A sea turtle nest gains an advantage from being between several inches (15 cm or more to top) to a yard (1 meter) under the surface of the sand. Temperature moderates as depth increases both in terms of absolute value and daily fluctuation. Water content of the sand stays stable of depths of the nest even though the sand dries near the surface. The main problem for a clutch of eggs is obtaining enough oxygen to carry out metabolism and getting rid of carbon dioxide produced in respiration. Oxygen is transported in the air and sand surrounding the nest to the clutch inside the nest by the process known as diffusion. Carbon dioxide is transported away in the same way.
Fick’s law of diffusion defines the process. The movement of material by diffusion depends upon the driving force that exists between an area of high concentration and one of low concentration and the resistance of the pathway between the source and the sink. In the case of a turtle nest the sand provides most of the resistance because the eggshell is relatively porous to the flow of gases. In some cases oxygen can drop from 20.9% in the air to 20.4% in the sand due to the metabolism of bacteria in the sand and to 12 – 14% in the middle of the clutch just before hatching. However, the level of oxygen in the clutch is similar to alveoli in the human lung.
One of the reasons that leatherback turtles lay their eggs in the dry season is that the dry layer that forms at the surface of the sand helps to transport gases more readily between the air and the nest. Olive Ridley nests on arribada beaches suffer from low oxygen levels because of the high density of the nests in the beach and the decay of the eggs broken during arribadas.
It is amazing that a clutch of sea turtle eggs can survive buried 10 – 36 inches under the sand. Oxygen must diffuse from the air down into the sand and into the egg. Carbon dioxide must move in the opposite direction. A developing sea turtle embryo breathes through its shell as does a chicken embryo, which has the same concentrations of oxygen and carbon dioxide inside its shell as occur in the human lung. Sea turtle eggs have similar internal gas concentrations, but there is a difference. The turtle eggshell is very porous, facilitating gas movement, while the chicken eggshell is very resistant. The sea turtles egg’s gas concentrations are set by the rate at which air can move through the sand and into the egg. Oxygen filters down through almost three feet of sand, through the pores between the sand grains, then between the eggs in the clutch, and finally into the egg in the middle of the clutch. The main barrier is the rate of movement of air between the sand grains. That three foot layer of sand essentially functions like the chicken eggshell or human air passes into the lung. It serves as the respiratory pathway for the sea turtle egg.