Conservation

Dramatic declines in amphibian populations, including population crashes and mass localized extinction, have been noted in the past two decades from locations all over the world, and amphibian declines are thus perceived as one of the most critical threats to global biodiversity. A number of causes are believed to be involved, including habitat destruction and modification, over-exploitation, pollution, introduced species, climate change, endocrine-disrupting pollutants, destruction of the ozone layer (ultraviolet radiation has shown to be especially damaging to the skin, eyes, and eggs of amphibians), and diseases like chytridiomycosis. However, many of the causes of amphibian declines are still poorly understood, and are a topic of ongoing discussion. A global strategy to stem the crisis has been released in the form of the Amphibian Conservation Action Plan (available at http://www.amphibians.org). Developed by over 80 leading experts in the field, this call to action details what would be required to curtail amphibian declines and extinctions over the next 5 years—and how much this would cost. The Amphibian Specialist Group of the World Conservation Union (IUCN) is spearheading efforts to implement a comprehensive global strategy for amphibian conservation. Amphibian Ark is an organization that was formed to implement the ex-situ conservation recommendations of this plan, and they have been working with zoos and aquaria around the world encouraging them to create assurance colonies of threatened amphibians. One such project is the Panama Amphibian Rescue and Conservation Project that built on existing conservation efforts in Panama to create a country-wide response to the threat of chytridiomycosis rapidly spreading into eastern Panama.^[11]
On January 21, 2008, Evolutionarily Distinct and Globally Endangered (EDGE), as given by chief Helen Meredith, identified nature's most endangered species: "The EDGE amphibians are amongst the most remarkable and unusual species on the planet and yet an alarming 85% of the top 100 are receiving little or no conservation attention." The top 10 endangered species (in the List of endangered animal species) include: the Chinese giant salamander, a distant relative of the newt, the tiny Gardiner's Seychelles, the limbless Sagalla caecilian, South African ghost frogs, lungless Mexican salamanders, the Malagasy rainbow frog, Chile's Darwin frog (Rhinoderma rufum) and the Betic Midwife Toad

Respiration

The lungs in amphibians are primitive compared to that of the amniotes, possessing few internal septa, large alveoli and therefore a slow diffusion rate of oxygen into the blood. Ventilation is accomplished by buccal pumping. However, most amphibians are able to exchange gasses with the water or air via their skin. To enable sufficient cutaneous respiration, the surface of their highly vascularized skin must remain moist in order for the oxygen to diffuse at a sufficient rate. Because oxygen concentration in the water increases at both low temperatures and high flow rates, aquatic amphibians in these situations can rely primarily on cutaneous respiration, as in the Titicaca water frog and hellbender salamanders. In air, where oxygen is more concentrated, some small species can rely solely on cutaneous gas exchange, most famously the plethodontid salamanders, which have neither lungs nor gills. Many aquatic salamanders and all tadpoles have gills in their larval stage, with some (such as the axolotl) retaining gills as aquatic adults.

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

Amphibians

Amphibians are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic (or cold-blooded) tetrapods. Most Amphibians undergo metamorphosis from a juvenile water-breathing form to an adult air-breathing form, but some are paedomorphs that retain the juvenile water-breathing form throughout life. Mudpuppies, for example, retain juvenile gills in adulthood. When they make their transition into adulthood they grow lungs so they can breathe in air. They have thin and slimy skin. The skin is so thin that amphibians absorb water through it instead of drinking. But they can also lose water through their skin and easily become dehydrate. Amphibians are often called ecological indicators. In other words, unhealthy amphibians can be an early sign of changes in an ecosystem.