Water frogs

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Water frogs
Pond frog (Pelophylax "esculentus")

Pond frog ( Pelophylax "esculentus" )

Systematics
Row : Land vertebrates (Tetrapoda)
without rank: Amphibians (Lissamphibia)
Order : Frog (anura)
Subordination : Neobatrachia
Family : Real frogs (Ranidae)
Genre : Water frogs
Scientific name
Pelophylax
Fitzinger , 1843

As water frogs , also green frogs , mainly semi- aquatic representatives of the family of real frogs (Ranidae) are summarized. Many authors have recently started listing them in their own genus Pelophylax , thus separating them taxonomically more clearly from other real frogs of the genus Rana , to which the water frogs are traditionally included. Alternatively, Pelophylax is still only treated as a subgenus within Rana . In addition, the term "water frog" is occasionally used as a synonym for the hybrid pond frog ( Pelophylax "esculentus" ). Furthermore, the fuzzy common name “water frog” can also refer to other frogs that live near water all year round.

The frogs of the genus or sub-genus Pelophylax understood under the term here are common in Europe, North Africa and Asia. They are typically, but not necessarily, characterized by a green color and a way of life that is closely related to the banks of the water. It is a systematically very "difficult" complex of forms with kinship relationships that have not yet been fully researched and various hybridizations with one another. The phenomenon of the emergence of hybridogenic hybrids, which can also reproduce independently of their two parent species , is particularly remarkable.

features

Head of a pond frog in profile
Water frogs have a relatively long, pointed snout and closely set or slightly upward-pointing eyes
The heel hump (also: "metatarsal tubercle" or "callus internus") at the base of the first toe can be used to more precisely identify the species of water frogs; here the foot of a pond frog
A sea frog, Pelophylax ridibundus , in its element; between Canadian waterweed
Edible frog from northern France

Depending on the species, sex and age, adult water frogs reach a head-to-trunk length between 4.5 and 14 (in very rare cases: 18) centimeters, whereby the females are often slightly larger than the males. There can be a very strong variability of the coloring and drawing, also within the species. Often a green basic color dominates the upper side and the extremities, but brown and gray tones are also possible. The back is often irregularly provided with dark spots or spots; in many individuals of several species there is also a bright line from the snout to the cloaca . A raised back gland ridge runs along the lateral back on both sides. The snout is usually quite long and pointed. The eyes are a little closer together than with the brown frogs and are directed more upwards than with these. The "folded" hind legs, which are placed against the torso when resting, are very muscular and enable the animals to make long jumps or even powerful swimming and diving movements, supported by well-formed webs between the toes. Characteristic of the males are paired lateral sound bubbles under the corners of the mouth, which they protrude to generate various calls and use as a resonance amplifier.

For the external differentiation of the individual species, certain body proportions and biometric indices are also important, such as the arithmetical ratio of the head-trunk to the lower leg length, the ratio of the lower leg length to the width of the heel hump on the sole of the foot and the shape and size of the heel hump in relation to Length of the first (inner) toe of the hind foot. Nevertheless, due to complicated genetic issues within the water frog complex (greatly abbreviated and simplified: see below) a phenotypically reliable identification of the species is not possible in all cases , especially since mixed populations often occur.

Hybridization

In the mid-1960s, the Polish herpetologist Leszek Berger first discovered that the pond frog, which had been treated as a normal species up until then, was actually a hybrid form that emerged from natural crosses between the sea ​​frog and the small water frog . Numerous cross-breeding experiments and biochemical studies confirmed this hypothesis. Other hybrid forms were later identified among the European water frogs. Experiments have even shown that all water frog species can be crossed with each other and can produce fertile offspring. This is attributed to the fact that, from an evolutionary point of view, they are all relatively young species that do not yet have sufficient reproductive isolation mechanisms. Thus the representatives of the water frogs do not meet all the criteria of a biological species .

The three previously known hybridogenic hybrids, which can also reproduce permanently without the presence of both parent species, are particularly out of the ordinary. They succeed in doing this because a complete set of parental chromosomes is eliminated during germ cell formation and only the other (hemi-) clonal set is passed on to the next generation. As a result, the germ cells of these animals only contain the genetic information of one parent species (for example that of the sea frog; P. ridibundus ), from pairings with a representative of the other parent species (here: small water frog; P. lessonae ) hybrid forms (here: pond frogs) emerged.

In addition, there are triploid pond frogs that have the complete set of chromosomes from a parent species and can thus virtually replace them. If, for example, a triploid pond frog , which has a lessonae- dominant genome , mates with a "normal", diploid pond frog (which only passes on hereditary information about the sea frog), pond frogs are formed. This mode of inheritance, which is sometimes also referred to as “genetic parasitism”, enables water frog populations to exist especially in Central and Eastern Europe that consist almost exclusively or even exclusively of pond frog hybrids and, contrary to Mendel's rules , permanently viable offspring beyond the F1 generation witness. The hybrids are often even ecologically much more potent and adaptable and therefore more abundantly distributed than the parent species. In Central Europe, for example, this applies to the pond frog, which is found much more frequently than the little water frog and the sea frog. Either it occurs in pure populations without the presence of the parent species (see above) or is associated with one of the two (but only rarely syntopically with both at the same time). Conversely, pond frogs are very often present in stocks of the sea frog as well as those of the small water frog.

Distribution, species and hybridogenic hybrids

Water frogs of the taxon Pelophylax are widespread in Europe (largely excluding the British Isles and Scandinavia, but including various Mediterranean islands), in North Africa (also released on some Canary Islands ), Middle and Middle Asia and in the Far East of Asia as far as Japan. Isolated populations are even found in desert oases in Saudi Arabia and Bahrain; however, the main focus of distribution is in the temperate zones of Eurasia.

The systematics and taxonomy of the water frogs are constantly changing. Many questions have not yet been clarified and the status of some forms is controversial. This applies in particular to the species group around the sea frog ( Pelophylax ridibundus sensu lato; these include P. bedriagae , P. cretensis , P. epeiroticus , P. kurtmuelleri , P. ridibundus and P. terentievi ). Of the approximately 25 species and hybridogenic hybrids currently described, the ones listed below are common in Europe. (Note: The generic name Rana is still used instead of Pelophylax ; in this case, the male ending -us of the species name may have to be replaced by a female -a . Bold formatting: also occurs in Germany / Austria / Switzerland.)


The hybrid forms Pelophylax "esculentus" (originated from P. lessonae and P. ridibundus ), P. "grafi" (parent species: P. perezi and P. ridibundus ) and P. "hispanicus" (parent species: P. bergeri and P. ridibundus ) are neither biological species nor taxa in the classical sense. This can be expressed by spelling the species epithet in quotation marks or by adding a "kl." (For "Klepton"; compare: pond frog ) between the genus and species name part - however, Anglo-Saxon authors in particular have often made these formal suggestions so far not taken over. In some systematic reviews, the hybrids are not listed separately at all, but taxonomically only understood as synonyms of the respective parent species.

For the Near East, Central and East Asia as well as North Africa at least the following further species are distinguished:

Sea frog from Iran
  • Pelophylax caralitanus (Arikan, 1988) - Arikans water frog
  • Pelophylax chosenicus (Okada, 1931)
  • Pelophylax demarchii (Scortecci, 1929)
  • Pelophylax fukienensis (Pope, 1929)
  • Pelophylax hubeiensis (Fei & Ye, 1982)
  • Pelophylax lateralis (Boulenger, 1887)
  • Pelophylax nigromaculatus (Hallowell, 1861)
  • Pelophylax plancyi (Lataste, 1880)
  • Pelophylax porosus (Cope, 1868)
  • Pelophylax saharicus (Boulenger in Hartert, 1913) - Sahara water frog
  • Pelophylax tenggerensis (Zhao, Macey & Papenfuss, 1988)
  • Pelophylax terentievi (Mezhzherin, 1992) - Terentiev's water frog


Individual species, meanwhile also named Pelophylax , are now assigned to other genera such as Babina , Odorrana or Rana ( R. shuchinae ).

Way of life

Quacking male of the little water frog, Pelophylax lessonae . In this species, the paired sound bubbles look pure white
Water frog tadpoles (here the Iberian water frog, Pelophylax perezi ) can grow to be relatively large

The majority of the water frogs stay more or less permanently in the direct vicinity of open water. However, individual species such as the little water frog or, in general, the young animals also have longer phases of terrestrial life far away from ponds and ponds. The preferred places to stay for most water frogs are waiting areas on banks that are exposed to the sun and not overgrown with vegetation or on floating leaf vegetation. There the animals sunbathe - also sociable - and lurk among other things for insects such as dragonflies , which they capture by jumping up and with the help of their tongues flashing forward. But smaller amphibians - including their own species - are also eaten. Water frogs themselves are part of the food spectrum of numerous water birds, mammals or even snakes such as the grass snake . If there is any disturbance or danger, the frogs jump into the water with a long leap and dive into the mud or between herbaceous water plants. In winter, depending on the region, a freezing cold occurs either in holes in the ground and other frost-proof hiding places on land or - especially in the case of sea frogs - also in the sediment of the water bed. In aquatic hibernation, the gas exchange of the severely reduced metabolism takes place via skin respiration .

During the breeding season, water frog males of different species form loud chants that can be heard from far away during the day, but also at night. A distinction must be made between territorial calls for territorial delimitation from rivals and mating calls for attracting females. In Central Europe, the water frog species are among the most recent courtship and spawning amphibians - the focus is in May and June. The amplexus of males and females is axillary as in all higher frogs. The light brown colored eggs are attached to plants in several gelatinous balls per female under water. This spawn is much less noticeable than, for example, the large "flat cakes" of the common frog that spawn in early spring, floating on the water . The tadpoles can reach a considerable size in the course of the summer before they undergo a so-called " shrinkage metamorphosis " to become the transformed young frog. Sometimes real giant larvae with a total length of more than ten (in extreme cases over 18) centimeters can be observed. These are mostly hormonally disturbed specimens with delayed metamorphosis, which occasionally even hibernate in the larval stage.

Water frogs and humans

In Central Europe, too, water frogs have long been on people's menu, which is reflected not least in the scientific name of the pond frog coined by Carl von Linné in 1758: Rana esculenta means “edible frog”. And even today, large quantities of “ frogs' legs ” are consumed , especially in francophone regions such as Switzerland, Belgium and France, but also Italy . Besides other - especially Asian - species, water frogs are still used. The majority of these animals are imported from the Balkans and Anatolia. Turkey alone exported around 13.4 million water frogs to Western Europe in 2009. Countless water frogs had to lose their lives in universities, schools and laboratories as experimental animals and anatomical objects. Meanwhile, at least the Central European forms enjoy a certain legal protection, for example through the German Federal Species Protection Ordinance and the Federal Nature Conservation Act . According to this, these are “specially protected” species that cannot simply be caught, injured or killed. The little water frog is even “strictly protected” across the EU and has also been included in the Red Lists of endangered species. This is mainly a consequence of its more narrowly defined habitat requirements, which are negatively influenced by changes in the landscape of humans. The hybrid pond frog, on the other hand, is a common occurrence in many still and flowing waters, even smaller artificial garden ponds are colonized by it.

The anthropogenic release of alien water frogs in the open landscape is problematic . Native amphibian species can be displaced by competition and predation pressure ; In addition, the locally grown genetic structure of the naturally existing water frog populations is interfered with. In addition, exposure actions of this kind increase the risk of the introduction and spread of diseases such as chytridiomycosis , which is involved in the worldwide extinction of amphibians . In addition to nature and species protection aspects as well as economic use, water frogs are now primarily in the focus of evolutionary biologists and geneticists.

In residential areas, neighborhood conflicts can sometimes arise if residents feel disturbed by the nocturnal croaking of water frogs in garden ponds. However, these noises are to be accepted as natural, especially if they are animals that have immigrated by themselves.

swell

Individual evidence

  1. ^ Rainer Günther: European water frogs (Anura, Ranidae) and biological species concept. Messages from the Museum of Natural History in Berlin. Zoological Museum and Institute for Special Zoology (Berlin), Volume 67, Issue 1, pp. 39-53. Published online: April 19, 2008
  2. ↑ Amphibian Species of the World  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link / research.amnh.org  
  3. ↑ Torsten Ohst, Yvonne Gräser, Frank Mutschmann & Jörg Plötner: New findings on the endangerment of European amphibians by the skin fungus Batrachochytrium dendrobatidis. Zeitschrift für Feldherpetologie 18, Laurenti-Verlag, Bielefeld 2011: p. 4.
  4. ↑ for example: Federal Agency for Nature Conservation (ed.): Red List of Endangered Animals, Plants and Fungi in Germany 1: Vertebrates. Landwirtschaftsverlag, Münster 2009. ISBN 978-3784350332
  5. ↑ Heinz Tunner: The pond frog Rana esculenta - A frog that is unique in evolutionary terms. In: Stapfia. Volume 47, Linz 1996, also catalogs of the O.Ö. Landesmuseums NF 107, pp. 87-102, PDF on ZOBODAT

literature

  • Dieter Glandt: Pocket dictionary of amphibians and reptiles in Europe . - Quelle & Meyer Verlag, Wiebelsheim 2010. ISBN 978-3-494-01470-8
  • Rainer Günther: Europe's water frogs . Neue Brehm-Bücherei 600, A. Ziemsen-Verlag, Wittenberg Lutherstadt 1990. ISBN 3-7403-0234-8
  • Rainer Günther (Ed.): The amphibians and reptiles of Germany . G. Fischer-Verlag, Jena 1996. ISBN 3-437-35016-1
  • Jörg Plötner: The western Palearctic water frogs - from martyrs of science to a biological sensation . Supplement to the magazine f. Feldherpetologie 9, Laurenti-Verlag, Bielefeld 2005. ISBN 3-933066-26-3
  • Jörg Plötner: The Central European water frogs (Rana esculenta complex). - In: Hubert Laufer, Klemens Fritz & Peter Sowig (eds.): The amphibians and reptiles of Baden-Württemberg. Ulmer, Stuttgart 2007, pp. 451-458. ISBN 978-3-8001-4385-6

Web links

Commons : Pelophylax  - collection of images, videos and audio files