tadpole
Tadpoles are post-embryonic stages of development - the larvae - of the frog . Larvae of the tail amphibians are not called that. In many species, this phase takes place in an aquatic manner , i.e. freely swimming in a body of water (indirect development). These accumulations of water can also be very small phytotelmata such as bromeliads or pitcher plants . However, there are also special forms of brood care (see below). In various groups, such as the Papuan narrow-mouth frogs or the Strabomantidae , a direct development takes place within the eggs up to the finished young frog.
Shape change of the tadpole
After fertilization of amphibians - spawn lasts embryonic development of the zygote depending on the type and ambient temperature only a day or several weeks; for the common frog in Central Europe, for example, 10 to 14 days, but sometimes also three to four weeks. Then the larva hatches from the gelatinous egg shell. An enzyme helps her to dissolve the gelatinous layer; in addition, the hatchling moves very violently during this process. The small tadpole initially has three outer gill tufts on each side of its head , as well as an oar tail with a fin edge. With the help of adhesive organs in the mouth area, it clings to the old gelatinous shell, later to plants, branches or stones, and feeds on its egg yolk supply on the stomach.
After the attachment phase, which lasted several days, the until then slit-shaped mouth opens, the oar tail enlarges and gets wider skin seams. The gills are now overgrown by a fold of skin ( opercular fold ), i.e. lying on the inside. The gas exchange with the environment takes place via the so-called spiraculum (breathing hole) on the left side of the body (in the case of the types of Archaeobatrachia, on the belly). The larva is now buoyant and is actively consuming food. With horned jaws and lip teeth on the mouth (see illustration below), it grazes green algae , bacteria , diatoms and other single-cell microorganisms as well as organic suspended matter and sediment material ( detritus or pollen ) from stones, aquatic plants and the water surface. Older tadpoles in particular consume animal food in addition to plant-based food, including dead conspecifics or the spawn of their own species and other amphibians.
Later, the two form pairs of limbs , wherein initially the hind legs, until days later, which are visible from the outside in the gill bag grown forelegs (in salamander larvae, the order is reversed).
The final phase of larval existence is characterized by the breakthrough of the forelegs, the gradual regression of the oars tail, the reshaping of the mouth and digestive tract, the development of lungs and simultaneous regression of the gills, the development of eyelids and eardrums and a general change in shape. Only this phase between the formation of the extremities and the shore leave is called metamorphosis , of which the change in shape of the frog auger is a classic example. Depending on the species, tadpoles can reach very different body sizes in the end - the common frog mentioned above is around 40 to 46 millimeters (rarely up to 70 mm). The larvae of the common garlic toad can be 10 centimeters, in extreme cases even over 18 centimeters long.
Zoologists differentiate between the development of the frog egg until the completion of the metamorphosis (shore leave) 46 consecutively numbered stages of maturity according to the so-called Gosner table. This was published in 1960 and based on the development of the clawed frog Xenopus laevis . Thereby, 25 developmental stages of the embryonic phase and then 21 larval stages are defined and illustrated.
Finally, the young frog or young toad leaves the water, often with a small tail stump, which disappears after a short time. The animal now feeds on small invertebrates such as flies, mosquitoes and worms and has thus become a pure "carnivore". The initially somewhat fish-like shape gives way more and more to the features of a frog. A common frog reaches sexual maturity after three years of rural life at the latest (in other species it is sometimes faster). Then it returns to its native waters in spring to mate and takes care of new tadpoles itself.
Development time
The time it takes to develop from a tadpole to a metamorphosed frog depends on the species and the environmental conditions, especially the ambient temperature. For common frog tadpoles and many other species in Central Europe, it is usually about ten to twelve weeks. Tadpoles made from spawn that was laid in early April then leave the pond around mid to late June.
Some late-spawning species also overwinter as larvae in the water and do not complete their metamorphosis until the following year (this usually results in the aforementioned giant growth, which can also be hormone-related ).
"Record holders" with the shortest larval phase are some species of American paddock toads with in extreme cases only twelve days. But even with the natterjack toad native to Central Europe , a development time of only 17 days to the young toad has been observed.
On the other hand, there are species in which the aquatic tadpole phase lasts several years, for example in the North American bullfrog two to three years. The larvae of some other species also sometimes remain in this state for years - one speaks of temporary neoteny (which, however, occurs more frequently in tail amphibians and can also be permanent).
Special cases of brood care
In the midwife toads , the aquatic phase of the larvae is shortened by the fact that the males carry the eggs around their hind legs with them and finally release relatively well-developed tadpoles into the water.
Some Seychelles frogs (family Sooglossidae) lay eggs in small piles of gel on land, where they are guarded by the male for several days. When the larvae hatch, they crawl onto the father's back, adhere tightly and are carried around with him. The rest of the time up to metamorphosis they only feed on the yolk supply from the egg.
A bizarre example of unusual forms of brood care and larval development in amphibians were the small gastric breeder frogs (genus Rheobatrachus ) from eastern Australia (genus Rheobatrachus ) from eastern Australia , known only since 1972 but now considered extinct : the females ingested fertilized eggs (possibly young larvae) with their mouths, they swallowed them and after several weeks of development they spat out “finished” young frogs. Here the stomach was converted into a uterus , as it were, in which the tadpoles grew; larval water was not required.
The situation is similar with the Chilean Darwin nosed frog ( Rhinoderma darwinii ): It is the male, however, that takes the fertilized eggs into his throat sac. The tadpoles hatch there, are nourished by a viscous secretion that is formed on site, and finally leave the father's interior through the mouth as transformed frogs.
The reproductive behavior of the comb toads (genus Pipa ) is no less astonishing : when they mate, the toads swim "loops" so that the eggs released into the water land on the back of the female. There they are trampled on by the male with their hind feet. Over time, the back skin of the female encloses every single egg with a honeycomb . The tadpoles then grow in these chambers. However, during this time the female is in a body of water.
In the Panamanian fringe-toed tree frog Ecnomiohyla rabborum , which was only discovered in 2005 and described three years later, the male apparently feeds the tadpoles growing in a water-filled tree cavity with bits of his skin.
Some species, for example from the New Zealand primeval frogs (Leiopelmatidae), forego the so-called indirect larval development, i.e. the free larval phase in the water, by depositing their eggs on land. There the embryonic and larval development takes place within the egg shell - the finished frogs finally hatch. A number of other frogs do the same.
In the tree climber genus Dendrobates , the clutches deposited on land are kept moist by a parent animal. After hatching, the tadpoles are transported on the parents' back to a suitable water reservoir. Here they are then left to their own devices. In the Oophaga genus (e.g. strawberry frogs ), the larvae are transported individually in pools of water such as the leaf axils of bromeliads or bananas. The female visits these again and again every few days. It then settles in the pool of water and is stimulated by the tadpole's snaking movements to shed individual eggs. The larva only feeds on these.
Direct live birth is a rare exception in amphibians. In this context, only the tiny viviparous toads of the genera Nectophrynoides (highlands of Tanzania ) and Nimbaphrynoides (West Africa) should be mentioned among the frogs . These give birth to fully developed young toads. They previously developed their larvae within the uterus of the female.
Dangers to the tadpole
Amphibians are threatened by numerous predators in every stage of their development. Fish (so-called predatory fish, but also non-predatory fish), newts , waterfowl , water bugs and insect larvae, for example those of dragonflies or the fire beetle , chase the spawn and the tadpoles .
Tadpole populations are really endangered primarily by human interference:
- The draining of wetlands and backfilling of water destroys their habitat.
- The entry of fertilizers, pesticides and other pollutants into the waters can lead to deformities and poisoning. It can also accelerate the silting up of the water.
- The introduction of fish in small ponds and ponds results in the extinction of the amphibians there.
- Amphibian spawn and larvae are often removed from fry ponds in the belief that the tadpoles could be food competitors for the fry. However, this is not the case, partly because fish hardly feed on algae growth and detritus.
(For the dangers to transformed and adult amphibians see for example: Amphibians .)
Word origin ( etymology )
The Middle Low German quappe, quabbe probably means “slimy lump, wobbly animal, frog spawn” and thus belongs to the group of words “quabbeln, wobble, wobble”. There is also a bony fish ( burbot ) from the burbot family with the same spelling .
The word part Kaul - means "ball, thick head," derived from the early modern German fabric roll (see also: ruffe , Kaulkopf [= Bullhead ] and the Saxon court quarkkäulchen ).
Individual evidence
- ^ KL Gosner: A simplified table staging anuran embryo and larvae with notes on identification . Herpetologica 16 (1960): 183-190.
- ↑ Wolf-Rüdiger Grosse: The use of standards and tables for field determination of the developmental stage of native amphibian larvae. P. 349–364 in K. Henle & M. Veith (eds.): Methods of field herpetology relevant to nature conservation. Mertensiella 7 (1997). ISBN 3-9801929-6-2
literature
- Günther E. Freytag, Bernhard Grzimek, Oskar Kuhn & Erich Thenius (eds.): Lurche . In: Grzimeks Tierleben, Vol. 5: Fish 2, Lurche. Licensed edition in dtv, Munich 1980, ISBN 3-423-03204-9
- Andreas and Christel Nöllert: The amphibians of Europe . Franckh-Kosmos, Stuttgart 1992, ISBN 3-440-06340-2
Web links
- kidsweb.de: child-friendly photo series "From tadpole to frog"
- www.kaulquappe.de (PDF; 44 kB)
- The common frog - A fieldwork (PDF; 830 kB)