The Northern Dusky Salamander is slender-bodied and can attain a total length of 14 cm. Individuals are grey to dark brown with varying amounts of dark or light flecking. Younger individuals often have an irregular light-coloured stripe that that runs down the back and onto the tail, while older individuals tend to be uniformly dark brown or black. The belly is grey to cream-coloured and is often mottled with light and dark flecks. Some individuals have an irregular dark dorsolateral stripe on each side. This species has a distinctive light stripe that runs diagonally from the eye to the back of the jaw. The tail is laterally compressed, keeled (narrows to create an edge along the top) and has a triangular cross-section. Individuals have 13–15 costal groves and the hind legs are larger than the front legs. Larvae are aquatic and have gills and a tail fin. The larvae are brown to reddish in colour with five to eight pairs of light dorsal spots and a dark dorsolateral stripe on each side.
The Northern Dusky Salamander is most easily confused the Allegheny Mountain Dusky Salamander. The Allegheny Mountain Dusky Salamander typically has a well-defined dorsal stripe, sometimes with dark chevron patterning down the centre of the back, and the tail is round in cross-section and lacks the keel. Aside from the Allegheny Mountain Dusky Salamander, no other Plethodontid salamanders in eastern Canada have hind legs that are larger than the front legs or the distinctive light strip that runs from the eye to the back of the jaw.
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In Canada, the Northern Dusky Salamander primarily occurs in southeastern Quebec and in southern New Brunswick. This species also occurs at a single site on the Niagara Peninsula in Ontario. Northern Dusky Salamanders occur throughout the northeastern United States south to South Carolina and West to Indiana.
The Northern Dusky Salamander inhabits small, fast-flowing streams and seeps in forest habitats, as well as the stream banks and immediately adjacent moist terrestrial habitats. Cover objects, such as rocks and woody debris are important microhabitats that provide moist conditions and shelter. Eggs are attached to the underside of submerged rocks in streams or seeps, or they are deposited in other moist environments adjacent to streams. Individuals hibernate in the stream bed or underground in the adjacent terrestrial habitat.
Plethodontid salamanders, including the Northern Dusky Salamander, are also known as lungless salamanders because the adults do not have lungs or gills; instead, they absorb oxygen directly through their skin. They must remain moist at all times so that oxygen and carbon dioxide can diffuse through the skin. Northern Dusky Salamanders breed in the spring or the fall, and females lay between eight and 45 eggs in grapelike clusters in the late spring or early summer. The female guards the eggs until they hatch about 45 to 60 days later. The larvae undergo metamorphosis after approximately one year, and individuals reach sexual maturity about two (males) or three (females) years after metamorphosis. Northern Dusky Salamanders can live for at least ten years. Juvenile and adult Northern Dusky Salamanders primarily forage around the margins of streams and on the forest floor for insects, spiders, worms and other terrestrial or aquatic invertebrates, while larvae primary eat aquatic benthic invertebrates.
The primary threat to this species is the loss of mature forest, primarily through urbanization, clearing of land for agriculture and logging. Pollution (e.g. herbicides, agricultural effluent and road salt) of the streams and seeps that this species inhabits can be detrimental since toxins are easily absorbed though their skin. Climate change and introduced pathogens pose potentially serious future threats to Canadian salamanders.