Biology Of Salamanders
Species of salamanders display a wide range of body plans and life histories. The smallest salamander is an unnamed species of Thorius from Mexico, mature males of which have a total body length of only 1 in (2.5 cm). The world's largest living salamanders can be as long as 5.5 ft (1.7 m). These are the giant Asiatic salamanders, Andrias davidianus and A. japonicus, which can achieve body weights of 88 lb (40 kg) or more. One individual giant Japanese salamander (A. japonicus) lived for an extraordinary 55 years in captivity.
Adult salamanders have four relatively small, similar-sized walking legs, and a long tail. The skeletal structure of living salamanders is relatively little modified from their geologically ancient relatives, and among the tetrapod vertebrates is considered to be relatively primitive.
Some species of salamanders have lost key elements of the skeleton during their evolution. For example, species within the salamander family Sirenidae have lost their limbs, and are eel-like in appearance. Remarkably, the numbers and shapes of limb bones are not necessarily the same within some salamander species, as is the case of the red-backed salamander (Plethodon cinereus) of North America. Within the same population, individuals of this species can have varying numbers of limb bones, and these structures can vary significantly in size, shape, and degree of calcification.
Salamanders have a protrusile tongue, used for feeding and sensory purposes. Many species are very brightly colored, usually to warn predators of the poisonous nature of the skin of these animals. Some salamanders secrete a chemical known as tetrodotoxin from their skin glands. This is one of the most poisonous substances known, and it can easily kill predators that are intolerant of the chemical, as most are.
Salamanders vary greatly in their reproductive biology. Salamanders typically have internal fertilization, meaning the ova are fertilized by male sperm within the reproductive tract of the female. During the breeding season, male salamanders of many species deposit packets of sperm, known as spermatophores, on the surface of aquatic sediment or debris. The male salamander then manipulates a female to pass over the spermatophores, which are picked up by the slightly prehensile lips of her cloaca, and stored in a special, internal structure known as a spermatheca. The sperm then fertilize the ova as they are laid by the female, producing fertile zygotes. These are then laid as single eggs encased in a protective jelly, or sometimes as a larger egg mass that can contain several or many eggs within a jelly matrix.
Hatched larvae of typical salamanders look rather similar to the adults, but they are fully aquatic animals, with gill slits and external gills, a large head, teeth, a flattened tail used for swimming, and initially they lack legs. The metamorphosis to the adult form involves the loss of the external gills, the growth of legs, and the development of internal lungs, which, together with the moist skin of the body, act in the exchange of respiratory gases. Adult salamanders also have eyelids that can close.
Salamanders in the family Plethodontidae show direct development. For example, the aquatic larval state of the fully terrestrial red-backed salamander occurs within the egg. What hatches from the egg is a miniature replica of the adult salamander. The red-backed salamander lacks lungs, so that all gas exchange occurs across the moist skin of the body and mouth.
The female of the European salamander, Salamandra atra, retains the eggs within her body. There they develop through the larval stage, so that the young are born as miniature adults.
Some salamanders do not have a terrestrial adult stage, and become sexually mature even though they still retain many characteristics of the larval stage. This phenomenon is known as neoteny, and occurs in species such as the mudpuppy (Necturus maculosus) of central and eastern North America. Neoteny also occurs in the axolotyl (Ambystoma mexicanum), a rare species found in Mexico, in which the breeding adults have external gills, a large head, a flattened tail, and other typically larval traits. The axolotyl is a common species in laboratories where developmental biology is studied, and sometimes individuals of this species will undergo metamorphosis and develop more typical, adult characteristics. Often, particular populations of other species in the genus Ambystoma will display neoteny, for example the tiger salamander (A. tigrinum), common in small lakes and ponds over much of North America.
The red-spotted newt (Notophthalmus viridescens) of North America has two distinct, adult stages. The stage that follows from transformation of the aquatic larva is known as the red eft. This is a bright-red colored, adult form that wanders widely for several years in forests, especially on moist nights. The red eft eventually returns to an aquatic habitat, adopts a yellowish color, and becomes a breeding adult.
Salamanders with a terrestrial adult stage generally have a keen ability to home back to the vicinity of their natal or home pond. One study done in California found that red-bellied newts (Taricha rivularis) were capable of returning to their native stream over a distance of 5 mi (8 km), within only one year.
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