We all know marine turtles lay eggs and don’t provide any parental care for their turtle hatchlings. The mothers do leave some food for the hatchlings though, as yolk in the eggs. But how healthy is this yolk?
Mammals suckle their young and when they do, they can pass environmental pollutants from their bodies to their offspring’s. But are toxins that maternal turtles accumulate when feeding, passed on to their turtle hatchlings within the eggs? And if so, does it affect the turtle hatchlings’ chances of survival?
These are important questions when conserving turtles like Chelonia mydas, because it means you have to conserve the ecosystem. Dr Jason van de Merwe decided to find out the answer and determine if green sea turtles (Chelonia mydas) pass on organic pollutants to their eggs and subsequently their hatchlings. But first here’s some background on pollution and marine turtles.
Ever since Rachel Carson’s book “Silent Spring” we’ve known that toxins in our environment are not good.
But as the human population pushes past 7 billion people and industrialisation spreads around the globe, keeping toxins out of our environment is getting harder.
Six of the seven species of sea turtles can be found within Australian waters, here’s a list of our native turtles
- Flatback turtle (Natator depressus)
- Green turtle (Chelonia mydas)
- Hawksbill turtle (Eretmochelys imbricata)
- Leatherback turtle (Dermochelys coriacea)
- Loggerhead turtle (Caretta caretta)
- Olive Ridley turtle (Lepidochelys olivacea)
Australia has the only known nesting sites for the flatback turtle. All marine turtles are protected species in Australia. The only species Australia doesn’t have is the Kemp’s Ridley turtle (Lepidochelys kempii) which is distributed between the northwest Atlantic and the Gulf of Mexico. Kemp’s Ridley turtle is only known to nest on one beach, Rancho Nuevo, Mexico!
Moreton Bay in Queensland (just next to Brisbane city), has resident populations of green, loggerhead, flatback and hawksbill turtles. These turtles feed on the plants and animals within the bay but migrate to nesting sites elsewhere. Having resident populations of turtles so close to a major city makes Dr van de Merwe’s toxin research important for turtle conservation. Removing toxins from our environment may ensure healthy turtle hatchlings.
When they migrate to the nesting sites to lay their eggs, the mother turtles don’t eat .
As the eggs develop inside the mother, she mobilises fats and oils from her own reserves and puts them into the eggs as yolk for the hatchlings.
Dr van de Merwe looked at green turtles nesting at the Ma’Daerah Turtle Sanctuary, Terengganu, Malaysia.
These turtles feed throughout the waters of southeast Asia. Several of the toxins which turtles ingest while feeding are soluble in fats, they’re lipophilic (e.g. some polychlorinated biphenyls or PCB’s). So as the mother transfers her fats to the egg yolks, she may also be passing on the toxins she’s accumulated while feeding in places far from the nesting site.
By taking blood samples from the nesting mothers and from the new hatchlings Dr van de Merwe showed that these toxins do pass from the mother to the turtle hatchlings. But what effect does it have?
Hatchlings don’t have an easy time!
The eggs are layed in a hole dug by the mother. When they hatch they have to dig their way up through the sand to the surface of the beach. Then they have to race to the waters edge trying to avoid predators such as herons, egrets and pelicans. The faster they can get across the beach the better. But once in the water they face another suite of predators like fish and sharks.
You’ve got to be a fit and lucky turtle to survive and make it to adulthood.
To examine the fitness of turtle hatchlings, Dr van de Merwe looked at their weight relative to their size (Weight: Straight Carapace Length). He found that turtles which inherited more toxins from their mothers had lower weight to size ratios. So the turtles with more toxins were skinnier.
When the hatchlings emerge from their nests they don’t eat for 3-5 days as they swim towards open water. Therefore, they need a large energy reserve when they hatch. It’s better to be a fat little turtle and not good to be skinny – there’s a lot of exercising to do before you get to eat.
So the toxins may be reducing the energy reserves of the hatchlings and subsequently reducing their chances of survival.
Conservation of these rare animals is more complex than just protecting nesting beaches and reducing the numbers killed by fishing nets and by boat strikes. Saving the adult turtles is important, but giving them clean, unpolluted feeding grounds is probably very important for the survival of the hatchlings and subsequently the next breeding generation of turtles.
van de Merwe (2010) Persistent organic pollutants in the green sea turtle Chelonia mydas: nesting population variation, maternal transfer, and effects on development. Marine Ecology Progress Series, 403: 269-278 DOI: 10.3354/meps08462