Sea turtles are ancient marine reptiles with a reproductive process distinct from mammals or birds. The fertilization of their eggs involves a specific sequence of events that accommodates their life at sea and their need to nest on land. This process maximizes reproductive efficiency, ensuring eggs are ready for development before they are laid on the beach. Understanding how fertilization occurs requires examining mating, sperm management, and the final packaging of the egg.
Mating and Sperm Transfer
Mating occurs in the water near nesting beaches several weeks before the female lays her first clutch. The male initiates copulation by using the curved claws on his front flippers to secure a grip on the female’s carapace, sometimes remaining mounted for hours. Sperm transfer occurs through the cloaca, a single opening that serves the digestive, urinary, and reproductive tracts. The male extends his reproductive organ from his cloaca to inseminate the female. Females often mate with multiple males during the breeding season, transferring sperm from several fathers.
Sperm Storage: The Key to Multiple Clutches
Following mating, the female stores transferred sperm within specialized structures in her oviduct, known as oviductal crypts. This storage capacity separates the timing of copulation from fertilization, which is necessary for species that lay multiple clutches with long intervals between nesting events. The ability to store viable sperm, sometimes for the entire nesting season, means the female does not need to re-mate between clutches. Sea turtles commonly lay between two and seven clutches of eggs per season, with about two weeks between each nesting. The stored sperm acts as a continuous reservoir, ensuring subsequent egg batches are fertilized and contributing to the genetic diversity of the offspring through multiple paternity.
Internal Fertilization and Shell Formation
Internal fertilization occurs high up in the oviduct as the egg begins its journey after release from the ovary. Fertilization must take place before the egg is encased in its protective layers. Sperm is released from the storage glands to merge with the yolk-filled ovum, forming the zygote. As the fertilized egg moves through the oviduct, it sequentially receives the albumen, which provides nutrients and moisture. This is followed by the deposition of the soft, flexible shell, which prevents breakage when the eggs drop into the nest cavity.