding its longevity difficult.
Lifespan Estimates and Measurement
The potential lifespan for a green sea turtle is estimated to be 70 years or more in the wild. This longevity is difficult to measure precisely because these animals spend much of their lives in the open ocean and lack external features that reliably indicate age. Growth rates vary greatly between populations and individuals, meaning a turtle’s size is not a reliable predictor of its age.
Scientists rely on indirect methods, such as a technique called skeletochronology, to determine how long these turtles live. This method involves analyzing growth rings, known as Lines of Arrested Growth (LAGs), found in cross-sections of the humerus bone. The LAGs are deposited annually as the turtle’s growth rate slows during less favorable seasons, providing an estimate of chronological age.
Another approach involves long-term tag-and-recapture programs. Studies of nesting females have documented individuals returning to the same beach to lay eggs up to 38 years after they were first tagged. Combining skeletochronology with data from these tagging projects helps researchers build a more accurate picture of the species’ extended lifespan.
The Journey Through Life Stages
The life of a green sea turtle begins when hatchlings emerge from their nests on sandy beaches and make a frantic dash to the sea. Once they enter the water, they begin the pelagic “lost years,” a phase that can last up to a decade. During this time, the young turtles live in the open ocean, often associating with floating seaweed mats like sargassum, where they feed and drift on ocean currents.
The juvenile stage begins when the turtles shift from a pelagic existence to coastal habitats like shallow lagoons and seagrass meadows. This habitat change is accompanied by a dietary shift, moving from an omnivorous diet of small invertebrates to becoming primarily herbivorous, feeding on seagrass and algae. The switch to a lower-protein, plant-based diet contributes to their notably slow growth rate.
This slow growth results in a late onset of sexual maturity. Green sea turtles typically reach reproductive age between 25 and 35 years. Once mature, females initiate long-distance reproductive migrations, returning every two to five years to the general area where they were born to lay their clutches of eggs.
Threats to Longevity
The average turtle in a wild population lives a much shorter life than its potential lifespan due to numerous threats. The greatest reduction in longevity occurs immediately after hatching, as only about one percent of hatchlings survive the initial journey from nest to ocean and through the precarious “lost years.” Early predation by birds, crabs, and fish accounts for a massive mortality rate.
Human-caused factors significantly shorten the lives of juvenile and adult turtles. Entanglement in commercial and recreational fishing gear, known as bycatch, frequently leads to drowning or severe injury. Habitat degradation also reduces life expectancy by eliminating foraging grounds or by destroying nesting beaches through coastal development and rising sea levels.
Older turtles are susceptible to injuries from vessel strikes, which occur when they are hit by boats. The ingestion of marine debris, such as plastic bags mistaken for food, can cause fatal blockages and nutritional deficiencies.