Sea turtles are generally found in the waters over continental shelves. After taking to the water for the first time, males will not return to shore again. During the first three to five years of life, sea turtles spend most time in the pelagic zone floating in seaweed beds. Green sea turtles in particular are often found in Sargassum beds, a brown seaweed in which they find shelter and food. Once the sea turtle has reached adulthood it moves closer to the shore. Females will come ashore to lay their eggs on sandy beaches during the nesting season.
It takes decades for sea turtles to reach sexual maturity. After
mating at sea, adult female sea turtles return to land to nest at night.
Different species of sea turtles exhibit various levels of philopatry.
In the extreme case, females return to the beach where they hatched.
This can take place every two to four years in maturity. They make from
one to eight nests per season. The mature nesting female hauls herself
onto the beach, nearly always at night, and finds suitable sand on which
to create a nest. Using her hind flippers, she digs a circular hole 40
to 50 centimetres (16 to 20 in) deep. After the hole is dug, the female
then starts filling the nest with a clutch of soft-shelled eggs one by
one until she has deposited around 50 to 200 eggs, depending on the
species. Some species have been reported to lay 250 eggs, such as the
hawksbill. After laying, she re-fills the nest with sand, re-sculpting
and smoothing the surface until it is relatively undetectable visually.
The whole process takes thirty to sixty minutes. She then returns to the
ocean, leaving the eggs untended.
Turtle gender depends on sand temperature while the egg is incubating.
The hatchling's gender depends on the sand temperature. Lighter sands
maintain higher temperatures, which decreases incubation time and
results in more female hatchlings.
It takes several decades for adult sea turtles to reach sexual
maturity. The mature turtles migrate, sometimes for thousands of miles,
to reach breeding sites. Male and female turtles mate in the water, and
the males return to deep sea to feed. For several weeks, female sea
turtles alternate between mating in the water and laying their eggs on
land. Before laying her eggs, a female turtle will dig a hole in the
sand with her hind flippers. She covers it with sand and returns to the
ocean. About two months pass for the eggs to incubate under the sand.
Afterwards, the eggs hatch, generally at night to avoid predation, and
the hatchlings crawl to the water. They then swim out to sea to begin
their own cycle of maturing and reproducing. Sea turtles can continue
this cycle until they are 80 years old.
Incubation takes about two months. The eggs in one nest hatch
together over a very short period of time. When ready, hatchlings tear
their shells apart with their snout and dig through the sand. Again,
this usually takes place at night. Once they reach the surface, they
instinctively head towards the sea. If, as happens on rare occasions,
hatching takes place during daylight, only a very small proportion of
each hatch succeed (usually 1%), because local opportunist predators, such as the common seagull,
gorge on the new sea turtles. Thus there is an obvious evolutionary
drive to hatch at night, when survival rates on the beach are much
higher.
