This slim-bodied species is characterized by the black interior of its mouth, a marbled pattern of pale-edged brownish saddles or blotches along its back and tail, and a prominent saw-toothed crest of enlarged dermal denticles along the upper edge of its caudal fin.

The International Union for Conservation of Nature (IUCN) has listed this species under Least Concern, as there is no indication that its numbers have declined despite fishing pressure.

Constantine Samuel Rafinesque briefly described the blackmouth catshark in his 1810 Caratteri di alcuni nuovi generi e nuove specie di animali e piante della Sicilia: con varie osservazioni sopra i medesimi, wherein he referenced the distinctive black interior of its mouth (from which the specific epithet melastomus is derived).

[5] The blackmouth catshark is widely distributed in the northeastern Atlantic Ocean, from southwestern Iceland and Trondheim, Norway southward to Senegal, including the Faroe Islands, the British Isles, the Azores, and the northern portion of the Mid-Atlantic Ridge.

[16] The body is grayish-brown above, with 15–18 dark, rounded saddles, blotches, and/or spots that run onto the tail; each marking is highlighted by a paler border.

[7][13][20][21] Known predators of the blackmouth catshark include the kitefin shark (Dalatias licha) and the European flying squid (Todarodes sagittatus).

[7][26] Its diet is dominated by decapods, krill, bony fishes (including lanternfishes, bristlemouths, dragonfishes, and moras), and cephalopods.

The most significant prey species generally reflect what is most available in the environment, for example the shrimps Calocaris macandreae and Pasiphaea multidentata off southern France and the prawns Sergestes arcticus and Sergia robusta off the Iberian Peninsula.

Juveniles consume a greater amount and variety of crustaceans than adults, including smaller types such as mysids and hyperiid amphipods.

[8][10][27] The stomachs of some blackmouth catsharks have found to contain pieces of animals too large for a single shark to overwhelm, suggesting that it may sometimes attack in groups.

For electroreception, the blackmouth catshark has a high number of ampullae of Lorenzini that are evenly arranged, which enhances spatial resolution and is best suited for localizing fast-moving prey.

[28][29] Unlike most members of its genus, the blackmouth catshark exhibits multiple oviparity, in which more than one egg can mature within each oviduct simultaneously.

[9] Mating and egg-laying proceeds year-round; reproductive activity is highest in winter and summer, though not all studies have found such a seasonal pattern.

[13][15] Harmless to humans and of little economic value,[3] the blackmouth catshark is caught incidentally in large numbers by commercial bottom trawl and longline fisheries.

In particular, it is among the most commonly bycaught sharks in trawls targeting deepwater lobsters and shrimps (Nephrops norvegicus, Parapenaeus longirostris [es], Aristeus antennatus, and Aristaeomorpha folicea), operating off Portugal and in the Mediterranean.

Some fisheries, such as those off Portugal and Italy, retain and utilize a small subset of the largest individuals for human consumption fresh or dried and salted, and for leather; the fishing fleet of Viareggio, Tuscany reported landing 700 kg (1,500 lb) in 2005.

[1][31] However, the species remains extremely abundant in a number of areas, and survey and fishery data have not shown any evidence of overall population decline.

The wide range of depths it occupies likely afford it some protection against fishing, particularly given a 2005 ban on trawling deeper than 1,000 m (3,300 ft) in the Mediterranean.

In the waters of the European Commission, fishing for this species is managed as part of the Total Allowable Catch (TAC) for deepwater sharks.