Water holds little oxygen, so gills must be huge surfaces
Gills let aquatic animals pull dissolved oxygen from water and dump carbon dioxide. Except in some insects, they are the classic water-breathing organ—thin filaments or plates folded into vast surface area because water carries only a fraction of the oxygen air does.
The numbers explain the design. A cubic metre of air holds about 275 grams of oxygen, while fresh water carries under a twenty-fifth as much: roughly 8 cubic centimetres per litre against 210. Water is also 777 times denser and 100 times more viscous than air, and oxygen diffuses through it 10,000 times more slowly. A baggy lung could never pull enough from such a medium, so gills fold thin filaments, plates or tufts into enormous surfaces, held apart by the water itself; out of water they collapse onto each other.
Most gill-bearers keep water moving one way, whether by current, swimming, beating cilia or a pump. Fish and some molluscs add countercurrent exchange, sending blood through the lamellae opposite to the water flow, and can recover as much as 90 percent of the dissolved oxygen. A fish takes in a mouthful, then squeezes the sides of its throat so the water exits through slits behind the head. Ancestral fish had many slits; today's species usually keep five pairs, with eight the ceiling. Sharks and rays usually show five external pairs, a few primitive sharks six, and the broadnose sevengill shark alone exceeds that among cartilaginous fish.
Gills turn up in molluscs, crustaceans, horseshoe crabs, polychaete worms, aquatic insects, fish and tadpoles, while tiny or sluggish animals often breathe through their whole skin instead. Crabs, land hermit crabs and mudskippers can breathe air ashore if their gills stay moist, some storing water in gill chambers. In spiders and scorpions the ancestral gills became book lungs, and silver and bighead carp use comb-like gill rakers to strain food. Embryonic gill pouches may even be the evolutionary source of the thymus and parathyroid glands.
Early naturalists disagreed. Galen described openings fine enough to pass gases but not water, and Pliny the Elder held that fish breathe through their gills while noting Aristotle thought otherwise.
Source: Gill
