The mouth of a fish has anatomical features that are related to the diet of the respective fish species.
Position of the fish's mouth
- A terminal mouth can often be observed in fish from the central water region. It's at the top of the snout; The upper jaw and lower jaw are the same length.
- An upper mouth can be found in fish species that hunt on the surface of the water. The lower jaw is longer than the upper jaw. However, this characteristic also occurs in some herbivorous fish species.
- A lower mouth is typical for fish species that mainly stay on the bottom of the water and look for food there. Here the lower jaw is slightly shorter than the upper jaw. The mouth opening therefore points downwards. An under constant mouth that also as a suction mouth is formed, have the armored catfish . This enables them to attach themselves to the subsurface without being carried away by the current. However, there are subordinate suckers already in the "most primitive" living fish, the lampreys , which they also use as ectoparasites for food intake as part of their way of life (whereby lampreys do not have jaws in the true sense), as well as carp fish and catfish from tropical mountain streams even sucked in, scrape off rubble in the stream bed while still being able to breathe.
- Almost all fish have teeth: the pines and other skeletal parts of the pharynx, especially the throat bone ( pharyngeal jaw ) - in almost unmanageable abundance of forms. The carp fish have no jaw teeth.
- An elongated snout is mainly found in specialized fish species that live in coral reefs , for example . They include the bird wrasse , forcipiger and chelmon , which with their long snouts are able to look for food in the cracks of a coral reef.
- The mouth can be extended especially in predatory fish. They are able to advance it when they catch prey. When the lower jaw is lowered (usually by lifting the gill cover ), the upper jaw (premaxillary) is pulled forward by the leverage of the maxillary ("maxillary apparatus" - see below under "Functional", especially well developed in carp and perch species ). The mouth becomes smaller, tubular, but is suddenly close to the prey, which is sucked in ("sucking snap") before it can escape. Most Teleostei are such predatory fish; however, fish eaters often lose their ability to stretch themselves because they do not snap on the suction, but instead grab the prey with their teeth (e.g. pike). Devouring occurs through the pharyngeal mechanism, by means of the pharyngeal teeth on the gill arches.
- The mouth of most fish (exceptions: e.g. herring) includes the buccal valves . These are two delicate, sickle-shaped folds of the mucous membrane on the back of the upper and lower jaw (maxillary and mandibular valve) with a valve function for breathing, without their own muscles (exceptions: e.g. some fish with a sucking mouth). The valves save the fish from having to completely close the mouth during the pressure pump phase.
In contrast to the mouth of the mammals, the mouth of the other vertebrates , but especially that of the "fish", can not only be opened and closed, but can also be extended transversely, which results from both the gill intestine function (breathing!) And the suction snap -Function (see below) of aquatic animals with internal skeletons results. However, this means that the skeletal and muscular apparatus in the mouth must be much more complicated than in mammals.
In mammals, the lower jaw is guided on a bone called the squamosum (secondary jaw joint), in all other vertebrates with jaws, however, on the quadratum (primary temporomandibular joint; in mammals the quadratum corresponds to the anvil, incus , in the middle ear). In birds and many reptiles, the quadratum is mobile and even steered on the skull - Jan Versluys called this condition streptostyly . Other parts of the skull in these animals can also move against one another. This resembles to a certain extent the state of many "fish" and is called a kinetic skull . In mammals, however, the skull is relatively rigid and compact, they have an akinetic skull.
Types of lower jaw suspension (suspension)
Traditionally, the upper and lower jaws of vertebrates are traced back to "gill arches". Such an “idealistic” derivation is neither compelling nor clear. Certainly there were agnathes with skeletal formations to support the gill slits, which did not yet have a jaw, but cartilage structures around the mouth (which were not previously gill arches!) - the jaw forceps may have emerged from this through lateral joint formation. However, fossils that prove this are still unknown. With these early vertebrates , we always have to reckon with a skin-bone armor whose (toothed) elements were mixed in at the front end of the “primal fish” from the start. Some theorists have even suggested that the upper and lower jaws can be traced back to two arches lying one behind the other; the majority derive both parts (each side) from the two legs of an arch. The hypothetical "initial state" (jaw not attached to the brain-skull) was called " paleostyly " (H. Hofer 1945). Usually, however, the upper jaw is more or less firm (through connective tissue, especially ligaments ), more or less flexible, articulated , connected to the skull. (The lower jaw always steers on the "upper jaw" - except in mammals.)
In the cartilaginous fish , the bone armor has disappeared (to reduce weight), the two (cartilaginous) jaws are therefore "uncovered" - but there is certainly no primitive condition here. Depending on the diet, the upper jaw (the palatoquadratum ) is connected to the (brain) skull in different ways , so that the "jaw forceps" (for gripping) are moved as a whole (whereby the upper part of the following arch, the hyomandibula , as Guide link serves, e.g. in rays : (eu) hyostyle state of the upper jaw; hyostyly ), or (as the other extreme of a series of transitional forms) the upper jaw is completely fused with the skull (similar to the mammals: holostyle, holostyly) ): among the chimeras (as an adaptation to shellfish food: crush teeth).
Ancient sharks such as the Hexanchidae (e.g. the collar shark ) are autostyl ( autostyly ), i. H. the upper jaw itself is directed on the skull (in front, in the nasal region, and in the back on the base of the skull, basitrabecular: autodiastyly - or behind only with the mediation of the hyomandibel: amphistyly ). Latimeria is diastylic because the hyomandibel does not act as a pine stem. The hyomandibel (it corresponds to the stapes in humans , stapes , in the middle ear) is more likely to be interpreted as a (transformed) part of a former gill arch. Between it and the upper jaw there is sometimes the injection hole , spiraculum , which has long (and probably wrongly) been regarded as the “ rudiment ” of a previously fully developed cleft gill. (In humans, the ear canal and the Eustachian tube correspond to it .)
All other recent jaws have skin or cover bones that - in quite different ways - are involved in jaw formation. The sturgeons are (met) hyostyl (their jaw apparatus is similar to that of “modern” sharks and rays), the lung fish are holostyled (similar to the chimeras: “ systyly ”), all other bone fish ( Osteichthyes ) are amphistylic. What corresponds to the shark's upper jaw (with symphysis !) Is now the (also largely ossified) jockstrap (it consists of the bones palatinum, three pterygoids and quadratum [on which the lower jaw is guided]; it is also found in more primitive bony fish ( Teleostei ) still dentate; there is only a symphysis in the sturgeon - probably already secondary) including the hyomandibular. In evolution, however, the “focus” of the dentition shifts to the mandibular margin bones premaxillary (upper) and dental (lower jaw; see bony fish : skeleton. The lower jaw of the teleostei usually only consists of three bones each, of which the dentate is the is largest - in mammals, as is well known, the lower jaw consists only of the two dentalia, while articular and angular have got back into the ear). The jockstrap enables (as with the terrestrial vertebrates (Tetrapoda) except for the mammals ) the widening of the oral cavity to the sides and thus - in the water - the " sucking " (prey is sucked in) - which is then greatly increased in its efficiency by an "automatic “Grasping movement with the premaxillaries, which slide back and forth on the nasal region of the skull using the rostral cartilage like on a splint. This movement is mediated by the jockstrap through a complicated unrolling of the maxillary , which in the more primitive bony fish (and bony fish) was still the toothed rim of the mouth (and remains in tetrapods). The amphistyly of the bony fish is called ethmohyostyly (we will not go into more detailed subdivisions) because the jockstrap is directed at the back by means of the hyomandibular on the skull, in front (in the palatine) directly on the nasal capsule, which ossifies (among other things) as an ethmoid. (The ethmoid corresponds to our ethmoid bone .)
The maxillary apparatus of the teleostei
The maxillary of the acanthopterygii only supports the skin fold, which makes it possible for the premaxillary to reach in advance - this narrows the gap in the mouth, but the sucked in water reaches a higher speed when sucking, which makes it very difficult for agile prey to escape. The two skulls joints of the suspensory themselves are usually double-headed and not parallel, resulting in (sensible!) Twists leads (made possible by a mostly wide cartilage zone, limited by the always striking at Teleostei Praeoperculare ).
The anatomical basis of the pre-extensibility of the functional upper jaw of the teleostei is called the maxillary apparatus . It has also emerged several times ( convergence ; e.g. also in Cyprinidae , Loricariidae , Veliferidae , Zeidae ) and has a very large number of special forms. The opening of the mouth (which usually initiates the activation of the maxillary apparatus ) occurs, for example, by dorsad rotation of the operculum and then (usually) by contraction of the ventral trunk muscles (especially in front of the shoulder girdle ), which also spreads the hyoid laterad and abducts the suspensoria ) (enlargement of the oral cavity to suckle). In the case of some extreme suckers, the jockstrap will even dissolve so that the lower jaw can also be moved forward (Labridae: Epibulus insidiator , Cichlidae: Petenia splendida , the latter, however, without suckers).
Different types of food intake
To catch fleeting prey (the majority of the Teleostei are predatory fish), three distinguishable methods are used, which cannot be clearly delimited:
- Carnivores pack the prey from a wait position or tracking with long, well-toothed jaws ( Hecht , pikes , belonesox belizanus , Zander , barracuda ). The Malacosteinae only hit the sharply toothed lower jaw into the prey. Fish with exactly matching teeth can accrue larger prey and “carve” (saw tetra ).
- Predators with mouthpipes surprisingly run over their prey with a suddenly extended, quite delicate maxillary apparatus like a sack pulled over ( ram feeding ; see Petenia splendida , Luciocephalus pulcher ).
- The suckers suck the fleeing or clinging prey into the oral cavity by means of sudden pharyngeal expansion and activation of the maxillary apparatus, whereby strong negative pressure is applied ( suction feeding ; see e.g. Epibulus insidiator , Stylephorus chordatus ; also with the Syngnathidae with no pipette mouth) Maxillary apparatus). The sucking snap is like a strong inhalation.
Fish with non-volatile food have also developed numerous specializations in the mouth area.
- The Ayu ( Plecoglossus altivelis ) scrapes algae growth with the jaws of stones, while Poeciliidae such as the black-mouthed larva do it frontally because the lower jaw is so flexible that the tooth surfaces like lips can be pressed against the substrate (see also kissers Gurami ). The scraping of carp fish works differently again .
- Parrotfish have a joint in their lower jaw to increase the bite pressure (on hard corals ).
- Sea wolves and sea bream , among others, have simple bites .
- Many plankton eaters eat very easily: they strain the breath water with the dense gill trap , like the European anchovy . Similarly, see z. B. sturgeon and mullet organisms from detritus . Teeth are not important here.
- A. Gibb et al: Functional significance of intramandibular bending in Poeciliid fishes. In: Environmental Biology of Fishes. 83, 4, 2008, pp. 507-519.
- Helmut Hofer: To the knowledge of the suspension forms of the jaw arch and their connections with the structure and with the kinetics of the skull in the bony fish. In: Zool. Jbr Anat. 69, 1945, pp. 321-404.
- Wilfried Westheide, Reinhard Rieger (Hrsg.): Special zoology. Volume 2: Vertebrates or Skull Animals. Spektrum Akad. Verlag, 2004, ISBN 3-8274-0900-4 .
- Cheryl D. Wilga, Philip J. Motta, Christopher P. Sanford: Evolution and ecology of feeding in elasmobranchs. In: Integrative and Comparative Biology. 47, 2007, pp. 56-69.