The world of science: sea turtle eggs

Moving further along with the World of Science series, this field diary will aim to explore the story of turtle beginnings: The process of ovulation to development of the hatchling.

As a predominantly aquatic reptile, sea turtles (Cheloniidae, Dermochellidae) only depend on the terrestrial environment during the nesting phase of their life cycle¹. Sexually mature females show a high degree of site fidelity to their nesting area, meaning these females will migrate large distances to return to the same beaches.

Typically loggerhead turtle eggs (Caretta caretta) are medium-sized (4.0 cm in diameter, 36 g)², spherical eggs, composed of a flexible aragonite shell, a shell membrane, albumen, vitelline membrane, and the embryonic disk^2,3,4. Egg sizes vary between species of sea turtle, with the smallest egg size belonging to the Hawksbill turtle (Eretmochelys imbricata) at 3.8 cm diameter, weighing approximately 28 g, and the largest eggs belonging to the Leatherback turtle (Dermochelys coriacea) at 5.3 cm in diameter and weighing 90 g on average.

Before the female arrives in the nesting area, both males and females migrate to mating grounds. Females copulate with multiple males during this time, and store their sperm within the posterior portion of the albumen-secreting region of their oviducts^5,6. Egg production begins early on within the female’s ovaries as thousands of tiny follicles. By the time the female arrives in the nesting area, her ovaries contain enough mature follicles to produce all the eggs that she will need to lay throughout the whole season. These mature follicles absorb water to slightly increase in size just before ovulation. An increase in levels of luteinizing hormone and progesterone initiates ovulation, and approximately 110 follicles travel through to the infundibulum of the oviduct (a funnel shaped structure located at the start of the oviduct). Sperm stored earlier in the season is mixed into the folds of the infundibulum of the oviduct and is held there to fertilise the follicles as they travel through. This process leads each clutch to contain the genetics of multiple males, allowing for increased genetic diversity between siblings. The newly fertilised ovum continues into the oviduct, which is lined with specialized secretory cells. The ovum is initially layered with albumen, followed by a layer of inner shell membrane as it travels through the oviduct^7,8. Lastly, aragonite crystals form the outer lining of the shell⁸. The whole process takes at least seven days for the shell to fully form from the start of ovulation³. Loggerhead females repeat this process between 1 and 6 times during their nesting season and follicles left over at the end of the nesting season are reabsorbed by the female in the following months².

With a fertility rate exceeding 95 %², it is not common for loggerhead turtles to produce eggs that are not normal, such as yolkless eggs⁹. Yolkless eggs are infertile, as they do not contain a zygote or embryo and they also lack the vitelline membrane. With no evident purpose, it is most likely that these eggs are created as a result of debris or yolk fragments accidently entering the oviduct and being layered with albumen and shell in the same process as a normal, zygotic egg³.

Once the eggs have been deposited within the egg chamber, the female completes her motherly duties through covering and camouflaging the nest. The next stage of the life cycle begins as the embryo starts to develop within the egg and depends on the safety and consistency of the egg chamber for the entire incubation period.

Temperature has been known to influence turtle biology and behaviour through multiple stages of their life cycles. From influencing incubation duration through to determining the sex of the hatchlings, temperature plays a large role within embryonic development¹⁰. The main role temperature plays, however, occurs within sex determination of hatchlings. Within the egg chamber, temperatures vary highly throughout, providing the opportunity for eggs in one area of the nest to experience different incubating conditions to eggs in another area. Unlike humans, sea turtles do not have distinguished sex chromosomes that determine their sex. Instead, sexual differentiation is temperature-dependent and occurs during the second third of the incubation duration¹¹ with warmer areas of the nest producing more females than cooler areas that produce a higher percentage of males¹².

Written by Megan Soulsby

References
1. Ackerman, R. A. (1980). Physiological and Ecological Aspects of Gas Exchange by Sea Turtle Eggs. Zool., 20, pp. 575-583.
2. Miller, J. D., Limpus, C. J. & Godfrey, M. H. (2003). Loggerhead Sea Turtle: Nest Site Selection, Oviposition, Eggs, Development, Hatching and Emergence of Loggerhead Turtles. Smithsonian Institute.
3. Miller, J. D. (1985). Biology of Reptilia: Embryology of marine turtles. New York: Wiley-Interscience.
4. Packard, M. J. & DeMarco, V. G. (1991). Egg incubation: It’s effects on embryonic development in birds and reptiles: Eggshell structure and formation in eggs of oviparous reptiles. Cambridge, U.K.: Cambridge University Press.
5. Gist, D. H. & Jones, J. M. (1989). Sperm storage within the oviduct of turtles. Morphol., 199, pp.379-384.
6. Pearse, D. E. & Avise, J. C. (2001). Turtle Mating Systems: Behavior, Sperm Storage, and Genetic Paternity. The American Genetic Association, 92, pp.206-211.
7. Aitken, R. N. & Solomon, S. E. (1976). Observations on the ultrastructure of the oviduct of the Costa Rican green turtle (Chelonia mydas ). Journal of Experimental Marine Biology and Ecology, 21, pp.75-90.
8. Solomon, S. E. & Baird, T. (1976). Studies on the eggshell (oviductal and oviposited) of Chelonia mydas Journal of Experimental Marine Biology and Ecology, 22, pp.145-160.
9. Hughes, G. R., Bass, A. & Mentis, M. (1967). Further studies on marine turtles in Tongaland. Lammergeyer, 7, pp.4-54.
10. Mrosovsky, N. (1980). Thermal Biology of Sea Turtles. Zool., 20, pp. 531-547.
11. Mrosovsky, N. & Pieau, C. (1991). Transitional range of temperature, pivotal temperatures and thermosensitive stages for sex determination in reptiles. Amphibia-Reptilia, 12, pp. 169-179.
12. Rees, A. F. & Margaritoulis, D. (2004). Beach Temperatures, Incubation Durations and Estimated Hatchling Sex Ratio for Loggerhead Sea Turtle Nests in Southern Kyparissia Bay, Greece. C.G. Testudo, 6(1), pp. 23-36.