The Portuguese dogfish typically reaches a length of 0.9 m (3.0 ft) for males and 1.0 m (3.3 ft) for females, though specimens up to 1.2 m (3.9 ft) long have been recorded. Sharks in the Mediterranean are smaller, growing no more than 65 cm (26 in) long. This species has a flattened, broadly rounded snout that is shorter than the mouth is wide. The nostrils are preceded by short flaps of skin. The eyes are large and oval in shape, positioned laterally on the head and equipped with a reflective tapetum lucidum that produces a yellow-green "eye shine". The mouth is wide and slightly arched, with moderately thick, smooth lips and short furrows at the corners extending onto both jaws. The upper teeth are slender and upright with a single cusp, numbering 43–68 rows. The lower teeth have a short, strongly angled cusp and number 29–41 rows; their bases interlock to form a continuous cutting surface. The five pairs of gill slits are short and nearly vertical.
The body of the Portuguese dogfish is thick and cylindrical except for the flattened belly. The two dorsal fins are small and of similar size and shape, each bearing a tiny grooved spine in front. The first dorsal fin originates well behind the pectoral fins, while the second dorsal originates over the middle of the pelvic fin bases. The pectoral fins are medium-sized with a rounded margin. There is no anal fin. The caudal fin has a short but well-developed lower lobe and a prominent ventral notch near the tip of the upper lobe. The very large dermal denticles change in shape with age: in juveniles, they are widely spaced and heart-shaped with an incomplete midline ridge and three posterior points, while in adults they are overlapping, roughly circular, smooth, and flattened with a round central concavity, superficially resembling the scales of bony fishes. Young sharks are a uniform blue-black in color, while adults are brown-black; there are no prominent fin markings. In 1997, a partially albino individual, with a pale body but normal eyes, was caught in the northeastern Atlantic. This represented the first documented case of albinism in a deep-sea shark.
The deepest-living shark known, the Portuguese dogfish has been reported at depths of 150 m (490 ft) to 3,675 m (12,057 ft) from the lower continental slope to the abyssal plain, and is most common between 400 m (1,300 ft) and 2,000 m (6,600 ft). This species is found deeper in the Mediterranean, seldom occurring above a depth of 1,500 m (4,900 ft) and being most common at 2,500–3,000 m (8,200–9,800 ft). The deep Mediterranean has a relatively constant temperature of 13 °C (55 °F) and a salinity of 38.4 ppt, whereas in the deep ocean the temperature is generally only 5 °C (41 °F) and the salinity 34–35 ppt. The Portuguese dogfish is essentially benthic in nature, though young sharks can be found a considerable distance off the bottom. There is depth segregation by size and sex; pregnant females are found in shallower water, above 1,200–1,500 m (3,900–4,900 ft), while juveniles are found deeper. There may be several separate populations in the Atlantic, and sharks in the Mediterranean and off Japan appear to be distinct as well.
An active predator of mobile, relatively large organisms, the Portuguese dogfish feeds mainly on cephalopods (including Mastigoteuthis spp.) and bony fishes (including slickheads, orange roughy, lantern fishes, and rattails).