gill

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A gill is an anatomical structure found in many aquatic organisms. It is a respiration organ whose function is the extraction of oxygen from water and the excretion of carbon dioxide. The microscopic structure of a gill is such that it presents a very large surface area to the external environment.

Many microscopic aquatic animals, and those which are somewhat larger but inactive, are able to absorb adequate oxygen through the entire surface of their bodies, and thus they often can respire quite adequately without a gill. However, more complex or more active aquatic organisms usually require a gill or gills.

Gills usually consist of thin filaments of tissue, branches, or slender tufted processes which have a highly folded surface to increase surface area. A high surface area is crucial to the gas exchange of aquatic organisms as water contains only 1/20 parts dissolved oxygen compared to air. With the exception of some aquatic insects, the filaments and lamellae (folds) contain blood or coelomic fluid, from which gases are exchanged through the thin walls. Oxygen is carried by the blood to other parts of the body. Carbon dioxide passes from the blood through the thin gill tissue into the water.

Gills or gill-like organs, located in different parts of the body, are found in various groups of aquatic animals, including mollusks, crustaceans, insects, fish, and amphibians.

Invertebrate gills

Caribbean hermit crabs have modified gills to allow them to live in humid conditions

Respiration in the Echinodermata (includes starfish and sea urchins) is carried out using a very primitive version of gills called papulli. These are thin protuberances on the surface of the body containing diverticula of the water vascular system.

In crustaceans, mollusks and some insects, gills are tufted or plate-like structures at the surface of the body in which blood circulates.

The gills of other insects are of the tracheal kind, and also include both thin plates and tufted structures, and, in the larval dragon fly, the wall of the caudal end of the alimentary tract (rectum) is richly supplied with tracheae as a rectal gill. Water pumped into and out of the rectum provides oxygen to the closed tracheae. In the aquatic insects, a unique type of respiratory organ is used, the tracheal gill, which contains air tubes. The oxygen in these tubes is renewed through the gills.

Physical gills

Physical gills are a type of structural adaptation common among some types of aquatic insects, in which atmospheric oxygen is held within an area into which small openings called spiracles open. The structure (often called a plastron) typically consists of dense patches of hydrophobic setae on the body, which prevent water entry into the spiracles. The physical properties of the interface between the trapped air bubble and the surrounding water function so as to accomplish gas exchange through the spriacles, almost as if the insect were in atmospheric air. Carbon dioxide diffuses out into the surrounding water due to its high solubility, while oxygen diffuses into bubbles as the concentration within the bubble has been reduced by respiration, and nitrogen also diffuses out as its tension has been increased. Oxygen diffuses into the bubble at a higher rate than Nitrogen diffuses out. However, water surrounding the insect can become oxygen-depleted if there is no water movement, so many aquatic insects in still water actively direct a flow of water over their bodies.

The physical gill mechanism allows aquatic insects with plastrons to remain constantly submerged. Examples include many beetles in the family Elmidae, aquatic weevils, and true bugs in the family Aphelocheiridae.

Vertebrate gills

The red gills inside a detached tuna head (viewed from behind)

The red gills of this common carp are visible as a result of a gill flap birth defect.

The gills of vertebrates are typically developed in the walls of the pharynx along a series of gill slits opening to the exterior. In most species, a countercurrent exchange system is employed to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other. The gills are composed of comb-like filaments, the gill lamellae, which help increase their surface area for oxygen exchange. ^[1]

When a fish breathes, it opens its mouth at regular times and draws in a mouthful of water. It then draws the sides of its throat together, forcing the water through the gill openings, so that it passes over the gills to the outside.

The gill slits of fish are believed to be the evolutionary ancestors of the tonsils, thymus gland, and Eustachian tubes, as well as many other structures derived from the embryonic branchial pouches.

Cartilagenous fish

Sharks and rays typically have five pairs of gill slits opening directly to the outside of the body, although some of the more primitive sharks have six or seven pairs. Each slit is separated by a cartilagenous gill arch from which projects a long sheet-like septum partly supported by a further piece of cartilage, the gill ray. The individual lamellae of the gills lie on either side of the septum. The base of the arch may also support gill rakers, small projecting elements that help to filter food from the water.^[2]

In front of the first gill slit lies a smaller opening, the spiracle. This bears a small pseudobranch, resembling a gill in structure, but only receiving blood that has already been oxygenated by the true gills.^[2]

In most sharks ventilation of the gills is achieved by ram ventilation, forcing water into the mouth and over the gills while rapidly swimming forward. In slow-moving or bottom dwelling species, especially among the skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth.^[2]

Chimaeras are somewhat different from other cartilagenous fish, having lost both the spiracle and the fifth gill slit. The remaining slits are covered by an operculum, developed from the septum of the gill arch in front of the first gill.^[2]

Bony fish

In bony fish, the gills lie in a branchial chamber covered by a bony operculum. The great majority of bony fish species have five pairs of gills, although a few have lost some over the course of their evolution. The operculum can be very important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills, so that bony fish do not have to rely on ram ventilation (and hence constant motion) in order to breathe. Valves inside the mouth keep the water from escaping through the mouth again.^[2]

The gill arches of bony fish typically have no septum, so that the gills alone project out from the arch, supported by individual gill rays. Some species retain gill rakers, but this is not universal. Although all but the most primitive bony fish lack a spiracle, the pseudobranch associated with it often remains, being located at the base of the operculum. This is, however, often greatly reduced, consisting of a small mass of cells without any remaining gill-like structure.^[2]

Marine teleosts also utilize their gills for the excretion of electrolytes. The gills' large surface area tends to create a problem for fish seeking to regulate the osmolarity of their internal fluids. Saltwater is less dilute than these internal fluids; as a consequence, saltwater fish lose large quantities of water osmotically through their gills. To regain the water, they drink large amounts of seawater and excrete the salt. Freshwater is more dilute than the internal fluids of fish, however, so freshwater fish gain water osmotically through their gills.^[2]

Other vertebrates

Lampreys and hagfish do not have gill slits as such; instead the gills are contained within spherical pouches with a circular opening to the outside. Like the gill slits of higher fish, each pouch contains two gills. In some cases, the openings may be fused together, effectively forming an operculum. Lampreys have seven pairs of pouches, while hagfishes may have anything from six to fourteen, depending on species. In the hagfish, the pouches communicate, on the inner surface, with the pharynx, but in adult lampreys a separate respiratory tube develops beneath the pharynx proper, allowing for separation of food and water for respiration by closing a valve at its anterior end.^[2]

The tadpoles of amphibians have from three to five gill slits, but these do not contain actual gills. There is usually no spiracle or true operculum, although an operculum-like structure is found in many species. Instead of internal gills, they develop three feathery external gills, growing from the outer surface of the gill arches. Sometimes these are also retained in the adult, but they usually disappear at metamorphosis. External gills are also found in the larvae of lungfish, and of the primitive ray-finned fish Polypterus, although in the latter case, these have a different structure to those of amphibians.^[2]

An Alpine newt larva showing the external gills, which flare just behind the head

Branchia

Branchia (pl. branchiæ) is the name given by the Ancient Greek naturalists to the gills of fish. Galen observed that they have multitudes of openings (foramina), big enough to admit gases, but too fine to give passage to water. Pliny the Elder held that fish respired by their gills, but observed that Aristotle was of another opinion.^[3]

The word branchia comes from the Greek βράγχια, "gills", plural of βράγχιον (in singular, meaning a fin).^[4]

See also

Freshwater Fish Gills magnified 400 times

• Aquatic respiration
• Book lung
• Lung
• Artificial gills (human)

References