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Salamander

Caudata

Salamanders are lizard-like amphibians with slender bodies covered by smooth, moist skin, blunt snouts, short limbs, and a tail in both adult and larval stages. They belong to the order Urodela, under the group Caudata.  An exception to the salamander group, the newts (of the family Salamandridae) lack the costal grooves found on the sides of their bodies. 

They are usually aquatic throughout their lives, though some live in water intermittently, while others are completely terrestrial as adults. Their name is obtained from the old French word salamandre, which refers to fire salamander, a common species. 

The remarkable ability of salamanders to regenerate not only their limbs but even some vital organs like the heart, jaw, and spinal cord parts made them one of the ideal model organisms for scientific research.

Description

Size

Though most salamanders range between 10 and 20 cm (4 and 8 inches) in length, the world’s largest salamander, the Chinese giant salamander (Andrias davidianus), reaches 1.8 m (6 ft) and weighs up to 65 kg (145 lbs). In contrast, the pygmy salamander (Desmognathus wrighti), a small species, measures 1.5 to 2.0 inches.

Body Plan

An adult salamander shares the basic tetrapod morphology, having a slender, cylindrical trunk, four limbs, and a long tail.

Mouth

Most salamanders possess small teeth in the upper and lower jaws in both larval and adult stages. The teeth are characterized by a high crown with two cusps (bicuspid) attached to the pedicel by collagenous fibers. Sometimes, they are also found in patches attached to the vomer and palatine bones of the skull and are constantly replaced at intervals throughout a salamander’s life.

Skin

They have thin, slippery skin with highly cornified outer layers equipped with glands. However, newts have velvety or warty skin that is wet to the touch. At the same time, other members of the same family have several vertical depressions running from the mid-dorsal region to the ventral area, known as costal grooves.

The skin is often brightly colored with patterns, like stripes, spots, blotches, and dots. However, cave-dwelling salamanders lack pigmentation and possess a translucent pink color. Male salamanders become more vibrantly colored during the mating season to attract females.

Limbs

Their front and hind limbs are almost the same length, projecting sideways from the body. The digits are short, with four on the front feet and five on the rear. The shape of the foot is modified based on the salamanders’ habitat. For example, climbers like the tree-climbing salamander have plate-like, square-tipped toes, while the toes of rock-climbing Hydromantes species are short and fleshy.

Some aquatic dwellers, such as sirens and amphiumas, either have reduced limbs or completely lack them, giving them an eel-like appearance.

Tail

Aquatic salamanders and those in their larval stage possess laterally flattened tails with dorsal and ventral fins, which help them propel underwater.

Males of the families Ambystomatidae and Salamandridae have longer tails than their female counterparts, while it is prehensile in the arboreal salamander and other tree-climbing species that help in grasping or clinging to objects they hang around.

The tail is also used as an organ for storing lipids and proteins and in courtship and defense.

Sensory Organs

Olfactory

Salamanders have two types of sensory epithelia, which are used to sense the surrounding environment.

  1. The olfactory epithelium in the nasal cavity helps detect airborne and aquatic odors.
  2. The vomeronasal organ’s epithelium extends to the nasolabial grooves and detects nonvolatile chemical cues, such as taste. This extended epithelium helps them to identify conspecifics.

Visual

In aquatic species, the eyes are reduced to a simplified retinal structure, whereas in cave-dwellers, like the Georgia blind salamander, they are either absent or covered with a thin film of skin. Amphibious salamanders can adjust their lens to become nearsighted in air and farsighted in water. Like the fire salamander, other terrestrial species possess a flatter lens (to focus over a greater stretch) and a trichromatic color vision enabled by three photoreceptors (450, 500, and 570 nm).

Auditory

Although these amphibians lack most ear components, such as the middle ear cavity, eardrum, and eustachian tube, they possess an opercularis system (like frogs) comprising an operculum and a columella fused to the skull. An opercularis muscle connects the operculum to the pectoral girdle.

The auditory system in salamanders can detect low-frequency vibrations (500 to 600 Hz), which are then transmitted to the inner ear.

Respiratory Organs

Salamanders respire through gills, lungs, skin, and membranes of the mouth and throat.

They breathe using their external, feathery gills (visible as tufts on either side of the head) in the larval stage. Some species, like the mudpuppy, retain these gills even in adulthood. Similarly, the embryos of terrestrial varieties, like Ensatina, possess strikingly large gills lying close to the egg’s surface.

These external gills in salamanders are quite different from those in amphibians with internalized gills. They usually depend on pressure fluctuations within the buccal and pharyngeal cavities to ensure oxygen diffusion. In the Necturus, the movement of external gills is controlled by a special musculature called the levatores arcuum.

Salamanders also use their lungs (if present) to oxygenate their body fluid. For example, cold-water species like torrent salamanders possess small, smooth-walled lungs, while those living in warm waters, like the lesser siren (Siren intermedia), have voluminous and convoluted lungs.

A few species, like the olm, possess both gills and lungs, while lungless salamanders have neither and breathe through the skin surface (cutaneous respiration), which is highly vascularized with blood vessels underneath.

Taxonomy

The naming of the salamander order has remained uncertain, with scientists still debating the definitions of the terms Caudata and Urodela. While some propose using the term Urodela for the crown group and Caudata for the entire group of salamanders, many suggest the other way around. However, the first belief is most widely accepted.

Currently, around 760 living species of salamanders are classified into 3 suborders: Cryptobranchoidea,  Salamandroidea, and Sirenoidea.

Salamanders (Urodela)

Evolution

Distribution and Habitat

Salamanders are restricted to the Holarctic and Neotropical realms. About one-third of the species are concentrated in North America, especially in the Appalachian Mountains. While the southern gray-cheeked salamander (Plethodon metcalfi) occupies slightly cooler and wetter areas of southern Appalachians (above 900 m), the northern slimy salamander (Plethodon glutinosus) has a comparably wider range. For example, the northernmost Asian species, Salamandrella keyserlingii, inhabits the Siberian larch forests of Sakha. In North America, Ambystoma laterale, the northernmost species, is found no farther north than Labrador. Meanwhile, Taricha granulosa is restricted to the Alaska Panhandle. 

Although most salamanders occupy freshwater habitats, the Anderson’s salamander thrives in brackish or salt water.

Diet

Salamanders are opportunistic predators that feed on almost every prey of suitable size.

Behavior

Feeding

All terrestrial salamanders catch prey by flicking their sticky tongues out. This feeding pattern is well-observed in the tiger salamander (Ambystoma tigrinum), which positions its snout very close to the prey before making the attack. It then wide opens its mouth, flinging the tongue out, while the lower jaw remains stationary. The prey is instantly trapped in the mucous-laden trough in the protruding tongue, which remains as such when the tongue is retracted and the jaws closed. As it is swallowed, the eyeballs press into the roof of the mouth, followed by alternate contraction and relaxation of the throat muscles.

Lungless salamanders have developed a more effective feeding strategy. They use their muscles to shoot the hyoid bone, further elongating the tongue for better reach. While rolling the tongue back, they employ the pelvic muscles to retrieve the hyoid bone.

Since aquatic salamanders lack tongue muscles, they grasp the prey using their teeth. They then toss their heads and snap their jaws sharply, constantly letting water in and out of their mouths. This mechanism helps tear and mash the prey, after which it is gulped.

Defense

These amphibians employ various defense mechanisms, such as aposematism, mimicry, camouflage, and autotomy, to ward off predators or avoid them.

Aposematism

They have evolved various strategies to make themselves unpalatable to predators. For example, the fire salamander (Salamandra salamandra) squirts a toxic fluid at its enemies (up to 80 cm distance) from the ridge of granular glands along its spine.

Similarly, the Iberian ribbed newt (Pleurodeles waltl) rotates its sharply pointed ribs (between 27 and 92°) and exudes a poisonous fluid at its predator. After it recedes, the newt retracts its ribs to its original position.

Camouflage and Mimicry

Salamanders are cryptically colored to either dodge the predator’s eye or advertise themselves as potentially toxic prey. They flash bright shades like yellow, orange, and red against a black background for a striking contrast.

Autotomy

When captured by a predator, some salamanders snip off their tails instantly (autotomy). The autotomized part keeps wriggling on the ground and distracts the attacker, thus giving the salamander ample time to flee. They regenerate the tail within a few weeks.

Molting

Salamders shed their old skin (molting) and replace them with new ones. As the skin breaks around the mouth, the amphibian moves forward to slough the old skin in the front legs. It then uses its muscles to send a series of ripples towards the posterior part. The skin of the hind limbs is pushed farther back as the animal moves forward by pressing its tail against the ground, thus releasing the old skin due to friction during movement. It often consumes the skin it has shed.

Vocalization

Lifespan

Although most salamanders live for about ten years, some have lived as long as 55.

Reproduction and Life Cycle

Since both sexes look alike in most salamander species, they employ different olfactory and tactile cues to identify mates. In temperate regions, mating is seasonal, and salamanders usually migrate to breeding grounds, with the males arriving first and setting territories. They then attract potential females using chemical cues called pheromones produced by their abdominal and cloacal glands. In contrast, the Old World newts are sexually dimorphic, and the males display themselves in front of the females for sexual selection.

All salamanders except those in the families Plethodontidae, Ambystomatidae, and Salamandridae mate in water. Advanced salamander males indirectly transfer their sperm to the females, following which the gametes undergo internal fertilization. In this form of sperm transfer, the male courts the potential mate and deposits a spermatophore (sperm-packed capsule) on the substrate, which the female picks with her vent. The sperm are stored in the spermatheca of females for a long time until the eggs are ready to be laid in water. In the Asiatic salamanders, the giant salamanders, and sirens, the male releases sperm on the egg mass, and the gametes fertilize externally. In such cases, the males exhibit considerable paternal care.

However, in species like the Sardinian brook salamander, the Corsican brook salamander, the Caucasian salamander, and the Pyrenean brook salamander, the sperm is transferred directly to the female’s cloaca.

Based on the species, salamander eggs are laid in three different ways.

Some species, like fire salamanders, retain the eggs inside their bodies, where they continue to develop (oviparous) until they hatch into the larval stage or well-developed juveniles.

Usually, the eggs hatch into larvae or tadpoles with a long body, three pairs of external gills, and a laterally flattened tail having dorsal and ventral fins. They typically lack eyelids and sometimes possess limb buds. Those living in streams (Rhyacotriton and Onychodactylus) are slender, with short gill filaments and narrow fins, while the ones hatching in ponds possess a pair of rod-like balancers on the head, long gill filaments, and broad fins. The larval stage may persist for days to years, while it is completely bypassed in some species, like lungless salamanders.

Generally, the larvae metamorphose into terrestrial adults through the closure of gill slits and the loss of external gills and tail fins. Simultaneously, the eyelids develop, the mouth widens, and teeth are formed.

However, when terrestrial conditions are too harsh for adults to survive, some salamanders (neotenes) deviate from the usual developmental pathway and resort to a unique method called neoteny (paedomorphosis). In this form of development, an individual attains sexual maturity while retaining larval characteristics, like the presence of external gills. These changes are usually triggered by thyroid hormones (reduced activity of the hypothalamus-pituitary-thyroid axis) but may also be due to fluctuating temperatures, aridity, lack of resources, or insufficient iodine to form thyroid hormones.

Predators

Invertebrates, like crayfish and giant water bugs, prey on adult salamanders and tadpoles. Similarly, many vertebrates, such as turtles, snakes, skunks, and raccoons, feed on these amphibians.

Adaptations

Conservation Status

Salamanders are threatened by various factors, such as habitat loss and fragmentation, stream silting, pollution, climate change, and chytridiomycosis (a fungal disease).

The IUCN undertook religious conservation measures in 2005 by establishing the Amphibian Conservation Action Plan (ACAP), followed by the formation of more specialist groups, such as Amphibian Ark (AArk), the Amphibian Specialist Group (ASG), and finally, the Amphibian Survival Alliance (ASA), to undertake some precautionary measures to avoid the loss of their diversity.

References Article last updated on 31st May 2024
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