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Author: (Mitchell, 1815)

Field Marks:
An unspotted, large Mustelus with a short head and snout, broad internarial, large eyes, narrow interorbital, upper labial furrows somewhat longer than lowers, low-crowned teeth with weak cusps, buccopharyngeal denticles confined to extreme front of mouth, lateral trunk denticles usually lanceolate and with complete ridges, unfringed dorsal fins, 85 to 100 precaudal centra, and a non-falcate but moderately expanded ventral caudal lobe.

Diagnostic Features:
Body fairly slender. Head short, prepectoral length 17 to 21% of total length; snout moderately long and bluntly angular in lateral view, preoral snout 5.5 to 8.1% of total length, preorbital snout 5.9 to 8.3% of total length; internarial space broad, 2.7 to 3.7% of total length; eyes fairly large, eye length 1.9 to 2.3 times in preorbital snout and 2.2 to 4.2% of total length; interorbital space narrow, 3.6 to 4.6% of total length; mouth fairly short, subequal to or slightly longer than eye length, its length 2.3 to 3.5% of total length; upper labial furrows slightly longer than lowers, upper furrows 1.6to 2.7% of total length; teeth molariform and asymmetric, with cusp reduced to a low point, cusplets absent except in very young sharks; buccopharyngeal denticles confined to tongue and anteriormost part of palate. Interdorsal space 16 to 23% of total length; pelvic fins moderately large, anterior margin length 6.6 to 8.6% of total length; trailing edges of dorsal fins denticulate, without bare ceratotrichia; first dorsal somewhat falcate, with nearly vertical posterior margin, midbase closer to pectoral bases than to pelvics; pectoral fins moderately large, length of anterior margins 11 to 16% of total length, width of posterior margins 8 to 14% of total length; anal height 2.5 to 4.5% of total length; anal caudal space greater or subequal to second dorsal height, 6.3 to 9.2% of total length; ventral caudal lobe not falcate but somewhat expanded in adults. Crowns of lateral trunk denticles lanceolate, with longitudinal ridges extending at least half their length, and often their entire length. Skeleton not hypercalcified in adults; palatoquadrates not subdivided; monospondylous precaudal centra 34 to 42, diplospondylous precaudal centra 48 to 60, precaudal centra 85 to 100. Colour uniform grey above, light below, no white or dark spots or dark bars, but newborn young with dusky-tipped dorsal and caudal fins. Development viviparous. Size large, adults 82 to 150 cm.

Geographical Distribution:
Western Atlantic: Massachusetts to Florida, northern Gulf of Mexico, Venezuela, including Cuba, Jamaica, Barbados, Bermuda, Bahamas; southern Brazil to northern Argentina.

Habitat and Biology:
A common to abundant temperate and tropical shark of the continental and insular shelves and upper slopes, ranging from shallow inshore waters and the intertidal to 200 m depth, but occasionally down to 579 m. In US temperate waters it prefers inshore waters less than 18 m deep, with many coming into enclosed bays and harbours, especially with mud or sandy bottoms; in tropical areas it apparently avoids coral reefs. Some smooth-hounds penetrate the lowermost reaches of rivers and are said to occur in fresh water, though it is doubtful that this shark can live in fresh water for extended periods of time like the bull shark, Carcharhinus leucas. Caribbean island populations inhabit deeper water (most below 200 m depth) than continental ones, and prefer rocky bottoms. It is primarily found near or on the bottom, but may occur in midwater off Cuba.

Off southern New England and the middle Atlantic States of the USA this is the second most abundant shark, although falling far short of the spurdog, Squalus acanthias, in numbers. It is said to be the most common local shark in Uruguayian waters.

There is some indication that this shark is divided into several discrete populations, with few or no members of the species occurring in the broad gaps between them (Bigelow and Schroeder, 1948). The best known population is the possibly cold-adapted one which occurs from the Carolinas north along the US Atlantic Coast to New England and southern Canada. The species is uncommon south of North Carolina but is again common off Florida and turns up at many localities in the Gulf of Mexico to southern Mexico, off Bermuda, theBahamas, and various Caribbean islands and again from southern Brazil to Argentina.

Off the Atlantic coast of the USA the species is migratory, and responds to changes in water temperature by moving. It primarily winters in the area between souOthern North Carolina and Chesapeake Bay. In the springtime as water warms up on the bottom to at least 6 to 7 C, it moves northward along the coast to New England, and southward to South Carolina. As summer wanes smooth-hounds move offshore and withdraw centrally to their wintering area.

A behavioural experiment involving 10 smooth-hounds apparently of this species (Allee and Dickinson, 1954; Myrberg and Gruber, 1974) indicated that a dominance hierarchy was formed in the experimental population, based on size and not sex, and that individual smooth-hounds, regardless of their position in the hierarchy did not defend territories.

Viviparous, with a yolk-sac placenta; number of young 4 to 20 per litter. The gestation period of smoothhounds off New England is about 10 months, with mating in midsummer (July) and birth between early May and mid-July of the next year. It has been suggested from growth-curve calculations that the growth rate of this shark is very fast, with maturation after one to two years, but this remains to be confirmed from direct ageing techniques.

Feedsprimarily on large crustaceans, especially crabs, but also heavily on American lobsters (Homarus). It also takes small bony fish, including menhaden, stickleback, wrasses, porgies, sculpins and puffers, squid (in springtime in New England waters), gastropods, bivalves, marine annelid worms, and occasional garbage (chickenheads and other human debris has been found in stomachs of smooth-hounds caught in Cuban waters). Some have been found with quantities of eel-grass (Zostera) in their stomachs, but this may be incidentally taken in while the smooth-hounds are capturing animal prey.

This is a very active shark, constantly patrolling the bottom for food, which can be located when hidden as when in sight, indicating use of other senses including olfactory and electrosense. In captivity they readily attack crabs and shake them vigorously sideways before devouring them, but rarely molest active bony fishes; however, sick, injured or dead fish are quickly devoured. Dusky smooth-hounds are harmless to humans, except for competing with them for crustaceans, especially the valuable American lobster. One estimate quoted in Bigelow and Schroeder (1948) suggested that in Buzzard's Bay, Massachusetts, at the turn of the century, these sharks annually ate 200 000 crabs, 60 000 lobsters, and 70 000 small fish.

Size:
Maximum 150 cm, males maturing at about 82 cm, females maturing at about 90 cm and reaching at least 122 cm; size at birth between 34 and 39 cm.

Interest to Fisheries:
A locally abundant shark that is primarily fished off Cuba, Venezuela, and Brazil, but no doubt utilized elsewhere in the Caribbean; it is caught with bottom and floating longlines, occasionally with bottom trawls, and utilized fresh and dried salted for human consumption.

Remarks:
This species is very close to the allopatric Mustelus mustelus and sympatric M. norrisi; the former differs from it in having somewhat shorter labial furrows, a slightly narrower internarial, and less precaudal centra, while the latter is a smaller, slenderer, narrower-headed shark with a narrower internarial, shorter labial furrows, more extensive buccopharyngeal denticles, more diplospondylous precaudal centra, and usually a more falcate ventral caudal lobe in adults (Heemstra, 1973). In addition, there are two new allopatric species of canis-norrisi-like smooth-hounds in the western Atlantic, one of which has often been confused with M. canis, which will be described by P.C. Heemstra.

Type material:
Holotype: ?. Type locality: New York.

Dusky smooth-hound (Mustelus canis)