Will rising seas drown sea turtle eggs?

Raine Island is really nothing more than a sandy speck located on the fringes of Australia’s Great Barrier Reef. Barely a third of a square kilometer in surface area and some 630 kilometers from the nearest major city, Cairns, the island is home to the world’s largest green sea turtle (Chelonia mydas) rookery. For at least 1,000 years, as many as 18,000 female sea turtles show up on the cay’s sandy shores to deposit their eggs each year. For just under two months, their offspring develop inside the leathery shells. Then the hatchlings bite their way out into the world, scoot down the beach, and slip back into the water.

But the story actually goes a little differently on Raine Island. For at least the past 20 years, nesting success and hatchling production have been decreasing there, even when nesting conditions appear ideal. And nobody knows why.

That’s where James Cook University researcher David A. Pike comes in. Sea turtle mothers need to strike a balance between laying their eggs far enough on shore to protect them from inundation, while keeping them close enough to the sea that the young hatchlings can safely find their way after breaking out of their shells. Pike wondered if the eggs might have been drowning.

If eggs are too close to the water, they could drown, especially in high tides. Developing turtle embryos need to exchange gases with the outside world through their shells. That is, they need to breathe. If the eggs are immersed in water for too long, they might not be able to breathe as efficiently, and that could reduce their chances of survival. Field observations suggest that inundation can reduce egg viability, but nobody knows at what point inundation overwhelms the developing turtle. Is just a few minutes enough? Can the embryos survive a few hours underwater? What about a few hours every day?

Together with Elizabeth A. Roznik, also of James Cook University, and Ian Bell from the Australian Department of Environment and Heritage Protection, Pike rounded up nearly 300 eggs from Raine Island and brought them into his lab where he set about finding out how much inundation green sea turtle eggs could withstand.

After dividing the eggs among incubation trays (“haphazardly”), the researchers subjected them to one, three, or six hours of inundation. On the beach, a nest inundated for six hours would be closer to the water, while one inundated for just one hour would be at the far boundary of the high tide line. A fourth group of eggs was kept from being inundated, to serve as a control group. Some of the eggs were inundated just after being brought to the lab; others were allowed to develop one third of the way to hatching, others half or two thirds of the way. After the inundation event ended, the eggs were left alone to continue developing as usual, surrounded by an appropriate amount of moisture and air.

The eggs that were submerged for six hours were 30% less likely to hatch than those left alone. Those that were dunked into seawater for just one or three hours were only 10% less likely to hatch, which was statistically indistinguishable from the undisturbed control eggs. It didn’t matter at what point in development the eggs got their seawater bath; as long as they were returned to their normal state fast enough, most went on to hatch just fine. Still, saltwater inundation wasn’t lethal for all turtles; some of the reptiles inundated for six hours still managed to hatch just fine. But it wasn’t clear whether even a quick inundation could result in sub-lethal effects on the turtles’ behavior, cognition, or physiology. Just because they survived long enough to hatch does not mean that they’d make it into the water, even on the most optimal of beaches. As is so often the case, more research is necessary.

In the wild, nearly half of all eggs laid at Raine Island don’t wind up producing viable offspring. Since inundation in the lab didn’t wipe out nearly so many sea turtles, something else must be going on as well. Those other factors could include maternal health, contaminants transferred from mothers to their eggs, or high levels of microbes in beach soil, which reduce oxygen available to the developing embryos. In addition, eggs in the field would be likely inundated more than once during the 60 day incubation period. It is possible that multiple, repeated inundations have more severe consequences than just one.

If sea levels rise as much as some have predicted, Raine Island could lose as much as 27% of its surface area, exposing more and more eggs to submersion, both because the sea level itself is higher, but also because storm waves would become more severe. Inundation might not explain all of sea turtle nesting failure, but if current trends continue, it could be responsible for a larger and larger chunk of the blame. – Jason G. Goldman | 24 July 2015

Source: Pike, D.A., E.A. Roznik, and I. Bell. 2015. Nest inundation from sea level rise threatens sea turtle egg viability. Royal Society Open Science 2:150127. DOI: 10.1098/rsos.150127.

Header image: Green sea turtle via Brocken Inaglory/Wikimedia Commons.