The Black Dogfish is a species of dogfish shark in the family Etmopteridae. The black dogfish contributes significantly to the bycatch of deep-sea commercial fisheries operating in the North Atlantic; it is of little commercial value and is usually discarded. As large portions of its range see little deepwater fishing activity and its northwestern Atlantic population seems to be stable, the International Union for Conservation of Nature has listed this species under Least Concern overall. It has been assessed as Near Threatened in the northeastern Atlantic, where its numbers may have declined from heavy fishing pressure.
Adult black dogfish typically measure 60–75 cm (24–30 in) in length and can reach 1.1 m (3.6 ft), making it the largest member of its family. Females attain a larger ultimate size than males. The shark has a rather stocky and laterally compressed body, with a moderately long, thick, and flattened snout that forms a very broad arch at the front. The sizable, horizontally oval eyes are a reflective green in life and lack nictitating membranes (protective third eyelids); they are followed a short distance behind by much smaller spiracles (accessory respratory openings). The nostrils are anteriorly placed and preceded by short flaps of skin. The mouth is wide and evenly arched, with thin lips and short but deep furrows around the corners. There are around 34 tooth rows in either side of both jaws; each tooth has three (occasionally up to five) slender cusps, with the central one the longest.
Both dorsal fins are immediately preceded by stout, grooved spines, with the second much longer than the first. The small first dorsal fin has a rounded apex and a nearly straight trailing margin, with its origin lying behind the pectoral fins. The second dorsal fin is rather angular and has about double the area of the first, with its origin located opposite the midpoint of the pelvic fin bases. The pectoral fins are small and rounded. The pelvic fins are about as large as the second dorsal fin, with rounded tips and nearly straight trailing margins. The caudal peduncle is short and leads to a broad caudal fin comprising less than a quarter of the total length; the upper lobe has a convex upper margin leading to a squared-off tip, while the lower lobe is indistinct. The skin is densely covered by tiny dermal denticles; each one is recurved and thorn-like, rising from an irregular star-shaped base. This species is a plain dark brown above, darkening to almost black below, with white dorsal fin spines. Juvenile sharks have white edges on the dorsal, pectoral, and pelvic fins. Minute, luminescent dots are scattered about the skin without a regular pattern.
Inhabiting the outer continental shelf and continental slope, the black dogfish is found mostly near the bottom in water 180–2,250 m (590–7,380 ft) deep. It is most common at depths of 800–1,200 m (2,600–3,900 ft) off Iceland, 1,250–1,500 m (4,100–4,900 ft) in the Rockall Trough, 500–1,300 m (1,600–4,300 ft) off Greenland, 350–500 m (1,150–1,600 ft) off northern Canada, and below 500 m (1,600 ft) off southern Africa. The species may venture closer to the surface in the northern extreme of its range, particularly during the dark, cold winter months. Depth segregation by sex has been documented in the North Atlantic, with females outnumbering males at depths greater than 1 km (0.62 mi). Depth segregation by size varies by region: larger sharks are generally found in deeper water off western Greenland, in shallower water off western Iceland, and without pattern with respect to depth off eastern Iceland. The black dogfish prefers water temperatures of 3.5–4.5 °C (38–40 °F), though off northern Canada, it is most abundant in water of 5–6.5 °C (41–44 °F); it can tolerate temperatures down to 1 °C (34 °F). There is some evidence that this species conducts seasonal migrations, spending winter and spring in shallower water. Sharks off northern Canada perform development-related movements (see below) not observed off western Greenland, suggesting the presence of two distinct in the northwestern Atlantic.
Apparently opportunistic in feeding habits, the black dogfish typically hunts in open water, but also scavenges off the bottom. The bulk of its diet consists of a variety of bony fishes, including rattails, whitings, rockfishes, lanternfishes, and barracudinas, as well as pelagic crustaceans such as krill and shrimp, and cephalopods. Fish become a progressively more important food source as the shark ages, while crustaceans become less important. Infrequently, polychaete worms and jellyfish are also eaten.