Aquatic respiration

In very small animals, plants and bacteria, simple diffusion of gaseous metabolites is sufficient for respiratory function and no special adaptations are found to aid respiration.

Passive diffusion or active transport are also sufficient mechanisms for many larger aquatic animals such as many worms, jellyfish, sponges, bryozoans and similar organisms.

In fully submerged aquatic higher plants specialised structures such as stoma on leaf surfaces to control gas interchange.

[1] Molluscs commonly possess gills that allow exchange of respiratory gases from an aqueous environment into the circulatory system.

Aquatic arthropods generally possess some form of gills in which gas exchange takes place by diffusing through the exoskeleton.

These filaments have many functions and are involved in ion and water transfer as well as oxygen, carbon dioxide, acid and ammonia exchange.

In species like the spiny dogfish and other sharks and rays, a spiracle exists near the top of the head that pumps water into the gills when the animal is not in motion.

In bony fish, the pumping of oxygen-poor water is aided by a bone that surrounds the gills called the operculum.

While the lungs are of primary importance to breathing control, the unique properties of cutaneous respiration supplements rapid gas exchange when amphibians are submerged in oxygen-rich water.

[7] All aquatic amniotes (reptiles, birds and mammals) have thick and impermeable cutes that preclude cutaneous respiration, and thus rely solely on the lungs to breathe air.

[12] Like their reptilian cousins, birds also lack a diaphragm and thus rely on the intercostal and abdominal muscles to change the volume of the entire thoracoabdominal cavity.

This mechanism maintains the concentration gradient thus increasing the efficiency of the respiration process as well and prevents the oxygen levels from reaching an equilibrium.

The latter view would imply that the brain possesses some kind of detection mechanisms that would trigger a respiratory response when muscular contraction occurs.