gill
The gill (mostly plural: gills ; from Middle High German : kimme "cut, notch", zoologically referred to as branchia ) is an organ that in many aquatic animals supplies the blood with the oxygen dissolved in the water . This form of breathing underwater is known as gill breathing .
Smaller animals do not need gills - skin breathing is sufficient for them . The gills of larger animals are therefore essentially covered with a very thin and permeable skin and allow the blood to circulate inside them either in special veins or in gaps so that it can exchange with the water as closely as possible by diffusion . They are located in different parts of the body: in worms and crabs on the extremities, in some mussels and water snails in the mantle cavity , in fish on the gill slits in the foregut. Usually the gills are exposed (but can often be pulled back under the skin), or they are protected in special hollows. In order to offer the water a large surface in a small space, they are comb, leaf, tuft or tree-shaped. The fish gills, which stand on the cartilaginous or bony gill arches, usually have a pink to reddish color. This is due to the fact that the blood vessels in the gills are very close to the surface. In almost all fish, the gill leaves, which can be clearly seen with the naked eye, and leaves perpendicular to them contribute to the increase in surface area. The total area of the gills corresponds to 10 to 60 times the skin surface of a fish (depending on the liveliness and habitat).
Gills are found in a great many aquatic animals and also in some terrestrial animals that live in moist air, i.e. snails (exception: lung snails ), mussels and other molluscs , various worms , crustaceans, etc., and more generally in fish and in the Larvae (and some adults) of amphibians . Even some aquatic turtles can (secondarily) breathe with gills.
Since the gill leaves dry out easily and stick together, the animals that rely on gill breathing can quickly suffocate outside the water. If the oxygen concentration in the water is too low, they can suffocate there too, especially since the amount of energy required to breathe in water is greater than in air.
Some fish and crustaceans are able to stay outside the water for a longer period of time due to special precautions (which for example supply the respiratory water in the gill cavities with oxygen from the air).
distribution
Tube worms
Tube worms have particularly noticeable gills.
Molluscs
Except for the barefoots , which have no developed breathing organs of their own, and the pulmonary snails , all molluscs have gills, mostly in a coat. The water flow can be increased with the help of the mantle muscles to increase the exchange of substances (especially with cephalopods ).
arthropod
Crustaceans
Crustaceans wearing gills - also well adapted to land life isopods (in a sort of mini aquarium ).
The real crabs have gills on their extremities. In the giant isopods , the tuft gills are located on the first two pairs of legs of the abdomen, the pleopods .
be crazy
Arachnids do not have gills. However, water-dwelling water spiders have developed mechanisms that enable them to exchange gas with the water: they carry an air bubble on their bristles and bring it into an air bubble under water. The air bubbles serve the water spiders as physical gills or plastron breathing .
insects
The tracheata breathing through trachea (this includes insects ) are only exceptionally equipped with gills (partly without, partly in connection with trachea), for example the aquatic larvae of dragonflies , mayflies , stone flies , large winged birds and some two-winged birds .
There is plastron respiration, especially in beetle larvae: the larva is (partly) covered by a layer of air that is held in place by specially developed, water-repellent hairs. The larva breathes from this layer of air as a physical gill by means of trachea.
Tracheal gills are the typical respiratory organs of aquatic insect larvae. Appendices to the abdomen (for example, damselflies (Zygoptera)) Rektalkiemen or intestinal gills (the dragonfly (Anisoptera) and the Urlibellen (Anisozygoptera)) are special respiratory organs of some aquatic larvae.
Vertebrates
In most species, water and blood flow in the gills in opposite directions (analogous to the flow direction in the nephron ) in order to improve the exchange of substances ( countercurrent principle). ( 1 )
fishes
Most bony fish can create an almost constant flow of water over the gills by moving jockstraps and gill covers and thus maintain or regulate the exchange of substances even without an external current and without swimming movement, as the gill cover sucks (or squeezes out) a little longer than the jockstrap. This is not equally possible for cartilaginous fish, they often need a stream of water against the open mouth to wash around the gills. The gill openings of the recent cartilaginous fish consist of a series of slits that open outwards from the throat and the gill pockets under water pressure. Only water absorbed through the mouth flows through these slits and washes around the gills, a backflow is hardly possible. As a result, many free-swimming sharks can only breathe while moving. The same applies to mackerel and tuna .
In fish, the gills also have an excretory function, for example they release ammonia (supporting the kidneys) and can also absorb ions (in the sea).
Amphibians
Amphibians are usually lung breathers and can exchange gases through the skin to a considerable extent in the water. In addition, many amphibians in the larval stage have outer gills, but also some adult stages ( Olme ); these gills are in the same place as in the fish, but consist of external gill tufts .
Reptiles
Reptiles do not have gills, they always have lungs. However, various water-dwelling turtles have also developed mucous membranes that enable them to exchange gas with the water.
Some snake-necked turtles (such as Mary River turtles ) living in rivers (Australia) have developed highly perfused mucous membranes near the anus. Although they breathe with their lungs outside of the water, their cloacal or rectal gill breathing provides their main breathing needs. This cloacal breathing is similar to that of the dragonflies (see above).
Some aquatic turtles, such as the softshell turtles , have developed oral cavity breathing or pharyngeal gill breathing . Your oral mucosa and throat are lined with villi that are well supplied with blood. By stretching and then shortening the neck or by moving the hyoid bone, as turtles do to smell, a water flow is generated and the gas exchange with the water intensified.
Evolution theories
Gill theory
According to the gill theory , the wings of the insects developed from the gills of the aquatic primordial arthropods (which were preserved as functional gills in crustaceans); this theory is in conflict with the sidelobe theory.
Gills from gill intestines
The gills of the cartilaginous and bony fish emerged from the original gill intestine of the early chordates ( tunicates and skullless animals ). The gill intestine serves there both as a filter organ to filter plankton from the surrounding water and for oxygen uptake.
Transformation of the gills with further development
Jaw formation
Evolutionary different organs emerged from the gill arches of the fish. This development began with the gnathostomata , when the pair of anterior gill arches were transformed into an original jaw. As a result, the bony fish only have 4 pairs of gill arches.
Gill slits
As land creatures evolved, the gill clefts necessarily closed to prevent moisture loss. The Eustachian tube is considered to be a rudiment of the former cleft gill between the arches of the jaw and hyoid bone, i.e. the injection hole (spiraculum). In humans, neck fistulas are viewed as an anatomical atavism for pathologically non-closed gill clefts.
New organs
The gills, which were superfluous as the lungs developed, undeniably regressed. The inner gills of the African and South American lungfish are greatly reduced and are not very suitable for oxygen supply. The jawbones, hyoid bone, parts of the larynx, auditory ossicles, tonsils and the thymus gland of the terrestrial vertebrates probably emerged from the regressive systems .
The ontogeny of terrestrial vertebrates seems to demonstrate this development. With regard to ontogenesis , the basic biogenetic rule established by Ernst Haeckel is generally observed (“This is how humans develop gill slits in the neck region a few weeks after fertilization. Some critics are of the opinion that this is an inadmissible interpretation of these untrained organs acts as supposed 'gills'. But there is no conclusive alternative interpretation for these structures, which occur exactly where gills would be expected. ”quoted from Biogenetic Basic Rule ). Since this theory was promoted by Charles Darwin , it is widely accepted.
Gill diseases (also fish diseases)
Gills are very fine mucous membranes and naturally exposed to a particularly large volume of the surrounding water. This inevitably goes hand in hand with an increased susceptibility of the gills to pathogens and other noxae .
The Koi herpes virus infection is a viral disease and can lead to a lethal disease with bleeding gills in infested river fish (for example in carp).
Gill worms (for example Dactylogyrus macracanthus Wegener ) are parasites that are responsible for fish diseases with economic consequences.
Artificial gills
Artificial gills are being discussed as a technical concept, so far they have only been implemented in aquariums for small terrestrial animals. The aim is to achieve greater independence when diving with the help of artificial gills.
literature
- David H. Evans, Peter M. Piermarini, Keith P. Choe: The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste. In: Physiological Reviews . Vol. 85, No. 1, 2005, pp. 97-177, doi : 10.1152 / physrev.00050.2003 .
Web links
Individual evidence
- ↑ Hans Thiele: Water spider (Argyroneta aquatica), breathing according to the principle of the physical gill
- ↑ Cronos: Underwater Rectal Breathing in Turtles, May 26, 2007 [1]
- ↑ Johannes Müller: The breathing technique of the turtles [2]
- ↑ Jarmila Kukalova-Peck
- ↑ Association of German Biologists, 2006 ... Haeckel showed how homologous structures, for example, the embryonic gill pockets change in the development of fish into gills or in humans to the Eustachian tube, which connects the middle ear and throat ( Memento from May 17, 2009 in Internet Archive )
- ↑ Nikolai Mette: Breathing of the bony fish: gills and gas exchange , 4. Functional change of the gill arches
- ↑ Charles Darwin: On the Origin of Species. 6. Difficulties in theory, modes of transition, 1859