Caudata

(Salamanders and newts)

Class Amphibia

Order Caudata

Number of families 10

Number of genera, species 61 genera; 502 species

Evolution and systematics

The order Caudata includes species that are generally called salamanders. Newts are terrestrial forms of some members of a single family, Salamandridae. Salamander classification is stable at the family level, but the relationship of the families is controversial. The 10 families typically are placed in three suborders. However, because the phylogenetic relationships of the species are uncertain, the suborders are seldom recognized or used by professional herpetologists. The suborder Sirenoidea includes only the Sirenidae, a small family restricted to eastern North America and generally considered the most basal lineage. The suborder Cryptobranchoidea and suborder Salamandroidea are sister taxa. Cryptobranchidae includes the small family Cryptobranchidae, of eastern Asia and eastern North America, and the large family Hynobiidae, which is restricted to Asia with the exception of one species that enters Europe in northern Russia. Salamandroidea includes most families and most species. These families are Ambystomatidae, Amphiumidae, Dicamptodontidae, Rhyacotritonidae, Plethodontidae, Proteidae, and Salamandridae. Dicamptodontidae and Ambystomatidae are restricted to North America and are thought to be sister taxa. Plethodontidae, by far the largest family, occurs in North, Central, and South America and has a few species in Mediterranean Europe. Plethodontidae has no close relatives but may be the sister taxon of the small North American family Amphiumidae. Some herpetologists consider the Rhyacotritonidae (a small family restricted to northwestern North America), Plethodontidae, and Amphiumidae relatively basal within the Salamandroidea. The small, gilled, permanently aquatic Proteidae occurs in North America and southern Europe. Its relationships are obscure. The Salamandridae, which is widespread in the Old World as well as in North America, may be the sister taxon to the Dicamptodontidae-Ambystomatidae.

Salamanders have evolved in fits and starts, and there has been a great deal of parallel and convergent evolution and even reversals of characters to more ancestral states. This indirect evolution has made determination of relationships uncertain, and many systematists have turned to biochemical characters. However, some features of the superorders and families are used to sort species. Sirenidae has many bizarre features, among them lack of teeth on the main jaw bones (premaxilla and dentary; the maxilla often is absent and when present is not articulated). Sirenids are permanently aquatic, have gills, and lack hind limbs; they are thought to practice external fertilization. The jaws are covered with a keratinized, beaklike structure. Cryptobranchoidea has a bone in the lower jaw, the angular, that was present in ancestral, Paleozoic forms and is lacking in all other salamanders. Cryptobranchids are the only salamanders that practice external fertilization, another ancestral trait. All Salamandroidea practice internal fertilization; the male deposits a spermatophore, which is picked

up by the vent margins of the female, and the sperm enter the reproductive tract.

Salamanders are thought more closely related to living anurans than to the gymnophionans (caecilians), mainly because of biochemical traits. The three lissamphibian groups are ancient, and there is little similarity among them. Although fossil sister taxa are known for each order, none of these fossils is of much use in assessment of relationships of the lissamphibians to Paleozoic amphibians. A number of fossil lineages are known from the Mesozoic and Cenozoic eras that fit well within the Caudata, but they do not help resolve phylogenetic relationships. A Jurassic taxon, Karaurus, usually is considered the sister taxon to caudates. The Caudata are monophyletic with respect to Karaurus, so discovery of this taxon was an important event.

Physical characteristics

Salamanders display great diversity in body form, but most of the species are small, generalized vertebrates that have a head about as wide as the body, a trunk that contains 12–20 vertebrae, four limbs that vary in length from long (overlapping when appressed to the side of the body) to very short, and a tail that usually is about the same length as the head and body combined. These generalized salamanders typically range in size from approximately 1.5 to 8 in (4–20 cm). However, some salamanders have a highly aberrant form. The permanently aquatic giant salamanders of eastern Asia may exceed 59 in (150 cm) in length and have broad, flattened bodies and heads and a strongly keeled tail. Sirens and amphiumas have very elongated bodies with many trunk

vertebrae, short tails, and diminutive limbs. Sirens lack hind limbs and have gills throughout life, whereas amphiumas have an open spiracle. Proteids are permanently aquatic, gilled forms, but they have relatively well-developed limbs that have lost some digits. The more bizarre salamanders often are known by colorful common names: hellbenders (North American cryptobranchids), mud sirens (North American sirenids), Congo eels (North American amphiumids), and olms (European proteids). Newts in the terrestrial stages are known as efts.

Herpetologists have become aware of the existence of large numbers of miniaturized salamanders, mainly within the family Plethodontidae. Most of these miniaturized species are terrestrial throughout their lives, and many are secretive. New species continue to be discovered at a rather high rate. Many of these species are less than 1.2–1.4 in (3–3.5 cm) in total length.

The most phylogenetically basal taxa have aquatic larvae with external gills and limbs that metamorphose after a growing season to produce a semiterrestrial to fully terrestrial adult. Approximately one half of the species of salamanders have abandoned the aquatic larval stage and lay eggs on land that develop directly into terrestrial juveniles. A few species are truly viviparous.

Salamanders include many species that are prototypical generalized tetrapods. The head is no broader than the trunk and usually is relatively small. The trunk is of moderate length and has 12–18 trunk vertebrae. The tail is approximately the same length as the head plus the body. The four limbs are of moderate length and just overlap when appressed to the trunk. These animals have large eyes used mainly in prey detection and capture and in predator detection. They have excellent olfactory capabilities. There is no external ear and although salamanders can hear, sound appears to play little role in their lives. Many people confuse salamanders and lizards. In the southern parts of the United States, salamanders often are called "spring lizards." Like frogs and caecilians, salamanders have moist skin that is well supplied with glands, both socalled poison or granular glands and mucous glands. Unlike that of frogs, the skin of salamanders is thick and tightly attached to underlying bone and muscle. Generalized salamanders may or may not have lungs, but in either case, most gaseous exchange takes place across the moist, vascularized skin. Salamanders that have this generalized shape can be fully aquatic, but they are more typically semiterrestrial or fully terrestrial. The terrestrial species often spend extensive periods underground, especially during nonbreeding seasons and during cold winters or dry summers.

Most salamanders fit the foregoing general description, but several are truly bizarre in appearance. The fully aquatic sirens lack hind limbs, have large, feathery external gills, and are extremely elongate. The mouth is a cornified beak that lacks teeth on the jaws, except for a tiny patch on the inside of the lower jaw that faces inward. Amphiumas, also aquatic but somewhat more terrestrial than sirens, are very elongate but generally slender. They have ridiculously small limbs with four, three, or even two toes depending on the species. Adult amphiumas lack gills but have an open spiracle, and they have formidable teeth that can produce a painful bite. Some salamanders in several families are permanently larval in morphology, but their gonads mature. Some of these animals (e.g., Proteus of south central Europe) are known to inhabit only underground water courses.

Unlike aquatic specialists, terrestrial salamanders can assume bizarre morphologies, such as becoming extremely slender and elongate with tails more than twice body length (e.g., the plethodontid genera Batrachoseps, Lineatriton, and Oedipina). Others have evolved fully webbed hands and feet, such as some species of Bolitoglossa and Chiropterotriton).

Biological attributes

Few predators specialize on salamanders, but a few snakes are known to prefer salamanders. Many salamanders are long-lived, and individuals dated by bone rings are thought to be between 20 and 30 years of age. Salamander skin is richly supplied with glands that produce bad-tasting, sticky, and sometimes dangerously poisonous secretions. Newts, especially those in the salamandrid genus Taricha in western North America, use a poison known as tarichatoxin, which is identical to tetrodotoxin (produced by puffer fish). This substance is one of the most potent natural toxins. One species of garter snake has evolved defenses and is capable of eating these newts, even though the poison has strong effects on the predator. Most salamanders have color patterns thought cryptic, but some salamanders have vivid colors. All these salamanders are poisonous or are Müllerian mimics of other poisonous salamanders. When bothered by a predator, a newt displays the unken reflex, depressing its trunk and raising its head and tail to expose bright ventral coloration. The newt often rocks back and forth and exudes a strong-smelling skin secretion that contains toxin.

Salamanders are generally thought to have a life cycle involving an aquatic larval and a terrestrial adult stage, but there are many variations. At least some species in all of the families except the Hynobiidae and the Rhyacotritonidae are permanently aquatic. Some species in the exceptional families are nearly completely aquatic. In contrast, true terrestriality has evolved in the Plethodontidae and to a limited extent in the Salamandridae. The most terrestrial plethodontids are found in diverse regions ranging from boreal forests to tropical rainforests. They also are found in mesic microhabitats in dry regions. The terrestrial species range

from those that are largely fossorial to cave dwellers and fully arboreal species.

Distribution

Salamanders are classic examples of organisms with a mainly Holarctic distribution. Only one family (Plethodontidae) has a significant presence in the tropics, and that exclusively in the New World, reaching nearly 20 degrees south. Few species reach high latitudes, and only one (Salamandrella keyserlingii) extends north of the Arctic Circle. The southernmost salamanders are unnamed species of the genus Bolitoglossa of central Bolivia. Many salamanders are montane, and some reach high elevations. The species that apparently reaches the highest elevation is the plethodontid Pseudoeurycea gadovii, which lives higher than 16,400 ft (5,000 m) on Pico de Orizaba in eastern Mexico. Several tropical species occur above 13,100 ft (4,000 m) in Mexico, Guatemala, and Colombia. The highest extratropical species is Hydromantes platycephalus, which reaches 10,500 ft (3,200m) in California.

Because nine of the 10 families occur almost exclusively in the North Temperate Zone (a salamandrid and a few ambystomatids enter tropical Mexico; a few salamandrids barely enter tropical Asia), the greatest lineage diversity is in northern regions, especially in North America and eastern Asia. However, Plethodontidae is abundant in both temperate and tropical zones. The plethodontids of the New World tropics are numerous (207 species) and diverse, and occur from sea level to above 16,400 ft (5,000 m). In contrast, far more anurans inhabit the tropical parts of the world than the northern temperate climates. Only 90 species live in North America and 116 species in temperate Eurasia. The greatest diversity of anurans is in the neotropical region (Central America, South America, and the West Indies), which is home to approximately 2,200 species. This number is approximately three times that in tropical Asia or tropical Africa and five times that in the Australo-Papuan region.

Feeding ecology and diet

Larval salamanders and many adults that are permanently aquatic feed by using suction. The buccal cavity of the salamander is expanded by the action of the gill bar system, and the mouth is suddenly opened, drawing in water and prey. The prey are grasped with the jaws and swallowed.

Adult salamanders feed primarily on small arthropods, although ambystomatids and some salamandrids are worm specialists. Prey are located with vision, but secondarily by olfaction. Prey typically are captured with a highly mobile tongue that varies greatly in structure from one species to the next, even within the same family. The tongue has a skeleton derived from the larval gill skeleton. As many as 11 skeletal elements can be found in a salamander tongue. The tongue is propelled from the mouth by specialized protractor muscles and brought back to the mouth by extremely long retractor muscles that may arise on the back of the pelvis. The longest tongues are found in bolitoglossine plethodontids, but other families have also evolved long and fast tongues. A sit-and-wait foraging strategy typically is used. Plethodontids in particular may remain motionless until the tongue explodes out of the mouth and the prey disappears. The tongue action is too fast to be resolved by human eyes.

Reproductive biology

Many salamanders court, mate, and deposit eggs in water, but most never go into water, choosing to court, mate, and deposit eggs on land. The ancestral life cycle involves the first strategy, which includes external fertilization and a larval stage that lasts for one season. A derived state is internal fertilization, which results from the deposition of a spermatophore by the male that is picked up by the cloacal walls of the female. A more derived state is deposition of eggs on land, hatchlings being either larvae that wriggle to nearby water or are overcome by rising spring waters. A still more derived state is complete development within the egg of a hatchling that is a miniature of the adult. Some members of the family Salamandridae give birth to larvae, which may be very immature or be large and near metamorphosis. A few salamandrids retain the larvae in the oviduct for one or two years or perhaps even longer. Very large juveniles are produced that have been nourished within the female reproductive tract, first on siblings and later on secretions of the female. Larvae usually can be classified as pond or stream type. The former usually metamorphose in one season and have large, feathery gills. Stream-type larvae have a much smaller tail fin that does not extend onto the body as in pond larvae; short, sometimes inconspicuous, gills; and a depressed body with stout limbs and cornified digital tips. These larvae may persist for several seasons. All salamander larvae are carnivorous, generally eating aquatic insects but sometimes small fish and even smaller salamanders.

Resources

Books

Deban, S. M., and D. B. Wake. "Aquatic Feeding in Salamanders." In Feeding, edited by K. Schwenk. San Diego: Academic Press, 2000.

Duellman, William E., ed. Patterns of Distribution of Amphibians. Baltimore: Johns Hopkins University Press, 1999.

Duellman, William E., and Linda Trueb. Biology of Amphibians. Baltimore: Johns Hopkins University Press, 1994.

Petranka, J. W. Salamanders of the United States and Canada. Washington: Smithsonian Institution Press, 1998.

Thorn, R., and J. Raffaelli. Les Salamandres de l'Ancien Monde. Paris: Societe Nouvelle des Editions Boubee, 2001.

Wake, D. B., and S. M. Deban. "Terrestrial Feeding in Salamanders." In Feeding, edited by K. Schwenk. San Diego: Academic Press, 2000.

Zug, G. R., L. J. Vitt, and J. P. Caldwell. Herpetology. 2nd edition. San Diego: Academic Press, 2001.

Periodicals

Crawford, A. J., and D. B. Wake. "Phylogenetic and Evolutionary Perspectives on an Enigmatic Organ: The Balancer of Larval Caudate Amphibians." Zoology 101 (1998): 107–123.

David B. Wake, PhD