Chordata-Amphibia

=**Amphibians**=

Alyssa Zisk



== (JJF)

Background
The first thing most people think of when they think of amphibians- members of the class amphibia- are the frogs and toads. The frogs and toads, however, make up only the largest of the three orders, Anura, meaning tail-less ones. The other two orders are Urodeles, meaning tailed ones and representing the salamanders, and Apoda, meaning legless ones and representing a worm-like creature that usually burrows. There are approximately 4,800 species of amphibian: 500 urodeles, 4200 anurans, and 150 apodans. [Early amphibians were the first animals to leave the sea and venture onto land, forming a crucial link from fish to terrestrial reptiles. RS]

Characteristics of amphibians
Amphibians are ectothermic (cold-blooded) animals which metamorphose from their juvenile water breathing form, to an adult air-breathing form, usually with four limbs. Most amphibians lay their eggs in water and are coldblooded (ectothermic)(MSR) Amphibians adapt to living on land in various ways. They have limbs with stronger bone support to allow them to regularly walk on land, and ears more specialized for hearing sound waves traveling through air (as opposed to water). Their eyelids are designed to retain moisture, and their skin is thin, smooth, and full of mucus glands for the same reason. (DRM)

Life Cycle
Amphibian means dual-life, which is represented by the common example of aquatic tadpoles going through a metamorphosis. The larvae of the other two orders, however, tend to resemble the adults more closely. In the case of salamanders, both stages are often carnivorous. Also, not all amphibians have portions of their life cycles both on land and in water. Some live only on land, and others live only in the water. The land-based amphibians tend to live in moister environments, partially because evaporation would cool them much more quickly in a dry environment. Young amphibians spend their first years in water. They use gills located on the side of their head to breathe. In their early years, they resemble a fish more so than their actual parents; they have no legs, and they swim in water. As amphibians develop, they lose their gills and acquire legs. However, some types of amphibians, keep their gills and remian in the water fro the rest of their life (they do develop legs too). (AK) The reproduction of amphibians usually involves the release of many eggs and sperm with a high mortality rate. However, there are a few species of frog that lay fewer eggs and carry the eggs on one parent's back. Some amphibians lay eggs, while others give live birth. Amphibians show the greatest variation in repr oduction than any other vertebrate group. Despite the gradient in birthing techniques, all amphibians must go through the metamorphosis of aquatic to terrestrial biomes (JL). Some amphibians exemplify **neoteny.** Neoteny is an evolutionary process in which adult organisms retain structures present in larva. Some species of amphibians retain their gills after adolescence. (APS)

Many amphibians release their eggs and sperm in standing water. The floating embryos (larvae) must obtain their own food and escape from predators; as a result, many do not survive into adulthood. The larvae that do survive develop their species' adult body plan and many leave the water. (SF)

Urodeles
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[Urodeles are the order of the amphibia that are characterized by a well-developed tail that persists throughout life, and usually two pairs of limbs.RS}[].The Urodeles are the salamanders. Salamanders have tails and walk with a side-to-side bending motion that may resemble that used by the earliest land-walking creatures. Urodeles are also the only vertebrates who can regenerate full limbs as adults. (Walker) A characteristic of the Urodeles is that they do not have scales or claws. They are most commonly found in North America in temperate climates. (JAS) Though urodeles reproduce in acquatic environments and spend their early childhood in the water, they go through a change to become land-breathing adults. Some salamanders do this with their lungs, however some do not have lungs and perform gas exchange through their skin. When salamanders do not go through this change and remain in the water throughout there whole lives it is called neoteny. (JAF)

Anura
The Anurans (frogs and toads) have legs but no tails in adult form. As they mature from tadpoles to frogs/toads, they lose their juvenile tail (AS). In larval form, aka the tadpole, many do have tails. However, these tails are resorbed when going through metamorphosis. Frogs are usually quiet, but in breeding season their calls can be heard both to defend territory and to attract mates. Migration to mating sites can involve vocal communication, navigation by stars, and chemical signaling. Anurans have several distinctions that separate them from the other amphibians. As mentioned earlier, one of these distinction is that adult frogs do not have tails. In addition, in frogs, both the radius and the ulna (the forearm bones) are fused together, as well as the tibia and fibula (shank bones). The ankle bones of frogs are also greatly elongated, allowing for an effective lever system that frogs can utilise in jumping. Anura also have external fertilization, and they have a special mating posture called **amplexus** to be sure that there is contact between the sperm and eggs. (SW) [This Bullfrog Tadpole is currently in its larvae form. It will lose its tail as it matures. (DG)]

Apoda
Apodans resemble earthworms, but are in fact amphibians. These worm-like creatures generally live in tropical areas. While most are burrowing, there are species in South America that live in freshwater lakes and streams. Apoda is the order within amphibia with the fewest component species. Typically they are less than 1 foot long. Some species lay eggs (mostly in water (AS)), and some of these species have an aquatic larval age. Other species do not lay eggs. (KTD) Because they are burrowers, apodans have a layer of skin protecting their eyes, which are sometimes deeply set into the skull. They can differentiate between light and dark and can identify prey, but other than that, their vision is not very good. Apodans can sense motion and chemical emissions using the antennae on their forehead (JM). The three main families include caecilidae (mainly terrestrial in adulthood and don't have tails), ichthyophiidae (have functional eyes and a small tail), and thpylonectidae (aquatic all their life and don't have a tail). (LD)

Reveiw Questions 2. In what ways have amphibians adapted so as to live both in the water and on land? [Ben Yudysky]
 * 1) Briefly describe how an adult amphibian differs from a tadpole.(CP)

"Regeneration In Urodeles." //Biology @ Davidson//. Web. 23 Oct. 2009. .(Walker) "Amphibian." Encyclopedia Britannica. 24 Oct. 2009. . (Sarah Fleming) [] (Ali Kirsch) [] (Sara Waugh) [][Rabya S.] [] (KTD) [|http://encyclopedia2.thefreedictionary.com/Urodele+amphibians](Jake Schwartz) [] (JAF) [|http://www.infovisual.info] (Leo Schwartz) [](MSR) http://upload.wikimedia.org/wikipedia/commons/6/68/Blue.poison.dart.frog.arp.jpg (KNS) http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookDiversity_9.html (Donna McDermott) http://www.britannica.com/EBchecked/topic/21445/amphibian (Jesse Landy) http://www.ucmp.berkeley.edu/vertebrates/tetrapods/frogskeleton.jpg (JJF) http://www.wisegeek.com/what-are-caecilians.htm (Jackson Murphy) [] (APS) [] (LD) [] (DG)
 * Sources:**