How Florida’s sea turtle hatchlings find the ocean in the dark

By Arielle Stevenson, University Communications and Marketing

Beneath the sand, the nest begins to move.

One sea turtle hatchling breaks free first. As it digs and shifts in the dark, the movement sends vibrations through the nest. Soon, others begin hatching too, crawling over one another towards the surface into what USF St. Petersburg Biologist Deby Cassill calls a "brand new world."

By the time they emerge at night, their mother is gone.

Hatchling emergence season is underway in Florida, part of the sea turtle season that runs from May through October. It's the time of year when baby sea turtles leave their nests and begin the short but dangerous crawl from the sand to the sea.

"The story starts with the mother," Cassill said. "She may travel hundreds or thousands of miles to return to the beach where she hatched. She comes ashore at night, digs a nest, lays her eggs, covers them and returns to the sea."

How she finds that beach is part of the mystery Cassill studies. Sea turtles imprint on the magnetic field of the beach where they hatch, creating a kind of biological map that helps females return decades later.

"It's all about reading the magnetic field," she said.

Once the mother reaches the shore, the work begins. Using her back legs, she excavates a narrow, column-like chamber deep enough to hold the clutch, or collection of eggs.

"She starts just with her back legs - dip down, fling out, dip down, fling out - and they go down as far as their back legs will reach," Cassill said.

The eggs, about the size of ping pong balls, drop into the nest. Afterward, the mother scrapes sand over the eggs and moves around the area, hoping to disguise the place where her babies lay.

"The female has laid her eggs and covered them up," Cassill said. "She then abandons them."

From the outside, it appears the mother turtle abandons the eggs and nest she worked so hard to lay. Cassill's research suggests something more complex: sea turtle nesting is a form of maternal risk management, or the way mothers manage the odds when predators, storms or catastrophe can erase an entire clutch of offspring.

In a 2021 paper in Scientific Reports, Cassill applied her theory of maternal risk-management to loggerheads nesting on barrier island off Southern Florida from 1988 to 2004, focusing on reproductive adaptations that mitigate clutch failure caused by storms and predators.

One strategy is spreading risk. Sea turtle mothers can lay multiple nests in a breeding season, moving them across the beach instead of placing every egg in one location.

"She's hedging her bets by diversifying where her nests are laid," Cassill said.

Weeks later, when her hatchlings finally reach the surface, they enter a world full of risk. The distance to the water may be short, but for a newly emerged turtle, it can be one of the most dangerous stretches of its life.

"There are raccoons, there are birds, there are crabs," Cassill said.

Storms, beach obstacles and artificial light can add to that risk. Hatchlings are trying to reach the ocean, but they are not following a parent. So, they look for ancient cues.

"They are orienting toward the horizon line," she said.

Depending on the night, that line may be shaped by moonlight, starlight, clouds or the reflection of the water. Cassill said hatchlings may also sense the vibration of the waves through the sand and smell the sea and sea grass to help lead them to the ocean. But developed coastlines can disrupt that system.

A glow from a building, sign or parking lot can pull hatchlings away from the water, exhausting them or making them more vulnerable to predators.

If the turtle is male, Cassill said, that first crawl may be the only time he ever touches land. If the turtle is female, she may not return for decades.

"The hatchlings go into the water and disperse," Cassill said. "Mom never ever sees them."

The result is a life cycle built around long odds: thousands of hatchlings may emerge, but only a few survive to return to their home beach and produce the next generation.

"Over her lifetime, she'll produce maybe 4,000 offspring," Cassill said. "Just a few of her daughters will ever grow up and mature and return to their mother's beach and produce her next generation of offspring, her grandchildren."