Biology of marine turtles

Green Turtle foraging on sea grasses
Photo courtesy of R.P. van Dam. Copyright©

Young marine turtles drift and feed in the open ocean. When they are about dinner plate size, turtles settle near inshore feeding grounds. Marine turtles grow slowly and take between 30 and 50 years to reach sexual maturity. They live for years in the same place before they are undertake the long breeding migration, of up to 3000 kilometres, from the feeding grounds to nesting beaches.

After reaching sexual maturity, marine turtles breed for several decades, although there may be intervals between breeding of two to seven years. When breeding, nesting females return to the same area, thought to be in the region of their birth. As hatchlings, they become imprinted with the earth's magnetic field and, possibly, with the smell of the waters adjacent to their ultimate nesting beach.

Courtship and mating take place in shallow waters near the nesting beach; females often mating with more than one male. After mating, the males return to the feeding grounds. Between nesting efforts, female turtles gather adjacent to the nesting beaches. They return to the same beach to lay consecutive clutches with female green turtles usually laying up to six clutches of eggs at two weekly intervals.

The moisture in the sand needs to be just right for nesting

When ready to lay eggs, the female turtle emerges from the sea, usually between one and two hours either side of the nocturnal high tide and makes her way to a location above the high water mark. Firstly, she scrapes away loose sand with all four flippers to form a body pit and then excavates a vertical pear-shaped egg chamber with her hind flippers. Often, the sand is unsuitable for nesting, especially if it is too dry, and she moves on to try another site.

For most turtle species, digging the nest takes about 45 minutes. It then takes another 10 to 20 minutes to lay the clutch of leathery shelled eggs. Each clutch contains about 100 white, spherical, "ping-pong" ball sized eggs.

After laying, she fills the egg chamber with sand using her hind flippers, and then fills the body pit using all four flippers finally making her way back to sea about one to two hours after emerging.

While on the beach, fluid is secreted from the turtle's eyes. This is a concentrated salt solution which helps to remove excess salt, ingested by the turtle from drinking sea water, and helps to rinse beach sand from her eyes.

During summer (and winter in the tropics) at selected turtle breeding areas, each nesting marine turtle is tagged with a non-corrosive metal tag carrying an identification number. Later recaptures allow scientists from the conservation agencies and universities to monitor marine turtle populations, migration patterns, breeding activity, growth and mortality factors.

Incubation time and the sex of the hatchlings depend on the temperature of the sand. Warm, dark sand produces mostly females and the eggs hatch in seven to eight weeks. Eggs laid in cool, white sand mostly result in males and the eggs take longer to hatch.

The hatchlings then take a few days to dig their way through the sand to the surface. When leaving the nest, usually at night, hatchlings head for the low elevation horizon of the ocean. Hatchlings can be easily disoriented and attracted to bright lights such as street and house lights. This contributes to many hatchling deaths.

Most hatchlings reach the sea although crabs and sea birds account for many before they reach the waters edge. During their first few hours in the water, the young turtles face heavy predation by sharks and other marine predators.

Research

Detailed research, population modelling, and turtle tagging and tracking activities are being carried out in Australia and the Indo-Pacific region to assist with turtle conservation programs. Genetic studies help to identify separate breeding populations and which population a particular turtle comes from, especially those captured in distant feeding grounds. They also provide information on the genetic variability within each population.

Links

Most of the information on this page has been sourced from The Department of Environment and Heritage website