Amphibians Way of Reproduction

For the purpose of reproduction most amphibians require fresh water. A few (e.g. Fejervarya raja) can inhabit brackish water and even survive (though not thrive) in seawater, but there are no true marine amphibians. However, there are reports of particular amphibian populations invading marine waters where their species is normally unable not survive. Such is the case^[9] with the Black Sea invasion of the natural hybrid Pelophylax esculentus reported in 2010.
Several hundred frog species in adaptive radiations (e.g., Eleutherodactylus, the Pacific Platymantines, the Australo-Papuan microhylids, and many other tropical frogs), however, do not need any water for breeding in the wild. They reproduce via direct development, an ecological and evolutionary adaptation that has allowed them to be completely independent from free-standing water. Almost all of these frogs live in wet tropical rainforests and their eggs hatch directly into miniature versions of the adult, passing through the tadpole stage within the egg. Reproductive success of many amphibians is dependent not only on the quantity of rainfall, but the seasonal timing.^[10]
Many amphibians exhibit different kinds of parenting behaviour. After their hatching, the tadpoles of different species of poison dart frogs (family Dendrobatidae) are carried by the adults to a sutable place where they can pass metamorphosis. Such places are the rosettes of many bromeliads in which water is gathered and used by the plant. The Surinam toad raises its youngs in pores at its back and after enough time they appear out of these pores fully developed. The ringed caecilian (Siphonops annulatus) has developed a unique adaptation for the purposes of reproduction. The progeny feeds on a skin layer that is specially developed by the adult. This phenomenon is known as maternal dermatophagy.
Several species have also adapted to arid and semi-arid environments, but most of them still need water to lay their eggs. Symbiosis with single celled algae that lives in the jelly-like layer of the eggs has evolved several times. The larvae of frogs (tadpoles or polliwogs) breathe with exterior gills at the start, but soon a pouch is formed that covers the gills and the front legs. Lungs are also formed quite early to assist in breathing. Newt larvae have large external gills that gradually disappear and the larvae of newts are quite similar to the adult form from early age on.
Frogs and toads however have a tadpole stage, which is a totally different organism that is a grazing algae or ongrowth or filtering plankton until a certain size has been reached, where metamorphosis sets in. This metamorphosis typically lasts only 24 hours and consists of:

• The disappearance of the gill pouch, making the front legs visible.
• The transformation of the jaws into the big jaws of predatory frogs (most tadpoles are scraping of algae or are filter feeders)
• The transformation of the digestive system: the long spiral gut of the larva is being replaced by the typical short gut of a predator.
• An adaptation of the nervous system for stereoscopic vision, locomotion and feeding
• A quick growth and movement of the eyes to higher up the skull and the formation of eyelids.
• Formation of skin glands, thickening of the skin and loss of the lateral line system
• An eardrum is developed to lock the middle ear.

The disappearance of the tail is somewhat later (occurs at higher thyroxin levels) and after the tail has been resorbed the animals are ready to leave the water. The material of the tail is being used for a quick growth of the legs. The disappearance of the larval structures is a regulated process called apoptosis.
The transformation of newts when leaving the water is reversible except for the loss of the external gills. When the animals enter the water again for reproduction changes are driven by prolactin, when they return to the land phase by thyroxin