How do turtle eggs breathe?
A sea turtle nest has the advantage of being between a few inches (15 cm or more up) to a yard (1 meter) below the surface of the sand. Temperature decreases with increasing depth both in terms of absolute value and diurnal variation. The water content of the sand remains stable at the depth of the nest, although the sand dries near the surface. The main problem for the clutch of eggs is obtaining enough oxygen to carry out metabolism and the removal of carbon dioxide produced during respiration. Oxygen is transported in the air and sand around the nest to the clutch inside the nest by a process known as diffusion. Carbon dioxide is transported in the same way.
Fick’s law of diffusion defines the process. The movement of material by diffusion depends on the driving force that exists between an area of high concentration and one of low concentration, and the resistance of the path 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 air to 20.4% in sand due to the metabolism of bacteria in the sand, and to 12 – 14% in the clutch environment just before hatching. However, the oxygen level in the clutch is similar to the alveoli in the human lung.
One of the reasons leatherback turtles lay their eggs during the dry season is that the dry layer that forms on the surface of the sand helps transport gases between the air and the nest more easily. Olive Ridley nests on arribada beaches suffer from low oxygen levels due to the high density of nests on the beach and the decay of eggs broken during arribadas.
Amazingly, a clutch of sea turtle eggs can survive buried 10 to 36 inches below the sand. Oxygen must diffuse from the air down into the sand and into the egg. Carbon dioxide must move in the opposite direction. The developing sea turtle embryo breathes through its shell, as does the chicken embryo, which has the same concentrations of oxygen and carbon dioxide in its shell as in a human lung. Sea turtle eggs have similar internal gas concentrations, but there is a difference. The shell of the turtle is very porous, which facilitates the movement of gases, while the shell of the chicken egg is very resistant. Gas concentrations in sea turtle eggs are determined by the rate at which air can move through the sand and enter the egg. The oxygen is filtered through nearly three feet of sand, through the pores between the grains of sand, then between the eggs in the clutch, and finally into the egg in the middle of the clutch. The main barrier is the speed of air movement between the grains of sand. This three-foot layer of sand essentially functions as the shell of a chicken egg or the human air passes into the lung. It serves as an airway for the sea turtle egg.
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