Aquatic respiration

Echinoderms have a specialised water vascular system which provides a number of functions including providing the hydraulic power for tube feet but also serves to convey oxygenated sea water into the body and carry waste water out again. In many genera, the water enters through a madreporite, a sieve like structure on the upper surface but may also enter via ciliary action in the tube feet or via special cribiform organelles.[1]

Molluscs commonly possess gills that allow exchange of respiratory gases from an aqueous environment into the circulatory system. These animals possess a heart that pumps blood which contains hemocyanin as its oxygen-capturing molecule. The respiratory system of gastropods can include either gills or a lung.

Aquatic arthropods generally possess some form of gills in which gas exchange takes place by diffusing through the exoskeleton. Others may breathe atmospheric air while remaining submerged, via breathing tubes or trapped air bubbles, though some aquatic insects may remain submerged indefinitely and respire using a plastron. A number of insects have an aquatic juvenile phase and an adult phase on land. In these case adaptions for life in water are lost at the final ecdysis. A number of orders of insects such as mayflies, caddis flies and stone flies have aquatic juvenile stages while some orders such as Lepidoptera have just a few examples such as China mark moths. A very few arachnids have adopted an aquatic life style including the diving bell spider. In all cases, oxygen is provided from air trapped by hairs[2] around the animals body.

Most fish exchange gases using gills on either side of the pharynx (throat), forming the splanchnocranium, the portion of the skeleton where the cartilage of the cranium converges into the cartilage of the pharynx and its associated parts.[3] Gills are tissues which consist of threadlike structures called filaments. These filaments have many functions and are involved in ion and water transfer as well as oxygen, carbon dioxide, acid and ammonia exchange.[4] Each filament contains a capillary network that provides a large surface area for the exchange of gases and ions. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. 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.[5] In some fish, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The muscles on the sides of the pharynx push the oxygen-depleted water out the gill openings. In bony fish, the pumping of oxygen-poor water is aided by a bone that surrounds the gills called the operculum.[6]

Both the lungs and the skin serve as respiratory organs in amphibians. The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of mucus from specialized cells. 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. When underwater, the animal is essentially holding its breath and has to routinely return to the surface to breathe in new air. Therefore, all amniote animals, even those that spend more time in water than out, are susceptible to drowning if they cannot reach the surface to breath.

The anatomical structure of the lungs is less complex in reptiles than in mammals, with reptiles lacking the very extensive bronchial tree found in mammalian lungs. Gas exchange in reptiles still occurs in alveoli, but reptiles do not possess a diaphragm, therefore ventilation occurs via a change in the volume of the body cavity which is controlled by contraction of intercostal muscles in all reptiles except turtles. In turtles, contraction of specific pairs of flank muscles governs inspiration or expiration.[8]

Diving birds and pelagic seabirds breath air using lungs like reptiles and mammals, but avian lungs are fairly rigid structures that do not expand and contract as elastically. Instead, the structures that act as bellows that ventilate the lungs are the avascular air sacs, which are distributed throughout much of the birds' bodies[9] and move air unidirectionally through the parabronchi, where gas exchange happens.[10][11] Although bird lungs are smaller than those of mammals of comparable size, the air sacs account for 15% of the total body volume, whereas in mammals, the alveoli (which act as the bellows) constitute only 7% of the total body volume.[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. The active phase of respiration in birds is exhalation, which requires contracting of respiratory muscles,[11] while the relaxation of these muscles causes inhalation.