Salamanders are amphibians belonging to the order Urodela (Caudata), distinguished from reptiles like lizards by their moist skin and unique life cycles. These creatures have evolved a specialized internal and external anatomy that allows them to successfully occupy a variety of ecological niches, from fully aquatic environments to fully terrestrial habitats. Their body structure and physiological systems enable them to thrive across diverse landscapes throughout the Northern Hemisphere.
External Body Plan and Skeletal Structure
The overall body form of a salamander features a distinct head, an elongated trunk, four limbs, and a tail. Most species maintain this consistent morphology, with the limbs typically projecting out sideways from the body, leading to a sprawling gait that barely lifts the trunk off the ground. The forelimbs generally possess four digits, while the hind limbs usually have five, though variation exists among species.
The underlying skeletal framework is relatively unossified, meaning the bone structure contains a higher proportion of cartilage compared to other vertebrates. This simpler, less-rigid skeleton supports their low-slung posture and undulatory locomotion, where the body bends side-to-side to aid movement. The tail is laterally compressed in aquatic species, functioning as a paddle for propulsion or as a counterbalance in terrestrial species. Along the trunk, many species exhibit vertical indentations known as costal grooves, which are thought to help channel water over the skin to maintain necessary moisture levels.
The Integumentary and Respiratory Systems
The salamander’s thin, smooth, and highly permeable integument, or skin, is linked to its respiratory function, requiring a consistently moist environment. This skin is rich in glands, including mucous glands that secrete a protective, lubricating layer to prevent desiccation and granular glands that may produce defensive or toxic compounds. The permeability of the skin facilitates cutaneous respiration, allowing for the direct exchange of oxygen and carbon dioxide across the surface.
This reliance on the skin for gas exchange has led to a diversity in respiratory strategies among different groups. Aquatic larvae and certain neotenic species, such as the mudpuppy, retain feathery external gills throughout their lives to draw oxygen from the water. Many terrestrial species possess simple, sac-like lungs, which they use for pulmonary respiration after metamorphosis. The lungless salamanders, belonging to the family Plethodontidae, represent a specialized adaptation, having lost their lungs entirely.
These lungless species rely solely on cutaneous and buccopharyngeal respiration, absorbing oxygen through their skin and the moist lining of their mouths and throats. This gas exchange is supported by an exceptionally dense network of blood vessels situated just beneath the skin’s surface. A visible pulsing or oscillation of the throat region can often be observed, an action believed to aid in pumping air across the blood-rich membranes of the mouth lining. The loss of lungs in this group is considered an adaptation to environments where efficient, skin-based gas exchange is sufficient.
Unique Feeding and Sensory Adaptations
Salamanders are predators that employ specialized sensory and feeding mechanisms to capture prey, which primarily consists of invertebrates. Aquatic species often possess a lateral line system, a sensory network similar to that found in fish, which detects subtle vibrations and changes in water pressure to locate nearby movement. Terrestrial species rely more heavily on acute chemoreception, using their sense of smell to track prey on the ground. Vision also plays a role, with some species exhibiting trichromatic color vision that extends into the ultraviolet range to assist in hunting.
The feeding mechanics vary significantly depending on the habitat, contrasting the rapid strike of terrestrial hunters with the engulfing action of aquatic ones. Fully aquatic salamanders capture prey using suction feeding, rapidly opening their mouths to create a negative pressure that draws in both water and the target organism. Many terrestrial species have evolved a specialized projectile tongue, which can be launched with speed and precision to secure prey. This tongue is often attached anteriorly and can be extended over a considerable distance in a fraction of a second.
All modern amphibians, including salamanders, share a unique dental feature known as pedicellate teeth. These teeth consist of a crown and a base, called the pedicel, separated by a layer of uncalcified dentine. This flexible joint allows the tooth to bend inward when pressure is applied, preventing prey from escaping once grasped. These teeth are found on both the upper and lower jaws and are continuously replaced throughout the animal’s life.
Tissue Repair and Regeneration
The salamander has a high capacity for tissue repair and complex structure regeneration. Unlike most other vertebrates, salamanders can regrow entire lost appendages, including limbs and tails, even as adults. This regenerative capability also extends to internal structures, such as portions of the jaw, the spinal cord, and the retina of the eye.
The process of regeneration begins with the formation of a structure called the blastema at the site of injury. The blastema is a mass of undifferentiated, proliferating cells that gathers beneath the wound epidermis. These cells are formed through a process called dedifferentiation, where mature cells from the stump tissue, such as muscle and cartilage, revert to a more stem-cell-like state. This cell mass then grows and differentiates into the precise tissues required to seamlessly reconstruct the missing part. This biological mechanism allows for perfect restoration of the original structure without the formation of scar tissue, a difference from the wound-healing process observed in mammals.