Amphibian Biology & Physiology: Caudata

Overview of Physical Appearance

Amphiuma (Amphiumidae) and sirens (Sirenidae) are elongate, mostly aquatic animals, with reduced limbs and digits. The sirens lack hind legs altogether, and possess reduced forelimbs. However, at one point, the sirens did possess hind limbs, and a homologous feature shared with other salamander species. The amphiuma possess all four limbs, but all are also reduced. Sirens and amphiuma are capable of exiting the water and traveling short distances, and apparently do so regularly on wet nights.   

Neoteny, sometimes called paedomophism, is apparent in all caudate families except Rhyacotritonidae. There are three types of neoteny, obligate, inducible obligate, and facultative. All members of the families Amphiumidae, Sirenidae, Cryptobranchidae, and Proteidae are obligate neotenes, meaning they never fully metamorphose, and retain larval characteristics in varying degrees into adulthood. Most of these species are insensitive to thyroid hormone doses, which make inducible obligate neotenes metamorphose in the laboratory. There are also several other neotenous species, subspecies, populations, or individuals in the families Ambystomatidae, Salamandridae, and Plethodontidae. Inducible obligate neotenes of the family Ambystomatidae, and some species of the family Plethodontidae are unique in that metamorphosis can be induced by manipulating the thyroid function in the laboratory. The most famous inducible obligate neotene is perhaps Ambystoma mexicanum (Ambystomatidae), the Mexican Axolotl. Facultative neoteny is observed in Salamandridae, Dicamptodontidae, Ambystomatidae, Hynobiidae, and Plethodontidae. Facultative neoteny occurs in individuals or entire populations as a result of environmental cues. For example, in extremely cold temperatures, where a terrestrial existence would be inhospitable, individuals or populations may remain aquatic, retaining larval characteristics into adulthood, including limb and tail structure, and branchial respiration. In the case of facultative neoteny, genetics may play a role in the individuals predisposition and affinity for neoteny, however, the cause of neoteny is based on environmental conditions, such that a permanently aquatic lifestyle would benefit the individual.

Caudates are generally secretive animals, with increased activity during the breeding season. Some species remain terrestrial except during the breeding season, whereas others remain mostly aquatic year round. Some do not display the typically biphasic lifestyle, with eggs, larvae, and young morphs, but are instead ovoviviparous or viviparous.

Many caudate species, especially the newts, are toxic, usually possessing bright coloration to warn of their toxicity. Others are simply noxious, or secrete thick, sticky mucous when disturbed. Larger species, such as Amphiuma and giant salamanders, are capable of inflicting painful, and serious bites. For more information about amphibian defense mechanisms, see article 0011 - Toxicity and Defense Methods of Amphibians.

Bone cells form linked cells that connect them to the nurturing blood vessels. Strong connective fibers permeate bone matter in an alternating arrangement, which mineral deposits, such as calcium phosphate and calcium carbonate, are stored. This alternating sequence makes bone the hard substance present in the skeleton.

Amphibian bone is formed in two ways: 1) Formation directly in the connective tissues. 2) "Replacement Bone", which replaces an existing cartilage element that is completely deconstructed. The latter type is commonly called "Hardening". Several skeletal elements are prefabricated from cartilage, including the axial organs, vertebrae of the spine, the ribs, the inner or primary girdle elements, the scapula and coracoid in the pectoral girdle, some elements in the pelvic girdle, the pubic bone, the ischium, and the ilium. It should be noted that the fore mentioned names are derived from mammalian anatomy, for which the functions of the actual skeletal structures may differ in amphibians.

The skeletal limbs are also formed from cartilage, for which the basic pattern is the same for all tetrapods: One skeletal element is connected to each girdle. At the shoulder (pectoral) this is the humerus, and at the hip (pelvic) is the femur. One might recognize these mammalian names as the upper arm and leg bones. The humerus and femur are both contacted by two other skeletal elements. For the humerus, these elements are the radius (interior) and the ulna (exterior). For the femur, these elements are the tibia (interior), and the fibula (exterior). The forearm and lower leg are then connected to the carpal bone and tarsal bone, respectively, followed by the metacarpus and metatarsus, and finally the radial arrangement of finger and toe bones.

The skeletal and musculature structure of members of the order Caudata differs from other amphibian groups in several ways, including the following: 1) The presence of a tail and two pairs of limbs of roughly equal size (except Sirenidae, which lack hind limbs), 2) The presence of a large footplate and short stylus on the columella, 3) The absence of an otic notch and middle ear, 4) The absence of postorbital, postparietal, tabular, supratemporal, jugal, quadratojugal (except fossil Karauridae), supraoccipital, basioccipital, and ectopterygoid bones, 5) Presence of ribs, 6) External gills in larvae and neotenic specimens (Tree of Life Web Project).

Typical salamander skeletons consist of vertebral columns poorly differentiated into cervical, trunk, sacral, caudal sacral, and caudal regions, with Sirens lacking pelvic girdles. The cranium of a typical salamander is intermediate between the solid structure of caecilians, and the reduced bone structure of frogs (Wright and Whitaker, 2001). Bolitoglossids may possess modified hyoid to allow the projection of the tongue to capture prey. 

The bones of amphibians (and reptiles) can tell the age of the animal through a process called Skeletochronology. The bones of amphibians develop growth rings every year, which can be stained and counted to determine the number of years a specimen has existed. This process is analogous to counting the annual growth rings of a tree trunk to determine its age. During the colder seasons of the year, the rate of bone growth slows and possibly stops in poikilothermic species. The slow-developed bone is denser, and forms a dark ring in the amphibian bone when viewed in cross-section. Because these dark rings correspond to winter, an annual season, they also correspond to one-years difference in time when compared to each other. These dark rings are called "lines of arrested growth", or LAG's (Hofrichter, 2000). 

The bones develop outward from the medullary cavity (center). Studies of the results of skeletochronology have showed that on average, caudates live longer than anurans, amphibians reach sexual maturity at a later age in higher elevations, cold temperatures promote slowed bone growth, male specimens often reach sexual maturity one or two years earlier than females, and the life expectancy is lengthened in captivity.

Fortunately, skeletochronology can be performed on tiny bones, such as the toes and fingers, and so can be performed on living species. This is especially useful with caudates because they can easily regenerate toes, and even tails and limbs. The tails are also used, however, vertebrate of regenerated tails do not retain the annuli of the original bone.

Cardiovascular, Circulatory, and Respiratory Systems

The white blood cells are comprised of nucleated agranular leucocytes (lymphocytes and monocytes), and necleated granular leucocytes (basophils, neutrophils, and eosinophils). Some salamanders also possess enucleated white leucocytes. 

Thrombocytes (spindle cells) are typically nucleated, however enucleated thromboplastids occur in salamanders, which essentially function as platelets.      

Caudates have three-chambered hearts, with two atria and one ventricle, positioned beneath where the pectoral girdle and sternum meet. The septum is fenestrated, except in Sirenidae, and Cryptobrqnchus alleganiensis. Plethodontid salamanders have an atrial division consisting of a membranous septum and a sinoatrial valve, which prevents stagnant blood flow through the left atria. There is no division of the sinus venosus in caudates, but a sinoventrical fold is present. Blood flowing through the renal portal vein system passes through the kidneys before entering the postcaval vein, whereas blood flowing through the hepatic portal vein system passes through the liver before entering the vena cavity. Both the renal and hepatic vein systems are present in the caudal portion of caudates. The renal portal system receives a large portion of blood from the tail.

Lymph vessels collect blood from the erythrocytes that seeps through capillary walls, and returns it to the veins. Intestinal lymph vessels also collect fat. Subcutaneous lymph sinuses empty into postcardinal veins, or other vessels, and visceral lymph vessels empty into the subclavian veins. Lymph is pumped through the system by lymph hearts containing valves that restrict the flow to one direction.

Cutaneous respiration is the absorption of oxygen, and disposal of carbon dioxide, through the skin. All caudates utilize this mode of respiration to some degree, usually coupled with another main form of respiration. Adult , terrestrial salamanders of the family Plethodontidae, however, rely solely on cutaneous respiration, as they lack lungs and gills. Species that rely mainly on cutaneous respiration are typically long, cylindrical in shape, with thin epidermal layers laden with dense capillary beds that pickup O₂ and expel CO₂. The long, cylindrical shape creates a high surface area to volume ratio, which enhances the amount of oxygen diffused. To further promote cutaneous respiration, these salamanders also have slow metabolisms, costal grooves that increase the surface area, and the ability to withstand oxygen debt through anaerobic glycolysis (energy metabolism). All of these features make for the successful utilization of cutaneous respiration in amphibians. Species of the families Rhyacotritonidae, and some of the family Hynobiidae also rely mainly on cutaneous respiration, as their lungs are greatly reduced in comparison to other salamanders that use pulmonic respiration. Aquatic species of the family Cryptobranchidae (Giant Salamanders) also possess large folds of skin that increase the oxygen-absorbing surface area, thus increasing the oxygen intake.  

Many caudate species also respire through gas exchange in the buccal cavity and pharynx (buccopharyngeal respiration). This is accomplished by rapid throat pulsations (buccal pumping) that move atmospheric gases over the buccopharyngeal membrane, a semi-permeable membrane lining the mouth and pharynx. This is often seen in newts, especially when excited or stressed. Other species, such as the Plethodontids, also partake in buccopharyngeal respiration in addition to their main source of respiration, cutaneous. In a sense, buccopharyngeal respiration can be considered a form of cutaneous respiration. Buccal pumping is also used in pulmonic respiration to receive and expel gases. 

Branchial respiration refers to the absorption of oxygen, and expulsion of carbon dioxide through gills. The gills of neotenous and larvae are identical in function to the gills of fishes, but can be described as "inside-out fish gills". Most fish possess internal gills, as do larval anurans (tadpoles), with dense beds of capillaries tightly folded to increase the surface area. Caudate gills are comprised of long, bushy-like fimbriae, protruding outward from near the gill slits. Caudate gills are covered in oxygen-absorbing red blood cells. Neotenous species rely mainly on branchial respiration, but may utilize cutaneous and/or pulmonary respiration, as well. Axolotls, for example, rely mainly on branchial respiration, but also utilize cutaneous respiration, as well as pulmonary through the use of rudimentary or advanced lungs. Sirens also possess lungs in addition to gills.

The shape and structure of the gills of any given species is dependent on the surrounding environment. Larvae and certain neotenes residing in flowing waters (i.e. streams, rivers, brooks) possess shorter, somewhat reduced gills compared to those inhabiting stagnant or low oxygen content water bodies. Stream dwellers possess these shorter gills for two main reasons: 1) Moving waters typically contain more dissolved gases, so less surface area is required to absorb sufficient oxygen; the longer gills found on stagnant water species are not required to absorb sufficient oxygen, 2) Shorter gills help reduce the drag of the current in running waters, allowing for more accurate and faster maneuvering. In contrast to stream dwellers, the gills of pond dwellers are long, and often flicked around to create a small water flow. 

Many salamanders, such as some of the family Plethodontidae, do not have an aquatic larval stage, but do possess gills in the embryonic stage. There are three typical types of gills found in embryonic Plethodontidae: 1) The "staghorn" type consists of fused gill rami and long fimbriae, and resembles the structure of a stag's horn, 2) The second type are consist of long rami with absent fimbriae, 3) The third are the "leaf" type, which are flattened and vasularized, resembling the shape and composition leaves.

Terrestrial and semi-aquatic species that possess lungs typically utilize cutaneous and buccopharyngeal respiration, as well. The lungs of caudates vary greatly in size, degree of partition, and degree of reliance, and in general, the right side is slightly smaller than the left. The lungs of many aquatic species, such as Necturus spp., lack partitioning or infolding, while the lungs of most terrestrial caudates have saccular structures, some with alveoli. Each lung is generally divided into two compartments, one containing the pulmonary vein, and the other containing pulmonary artery. Newts and other pond dwelling species also tend to possess alveoli. The lungs of newts, and obligate neotenes appear to function more as hydrostatic organs. Cartilaginous rings are present in the trachea, and in some species, the bronchi. In most caudates, the trachea is relatively short, however, many aquatic species, such as Amphiuma spp., possess greatly elongated trachea resulting from the position of the lungs. Pulmonary respiration is usually supplemented by rapid throat pulsations (buccal pumping) that push air over the vascular lining of the buccopharyngeal (mouth and throat) region, and into the lungs, while others use their nares for ventilation. This is sometimes called "forced-air" breathing, and can be compared to diaphragm contractions as a means of ventilation in mammals.

Urogenital System

Auditory and Vocal Function

Larvae and aquatic individuals possess mechanoreceptive neuromasts, or the lateral line system. The later line system is also present in most fish, and consists of hair cells appearing as pit-like depressions in the skin, running from the head to the tail. In dark colored salamanders and newts, and some larvae, the lateral line system can be seen as light colored, perforated lines running dorsolaterally along the individual. The hair cells register water current and pressure changes, allowing the animal to detect even slight water movements at, or under, the surface. In most aquatic caudates, ampullary organs are also present in the lateral line system, which function as electroreceptors. 

All salamanders, with the exception of one species, are incapable of producing sounds due to the lack of vocal sacs, folds, and chords. Dicamptodon spp., the only exceptions, possesses vocal folds and have been noted as making squeaking or growling sounds when disturbed. However, it is thought that the sounds actually result from air being forced through gill slits or nares, which creates a vacuum during inspiration.

Form and Function of the Skin, Pigmentation, and Glands

The poison found in in the granular glands of newts (family Salamandridae) is especially toxic, and was given the name Tarichatoxin* after its isolation in western newts of the genus Taricha. Tarichatoxin* is biochemically identical  to Tetradotoxin (TTX), the most potent non-protein neurotoxin known to exist. Upon entering the blood stream, TTX blocks the sodium channels of excitable membranes, causing paralysis in the nerves and muscles (Fuhrman, 1986; Yasumoto et al., 1986) (see article 0011 - Toxicity and Defense Methods of Amphibians for more information on TTX). 

Caudate skin is also comprised of mucus glands. Mucous secretions keep the skin moist, inhibit the entry of bacteria and other pathogens, and reduces friction in the water by creating a slippery surface, which aids in quick escapes from predators. Terrestrial caudates, or those in a terrestrial phase, experience rapid water loss as a result of the secretory activity of these glands, but the skin is replenished by dermal absorption through the porous skin, and consumption of certain food items. 

Amphibians are found in a variety of colors, from drab browns and grays to shocking reds and blues. Color plays an important role in the survival of amphibians, and is often used either as a camouflage or a warning to potential predators as to the presence of toxins. Amphibians that are cryptically colored blend into their environment and often go unnoticed by potential predators. For example, Desmognathus marmoratus blend nicely into algae covered substrate, and aquatic Dicamptodon aterrimus are nearly the same color and pattern as their brown substrate. Other species, such as Salamandra salamandra, are aposematically colored, meaning they display "warning colors" that boldly advertise their toxicity to potential predators.  Aposematic colors, such as yellow, red, and orange, are generally known throughout nature to indicate toxicity to predatory animals. Some species even have a combination of cryptic and aposematic coloration. This is common in several newt species, which display a cryptically colored dorsal surface, but can easily make known their toxicity by flashing or showing the entirety of the brightly colored underside.

Some non-toxic or noxious caudates use coloration to mimic toxic species in their range. This is the case with Pseudotriton ruber, whose striking red coloration is thought to be a form of anti-predatory mimicry of highly toxic efts of the species Notophthalmus viridescens viridescens. Another example is the yellow eyed subspecies of Ensatina, Ensatina eschscholtzii xanthoptica, whose yellow eye patches and light colored ventral surface are thought to be mimicry of highly toxic Taricha torosa in the same range.

The coloration of amphibian skin is due to the presence, abundance, lack or aberration of particular chromatophores, or pigment cells. In vertebrates, chromatophores arise from the neural crest as chromatoblasts, and subsequently migrate to the skin early in embryogenesis. In the skin of poikilothermic vertebrates, the chromatoblasts differentiate into three types of mature pigment cells, melanophores, xanthophores, and iridophores. Contrast this to homeothermic vertebrates, which develop only one type of chromatophore, the melanocyte. 

Melanophores, found in both the dermis and epidermis, are dendritic cells that synthesize melanin, or black and brown pigments. Melanin is contained in melanosomes, intracellular organelles within the melanophore. Melanosynthesis, the synthesis of melanin, essentially consists of the conversion of the amino acid tyrosine into melanin, through a series of chemical reactions. Tyrosine is first converted to dihydroxyphenylalanine (dopa), and dopa to dopaquinone, in the presence of a catalyst, o-Diphenol oxidase (tyrosinase). Tyrosinase is a copper-protein enzyme, and the conversions of tyrosine and dopa cannot take place in the absence of this catalyst. Both the synthesis of melanin and tyrosinase occur in the melanosome. From the melanosomes, synthesized melanin is transferred to the dendrites, projections found on the cell wall, at which point dopaquinone continues to undergo further metabolic elaboration to become melanin (Bechtel, 1995).

Two types of color change occurs in amphibians: rapid, and morphological (Bechtel, 1995), and each is controlled by melanophores in different layers of the skin. Dermal melanophores are found in the upper dermis, while epidermal melanophores are found among the keratinocyte in the lower epidermal layer. The epidermal melanophore is equivalent to the melanocyte in homeotherms, and it is the epidermal melanophores that are responsible for color changes associated with UV exposure. In poikilotherms, the function of the dermal melanophores is rapid color change and pattern productions. Morphological color changes are slow, and generally consist of increased melanosynthesis, and accumulation of melanin in keratinocytes. Morphological color changes are usually in response to consistent stimulation, such as seasonal changes. Rapid color changes, as the name suggests, occur quickly in response to hormonal cues, usually initiated by the presence of competing rivals, potential predators, or the presence of the opposite sex. In such species, chromatophores are arranged into a unit called the "dermal chromatophore unit", which consists of, in descending order from just under the epidermis, a stack of xanthophores, iridophores, and melanophores possessing upward extending dendrites. During rapid color change, melanin is flooded into the dendrites, which extend into the xanthophore and iridophore layers, causing the darker color of the melanin to overcome the colors and effects produced from the xanthophores and iridophores. Rapid color change is not known to exist in caudates, but is common in many anuran species, as well as many well-known reptile species.

Xanthophores are the chromatophores responsible for yellow, red, and yellow-red intermediate coloration in poikilotherms. Xanthophores contain more than one pigment, including pteridines, and carotenoids. Pteridines produce red and yellow pigments, while carotenoids produce intermediates of these two, including orange, reddish-orange, yellowish-orange, etc. Pteridines are synthesized by the xanthophores, and are contained in organelles called pterinosomes (analogous to melanosomes). Pteridines result from the formation of folic acid from purines, with the participation of the enzyme xanthine dehydrogenase in some cases. Carotenoids, as the name implies, are also found in certain vegetable oils and animal fats. Unlike pteridines, carotenoids are not synthesized in the xanthophores, but originate from the diet and are simply stored in the xanthophores. Pteridines are the first pigments to develop in the xanthophores, while carotenoids accululate later in ontogeny.

In contrast to melanophores and xanthophores, iridophores do not synthesize pigments, but participate in the color of poikilotherms by reflecting and refracting colors. Iridophores contain organelles called reflecting platelets, which are composed of crystallized purine deposits, specifically guanine, hypoxanthine, and adenine, that produce a light-scattering effect. Depending on the stacking composition, orientation, size, and form of the cells, iridophore reflections can produced blue, green, red, and khaki hues. This type of chromatophore is easily recognizable  in amphibians by their often iridescent and metallic hues.

Amphibians, and reptiles for that matter, are known to display a wide variety of color defects and abnormalities, such as leucism, melanism, and xanthism, which result in some rather remarkable phenotypes. Such abnormalities are usually resultant from inherited deficiencies or defects in the chemical processes undertaken to produce the actual pigments. 

Albinism, caused from an inherited recessive gene, is the absence or reduction of melanin in the skin, eyes, and mucosa, resultant from a defect in melanosynthesis. The terms albino, albinotic, and albinistic are used to describe animals with little or no melanin. The amphibian and reptilian phenotype is generally whitish with red eyes, however, those that possess patterns will still display their patterns, but usually in whitish, yellow, or red colors. This is because albino amphibians still possess normal xanthophores and iridophores. Albino species are highly sensitive to UV radiation, usually accompanied by sensitivity in the eyes and other ocular defects. 

Albinism is a congenital inherited condition, however, mutations at different loci can result in different specific causes of albinism, all affecting melanosynthesis. Although every type of albinism has not been identified in amphibians, Bechtel, 1995, classifies albinism into three types: area of involvement, degree of involvement, and genetic defect. 

Area of Involvement: Albinism may be localized to particular parts of the system, while others are unaffected. Oculocutaneous albinism, for example, occurs when melanin is absent in the eyes, skin, and hair (in mammals), while ocular albinism occurs when melanin is absent only in the eyes; the former type has been observed in man. Albinism may also occur irregularly in the skin, and usually goes by the names piebald or vitiligo.

Degree of Involvement: Other mutations at the albino locus may result in albinos that contain varying levels of melanin. Such animals may not fit the classical description of an albino, i.e. white with red eyes; instead, this type of albino may have melanin in the eyes or skin. Occasionally those animals called hypomelanistic based on phenotype alone are actually a type of albino.

Genetic Defect: There are several requirements to facilitate melanosynthesis, including, but not limited to, the existence of a functional melanophore, differentiation of the chromatoblasts in order to form chromatophores, and the ability to synthesize tyrosinase; Dozens of genes at different loci control these steps. More generally, a melanophore must be able to produce melanin, and so must first be able to produce tyrosinase, a process controlled by a pair of genes. A mutated recessive gene at this albino locus causes albinism by preventing the synthesis of tyrosinase, if both genes are recessive. This type of albinism is called tyrosinase-negative albinism.

Tyrosinase-positive albinism also exists. Tyrosinase-positive species possess tyrosinase and tyrosine, the essential building blocks of melanin, however, conversion of tyrosine and dopa cannot occur because the tyrosine is not transmitted into the melanosomes, where tyrosinase and melanin are synthesized. Recall that the conversions of tyrosine and dopa in melanosynthesis require the catalyst tyrosinase, and melanosynthesis cannot occur in its absence. With tyrosinase-positive animals, there exists the possibility of melanosynthesis occurring, if at some point tyrosine is transmitted to the melanosomes.

Tyrosinase-positive and tyrosinase-negative animals have identical phenotypes, although the genotype is different. Because albinism is caused from different mutations, breeding two albinos does not always guarantee the production of albino offspring. Skin biopsies can be tested for the tyrosinase-positive mutation. When incubated in a dopa solution, melanophores become visible in tyrosinase-positive skin due to the deposition of of granular intracytoplasmic melanin (Bechtel, 1995). In the same test, tyrosinae-negative melanophores remain clear after the treatment.

Axolotls, Ambystoma mexicanum, are perhaps the famous albino amphibians in existence. Generations of selective breeding has resulted in large numbers of albino axolotls; in fact the albino type may exist today in higher abundance than the wild type. Albino axolotls are sometimes referred to as "golden albinos" because they retain xanthophores and iridophores, and are therefore pearly yellow in coloration. The albino gene was introduced into this species by hybridization with a tiger salamander, Ambyatom tigrinum (Bechtel, 1995). 

Other salamanders, particularly the troglobitic species, have lost most or all of their pigmentation evolution. These cave-dwelling species live in virtual darkness, and have lost the ecological need for pattern and color. Such species are not albinos because their lack of pigment is not due to an inherited recessive gene, but has occurred by means of natural selection and evolution. It's interesting to note that many troglobitic species also possess reduced or absent eyes for the same reasons; they serve little purpose when living in virtual darkness.

Axanthism is another pigment-affecting condition observed in amphibians. Axanthism is a hereditary defect similar to albinism, but effects xanthophore metabolism instead of melanosynthesis. Axanthic animals possess reduced or absent red, yellow, or intermediate pteridines, but normal carotenoids. This is because pteridines are synthesized within the xanthophore, while carotenoids are acquired from the diet, so a defect in xanthophore metabolism would only affect the synthesized pigments. As with albinism, axanthic individuals retain their patterns. Axanthism, at least in corn snakes, is caused by an autosomal recessive gene mutation. Axanthism has been documented in frogs, which take on a blue coloration. Axanthism is also observed in axolotls, which take on a light or charcoal gray in the absence of yellow pigments. Axanthic axolotls are similar in appearance to melanoids (lacking xanthophores and iridiphores), but retain iridophores. Axanthism is also observed in several frog species, which take on a blue hue.

Leucisim, also caused by an autosomal recessive gene mutation, is an inherited defect affecting all chromatophores. Leucistic animals lack functional melanophores and xanthophores, and possess minimal iridophores. Leucistic invididuals are solid white, with no pattern, except for the eyes, which are dark blue or black. Leucism is also common in selectively bred axolotls, and these animals are easily distinguished by albinos by their solid white skin and dark eyes. Dark pigmentation is sometimes found about the body of leucistic animals, but again, melanophores are non-functional in such individuals. Some may develop dark pigment in various amounts at the time of sexual maturity, a phenomenon that is still not fully understood. Leucism is different from xanthism and albinism in that it is thought to result from a defect in the skin itself, such that the skin cannot support pigment cells. Xanthic and albonic skin is capable of gaining pigment when chromatoblasts are injected, however, such processes have no effect on leucistic skin.          

Melanism, the phenotypical opposite of leucism, is caused by an autosomal recessive gene mutation that results in excessive amounts of melanin, in the absence of iridophores and xanthophores. Xanthophores are present in early development, but diminish with age, while iridophores are never present. The phenotype is dark gray or black, with no pearly sheen. Again, melanism is common in axolotls and is often selectively bred into the species.

Table 1.1: Axolotl phenotypes and anomalies.
Axolotl's (Ambystoma mexicanum) are infamous for their amazing phenotypes. Laboratories and breeders are able to determine with varying accuracy the phenotype that will be produced by breeding certain types that carry the characteristic desired. The phenotypes above are, from left to right: Leucistic, albino, melanoid, leucistic with black spotting, wild type, and an anomalous copper individual. Photos © Jessica Miller (1,2,3), Johnny Jensen (4), Anika Diez (5), and Henk Wallays (6).

The cells of the shed skin are used to distinguish diploid and polyploid individuals in some species, as the nuclei is much smaller in diploids. This is a useful laboratory technique, as it does not require living tissue samples, thus sparing the lives of the specimens under research.

Thermoregulation & Metabolism

Poikilotherms have slow metabolisms, as reflected in their inability to efficiently regulate body temperature internally. This enables them to go longer periods of time without food. Some poikilothermic snakes can go nearly a year without feeding. However, most amphibian species eat every day, or every few days, except during dormant periods, when they may go a few months without feeding.

The dependence upon their environment for body temperature control restricts amphibian species to specific habitats. Because amphibians have little control over their body temperature, it is imperative to their health to remain in environments within the proper temperature range. Within a habitat, there are many micro-habitats where temperatures may be drastically different from the ambient temperature. Amphibians use body positioning to utilize such micro-habitats, basically positioning their bodies on surfaces in manners to either expose more of the body to the surface, or less. In general, it takes larger amphibians longer time to equilibrate their body temperatures depending on their surface-mass ratio.

Although they cannot fully regulate body temperature internally, amphibians do contribute to the thermoregulation process by utilizing a few physiological functions. Some amphibians can increase of decrease the rate of evaporative water loss from the skin, which is an important temperature lowering technique. Most amphibians can also change the skin color, which can increase or decrease the amount of solar radiation absorption or reflection. This is commonly seen in captive treefrogs of the family Hylidae, who are often a different shade of green during different times of the day. 

Alimentary Function, Nutrition, and Diet

Caudates show a number of "hunting" methods. Some are simply opportunistic feeders, waiting for small prey items to come into reach, while others may actively search for food items. More correctly, most caudates use both methods, depending on the circumstances. Once a food item is located, a short chase may ensue, however, most species will choose not to expend the energy in an active chase. In captivity, when a terrestrial or aquatic salamander gets a whiff of food items, they usually emerge from hiding places and begin searching for the source of the smell. In the wild, terrestrial and aquatic salamanders may use the same cues to begin searching for food.

Caudates mainly use eye sight for hunting, with the exception of the blind salamanders, cave salamanders, and those with reduced eye sight. Caudates are also capable of hunting by scent, as is a common practice in most species. Aquatics and semi-aquatics possess a lateral line system that enables them to detect surface vibrations with exceptional accuracy. The response to potential food is very strong in healthy amphibians, which can been seen clearly in captivity. Many types of caudates become accustomed to regular feedings in captivity, and can be persuaded to accept frozen foods, as well as live foods. Some individuals become very excited during feeding time, often snapping at anything moving, including other caudates, fingers, and forceps. In poor light, such as in murky waters or dark caves, caudates may rely more on olfaction food detection (sense of smell). In captivity, some species switch to olfaction detection as their primary function, especially if trained to accept non-living foods. 

Caudates generally capture food by lunging at it, as is common in terrestrial and aquatic species. In aquatic individuals, including larvae and neotenes, the hyobranchial apparatus functions to support and move the gill filaments, as well as expand and contract the buccal cavity during feeding. In aquatic phase terrestrial adults, the hyobranchial apparatus controls the expansion and contraction of the buccal cavity, as well. These are accomplished by the hyoid musculature and the depressor mandible. Amphiuma tridactylum, larvae, and some neotenes have adapted a form of suction feeding, sometimes called gulping, where prey are forced into the mouth as the buccal cavity expands and pulls in water. This is done in conjunction with lightning-fast lunges, and usually only with small prey items. In many terrestrials, especially those possessing lungs, the protractible tongue is attached anteriorly. In such species, the hyobranchial apparatus performs the roles of buccal pump in respiration, and as the main mechanism in tongue protrusion.

Some caudates are capable of extending the tongue to trap and retrieve prey. These species possess are free all around and attached to a pediceled base, forming a mushroom-like appearance, as shown in the illustration at right. In such species, the tongue skeleton is also projected forward. On average, the tongue can be extended to "hit" prey at a distance 50%-80% the length of the body. The entire process of opening the mouth, extending the tongue, and retrieving prey occurs in less than a tenth of a second. Other species may possess tongues that are attached to the floor of the mouth. 

Most caudates possess rows of teeth on the upper and lower jaws, which are shed periodically throughout their lives, as well as on the roof of the mouth. Bicuspid teeth are found in adults, while larvae, some male Plethodontids, and some netotenes typically possess monocuspic teeth. Caudate teeth are typically used for gripping prey, preventing it from escaping, and even shredding in some species. Some larger species, such as Amphiumids and Cryptobranchids, possess powerful jaws and strong teeth, capable of inflicting injury to humans.

Digestive System

Amphibians possess a pancreas, and connected liver and gall bladder. Like mammals, the liver functions as the central metabolic organ that regulates blood sugar, and is thus a main source of energy. The liver of amphibians also produces the final metabolic products and transports them through the vascular system to the kidneys, and finally to excretion. The liver in most amphibians is large, and bi-lobial. The size of the liver is determined by its vital function as a glycogen and fat storage unit. The size of the liver may change proportionally with the seasons, when activity is increased or decreased. In aquatic amphibians, the liver plays only a small role in processing Nitrogen for excretion, and ammonia is diffused mainly through the skin and excretion. The liver of terrestrial amphibians converts ammonia to urea, a less toxic, water soluble nitrogenous compound, as a means of water conservation. In some species, urea is further converted into uric acid. The liver secretions from the liver collect in the gall bladder, and flow into the small intestine. Salamanders lack a valve separating the small intestine from the large intestine. In the small intestine, enzymatic digestion and carbohydrate, fat, and protein absorption occur. Salt and water absorption occur in the large intestine, as well as mucous secretion to aid in the transport of fecal boluses (digested food). Fecal matter is excreted through the cloacal opening.  

In amphibians, the kidneys are located dorsally, near the roof of the body cavity, immediately below the dorsal musculature. The kidneys are relatively large, and paired. Like all vertebrates, the glomerular filtrate empties first into the renal corpuscle (Malpighian body). A small artery leads into this body, leading into a number of capillary loops, the glomerules, which are covered in many-branched cells, the podocytes. 

Mating Seasons & Courtship Behavior
For those species that lead biphasic life cycles, the mating season usually corresponds to the rainy season, or shortly thereafter. Many newts (Salamandridae) are Spring breeders, often emerging from dormancy to reproduce in any suitable water source when the temperatures begin to rise. Other species may choose to emerge from Summer hiding places, often from aestivation, to reproduce in the heaviest of rains. 

Some caudates, especially the semi-aquatic newts, may remain aquatic year round, reproducing in the permanent water sources they live in. Others may make the same migratory trek from upland hiding places to breeding sites every year, and subsequently return to their terrestrial habitat for the remainder of the year. 

Oviparous, Ovoviviparous, or Viviparous
Many caudates are oviparous, meaning they produce fertilized eggs that are nourished by a yolk sac, and that hatch outside the mothers body. Oviparity is observed in those internal fertilizers that produce eggs in the water, and those that produce eggs on land. Many terrestrial Plethodontids produce fertilized eggs on land, attaching them to the roofs of small caves, or in burrows, where they are sometimes guarded by the female. These types of eggs may be in strings, connected with constricted jelly, as is apparent in some Bolitoglossids, or are adherent to each other, as is the case in some terrestrial Plethodontids and Ambystomids. Although the development after egg deposition varies widely within many oviparous species, all caudates that produce externally developing eggs are oviparous.

Ovoviviparous species produce eggs that develop internally, i.e. inside the mothers oviduct. The internal egg casing is reduced to a thin membrane upon delivery, and is usually broken through by the emerging larvae, making the birth appear live. Ovoviviparous adults, such as Salamandra salamandra, typically deliver their offspring directly into a water source. 

Other caudate species are  viviparous. Viviparity occurs when the development of internal eggs is prolonged even further than with ovoviviparity, causing the larvae to emerge from their casings internally, and continue development to metamorphosis within the mothers oviduct. Viviparous young are able to exchange gases, waste products, and nutrients from the mothers blood, whereas ovoviviparous species are capable only of limited gas exchange across the egg membranes. A placental-like infolding of the oviduct and egg membranes may also develop in advanced viviparity. 

Viviparous amphibians, such as Salamandra atra, usually only produce one or two offspring out of a clutch of 20-30, which are delivered as fully morphed, miniature adults. The remaining, unfertilized eggs provide nourishment to the developing larvae when their yolk sacs have been exhausted. The larvae of Salamandra atra obtain further nourishment by scraping the mothers reproductive tract with specialized teeth, which provides them with enough nourishment to last through the 2-4 year gestation period.  Similar behavior is also observed in some populations of Salamandra salamandra, but with a slight twist; after all the unfertilized eggs have been consumed, some developing larvae may cannibalize other developing larvae within the mothers oviduct. All larval development occurs within the mother, making these species true terrestrials. 

Viviparity is also observed in the genus Mertensiella. M. luschani antalyana, in particular, produce young in a similar fashion to Salamandra atra, however, the gestation period is usually only one year. On the other hand M. luschani's close relative, M. caucasica, produce eggs on land after an aquatic courtship.

Some would argue the validity and definitions of ovoviviparous and viviparous when applied to caudates, and often times only the terms oviparous and viviparous are used to differentiate between egg laying and "live-bearing". The existence of some ambiguity between viviparous and ovoviviparous amphibians is generally accepted in practice today.  

Ovoviviparity and viviparity, or whatever terminology is peferred, are thought to be adaptations to the extreme seasons encountered in mountainous regions. At high altitudes, suitable development periods for larval amphibians may simply be too short for survival. This problem is resolved with internal development by eliminating or reducing the time larvae develop in the external climate. The same can apply to those species found in very dry areas, where rainfall may be severely limited. Another factor to consider is the abundance of aquatic food supply for larvae, which may be inadequate in some areas, and the pressures of predation from other animals. In general, ovoviviparity and viviparity are associated with the climatic and geological surroundings, which may inhibit larval development for multiple reasons. However, there are some holes in this theory, as some species seemingly do not encounter such environments, and so the advantages of ovoviviparity or viviparity for such species are unknown. Also, the mother puts herself at considerable risk of predation by carrying around such a heavy load, and considerably less offspring are produced compared to the hundreds of eggs produced by oviparous species every season. In summary, the advantage of, and reason for viviparity and ovoviviparity are unknown for every species, but it is assumed that the advantages of such development practices outweigh the disadvantages in all cases.

Embryonic Development & Biphasic Life Cycles
The majority of caudates are internal fertilizers, with the exception of the families Hynobiidae and Cryptobranchidae, which are external fertilizers. In many cases, fertilized eggs are deposited in water sources, including streams, ponds, lakes, reservoirs, vernal pools, and sometimes ditches, potholes, or other temporary water source, over a period of several weeks or months, depending on the breeding season of the species. Some species attach eggs individually onto plant leaves, often folding the leaf over for protection, while others may produce clumps of 10-30 eggs and simply place them on the substrate. Stream-dwellers typically deposit eggs on the undersides of rocks and wood where they will not be swept away with the moving water. Female salamanders of the family Hynobiidae produce gelatinous, crescent-shaped egg sacs containing several eggs, that are externally fertilized by males. Cryptobanchids are also external fertilizers, but produce strings of eggs. 

Unlike amphibians, reptiles, birds, and mammals are amniotes, which means their embryos are protected by an embryonic membrane called amnion. Amnion is developed early in the embryo, and serves as a protective layer of fluid, enclosing the embryo in the embryonic cavity. Amniotes essentially develop within an "internal pond" of amnion, and do not require an external water source. Amphibians, on the other hand, lack amnion and are called anamniotes. Their eggs are "naked", only protected by semi-permeable, gelatinous layers, and so rely on the water from external water sources. This is why biphasic type eggs are deposited into water sources, where they develop into aquatic larvae, and eventually metamorphose into terrestrial juveniles (in most cases).

The ovum is enclosed in a thin membrane, called the vitelline membrane (sometimes called the fertilization membrane or chorion). This semi-permeable membrane is produced by the ovary, and surrounded by up to 8 concentric egg capsules secreted by the oviducts. The capsules are comprised of acidic and neutral mucopolysaccharides, sometimes sulfated mucopolysaccharides, and mucoproteins. These mucoid capsules protect the developing embryo from injury, fungal infections, and ingestion by some fishes, as well as provide support by adhering to surfaces. The eggs of Taricha species, and probably other toxic species, possess Tarichatoxin, as do the adults, which reduces predation significantly. It is has been suggested that the capsules may also act as lenses, focusing sunlight on the eggs to increase temperature. In salamanders, the innermost capsule liquefies quickly after deposition, enabling the ovum to float freely in the surrounding chamber, as well as rotate immediately after deposition. In many frogs, the rotation of the ovum takes several minutes because it is subject to the viscosity of the innermost capsule. Expansion of the chamber is immediate in most species, however, some may only reach full expansion late in development. The capsules are a mix of soft and hard, with the hard capsules positioned between the soft capsules. The number, shape and size of the capsules, and the size of the ova vary interspecifically, and homologous formations have shed some light on the relationships of some species. The largest ova of Cryptobranchus alleganiensis are the largest at 6mm, with 18mm capsules. This is unusual because salamanders exhibiting direct development, such as Aneides lugubris and Batrachoseps wrighti, typically possess larger eggs. Newts produce the smallest eggs, and exhibit biphasic lifestyles.

In general, eggs deposited in sunlight typically possess melanin, a skin pigment that resists the effects of UV lighting, whereas those placed in the shade lack melanin. There are some exceptions, such as Euproctus asper populations at high elevations, whose eggs may also contain melanin, although they are typically placed in dark caves. The melanin found in eggs is thought to serve the same purpose as in adults; protection from UV exposure and heat retention.

Within a few hours after deposition, the egg capsules swell up with water. The pH, or ionic concentration of the water may affect the size of the eggs in some species such that higher pH relates to larger eggs. Oxygen is a crucial factor in egg development, and the consumption of oxygen increases with development. Colder water possess more dissolved oxygen than warmer water, which can play a role in egg development in captive and wild clutches. Aquatic, globular type egg masses can obtain sufficient oxygen in cooler waters because the higher oxygen level is able to penetrate to the innermost eggs in the mass. Single type eggs may fair better in warmer waters than globular egg masses because their semi-permeable outer membrane allows more water, and therefore more oxygen in. Terrestrial eggs are susceptible to desiccation, as they take up moisture from the moist surroundings are readily release moisture into the air. Phyllomedusine treefrogs have developed a unique method of providing additional moisture to their eggs; some species will produce eggs lacking embryos, but containing metabolic water, which are placed around the embryonated eggs to provide water when necessary.

Although smaller in surface area than those of hatched larvae, some embryonic amphibians possess gills to aid in respiration. Plethodontids that display direct development, may have one of three general types of gills in the embryonic stage; staghorn, elongate, and leaf type. The staghorn type of gills consist of fused gill rami with moderately long fimbriae. The elongate type lack fimbriae on the long rami. The leaf type gills are flattened, giving the appearance of a vascularized leaf. The embryonic gills of viviparous Salamandra salamandra possess long fimbriae, and adaptation thought to enhance oxygen absorption in the oviducts.

During development in biphasic species, embryos obtain nutrition from yolk sacs. Embryos of some viviparous species, such as some subspecies of Salamandra salamandra, obtain nutrients from the epithelial walls of the mother's oviduct. In Salamandra atra, only a few eggs out of the 20-30 produced are actually fertilized, and the remainder degenerate into yolk, which provides nutrients to the developing embryos after their yolk sacs have been depleted. The major energy source for developing embryos comes from lipids. Salamanders typically produces larger hatchlings that have undergone a longer development than most anurans, which results in larger yolks with more lipids. Embryos require a certain amount of calories in order to hatch; the exact amount varies with different species. 

Developing embryos expel nitrogenous waste in the form of ammonia. Aquatic eggs produce small amounts of urea, as the toxic ammonia is diluted through diffusion of water into the perivitelline fluid. In Ambystoma gracile, a green algae lives symbiotically in the perivitelline fluid of the eggs. The algae remove ammonia from the eggs, and store the excess nitrogen as proteinaceous bodies. The presence of the algae effects development and survival rates in this species, which decrease without the algae.

Garcia, Fuhrman, F.A. 1986. "Tetradotoxin, tarichatoxin, and chiriquitoxin: Historical perspectives." In C.Y. Kao and S.R. Levinson, eds., Tetradotoxin, Saxitoxin, and the Molecular Biology of the Sodium Channel. N.Y. Academy of Science 479: 1-14.