Within the egg - a tale of tiny turtles

Reproduced with permission from Tropical Topics - a second compilation by Stella Martin. (Originally adapted from articles in Coralines by Margaret Card EPA and Jeff Miller, QPWS).

Green Turtle laying eggs on Heron Island Photo: GBRMPA

The laying of eggs is not an annual event for female turtles. The average female spends several years in the feeding areas building up energy for the next reproductive effort. As she prepares to breed, the follicles (early egg yolks) in her ovaries grow from about 3mm to 2cm in diameter (depending on species). The turtle then migrates to the mating areas near nesting beaches.

The female mates with a number of males about a month before she begins laying eggs. The sperm is stored at the top end of the oviduct and fertilises the follicles as they are released from the ovary in batches. Each fertilised follicle then becomes the yolk of the egg, which is the food supply for the developing embryo. As the yolk moves down the oviduct special cells surround it with a gelatinous covering - the albumen. This watery protein mixture provides essential moisture and insulation for the embryo.

Now other cells begin to lay down the inner shell membrane which supports the shell itself. This stage is reached within 36 hours of the follicle leaving the ovary. The production of the egg shell, by another set of cells, requires a further 7-9 days. The inner membranous layer and the outer calcium layer together form a soft pliable wrapping around the contents of the egg. Like the harder shell of a bird's egg, it allows respiratory gases (oxygen and carbon dioxide) and moisture to pass through.

By the time the eggs are laid, about 14 days after fertilisation, the embryonic cells have divided repeatedly and number many hundreds.

On their own

Once the eggs have been laid in the sand, temperature, moisture and availability of oxygen have a great impact on the hatching success of the clutch as the embryo continues to develop. Temperatures in the egg chambers of between 23 ° and 33 ° [centigrade] are required, the higher temperatures resulting in all females hatching while lower temperatures produce males. Temperatures outside this range can result in the death of the embryos.

The egg gains weight during incubation by absorbing water through the egg shell. If water is not available the egg loses weight and the embryo may not survive. Too much water, on the other hand, may also cause problems. If the nest is flooded, especially towards the end of incubation when the embryo needs more oxygen, the embryo may literally drown inside the egg.

As it develops, the embryo draws food (energy) from the yolk, which becomes smaller as the embryo becomes bigger. By the time it is ready to hatch, the little turtle takes up nearly all the space inside the egg.

Into the world

Some 7-11 weeks after the eggs were laid, the hatching turtle, using the sharp point at the end of its nose, tears a hole through the membranes and shell to take its first lungful of air. After a 2-3 day struggle with the shell, the hatchling frees itself and its body flattens from the curled position it had inside the egg. During the next 2-5 days all the hatchlings burrow upwards together. They then wait, just under the surface of the sand, until the temperature of the sand falls (usually just after dark) before emerging all together and scrambling across the beach to the sea. They run the gauntlet of predatory crabs, birds, goannas, pigs and domestic pets. Once in the sea, many are lost to fish such as cod, grouper and sharks.

'Turtle hatchlings on Heron Island' R. Fitzpatrick. (GBRMPA)

The lost years

Very little is known about young turtles after they leave the nesting beaches. It is thought that they drift with the currents, feeding on plankton. When they are about 30cm in length (or bigger depending on the species) they appear in the adult feeding areas.