These small sharks are greyish brown in color and are countershaded. Males are identified by a pair of pelvic fins modified as sperm-transfer organs, or "claspers". The spiny dogfish has dorsal spines, no anal fin, and white spots along its back. The caudal fin has asymmetrical lobes, forming a heterocercal tail. The species name acanthias refers to the shark's two spines. These are used defensively. If captured, the shark can arch its back to pierce its captor. Glands at the base of the spines secrete a mild poison.

Spiny dogfish are bottom-dwellers. They are commonly found at depths of around 50-1490m, but have been found deeper than 7000m. Tolerates brackish water, often found in enclosed bays and estuaries. Reported to enter freshwater, but cannot survive there for more than a few hours. Highly migratory species, used to be observed in large foraging schools with up to thousands of individuals, usually segregated by size and /or sex, with schools of large gravid females preferentially targeted by fisheries.

Opportunistic predator and scavenger, Feeds on small squid, fish and crustaceans.

Reproduction is aplacental viviparous, which was before called ovoviviparity. Fertilization is internal. The male inserts one clasper into the female oviduct orifice and injects sperm along a groove on the clasper's dorsal section. Immediately following fertilization, the eggs are surrounded by thin shells called "candles" with one candle usually surrounding several eggs. Mating takes place in the winter months with gestation lasting 22–24 months. Litters range between 2 and 11 but average 6 or 7. Pup once a year between Nov and Feb. The Maori name 'Koinga' means 'a sharp point' after the sharp spine in front of each dorsal fin.
Daily limit of 15. Early Maori dried dogfish meat to eat later and used their skin for sandpaper.

Possibly the most abundant living shark.

Marine Fishes 2: Paul
Compagno, L.J.V., 1984. FAO Species Catalogue. Vol. 4. Sharks of the world. An annotated and illustrated catalogue of shark species known to date. Part 1 - Hexanchiformes to Lamniformes. FAO Fish. Synop. 125(4/1):1-249. Rome: FAO.