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Taxonomy
Mustelus canis can be known as smooth dogfish, Atlantic smooth dogfish, dusky smooth-hound, grayish, nurse shark, smooth dog, or smooth-hound. Mustelus canis were originally named Squalls canis but was later changed. In Latin, mustelus translates to weasel-like and canis translates to dog. Mustelus canis has an allopatric relationship with Mustelus mustelus (Common smooth-hound) and a sympatric relationship with Mustelus norrisi (Narrowfin smooth-hound).

Distribution and habitat
Mustelus canis are found in marine and brackish waters and are demersal and oceanodromous. They can be found between 42° N to 44°S and 100° W-46° W. Mustelus canis in the North Atlantic migrate in response to changing temperature. In winter, they can be found in the Carolinas to the outlet of the Chesapeake Bay and in Summer from the mid-Atlantic to southern New England. They are most abundant on the east coast, from Massachusetts to Florida, Brazil to Argentina, and in the Gulf of Mexico. They are mostly found in waters shallower than 60 ft (18 m), but can be found up to 665 ft (200 m).

Anatomy and appearance
Smooth dogfish are relatively small and slender. They have elongated, oval shaped eyes with a spiracle located directly behind on each side. They have triangular fins. Their first and second dorsal fin are well-serrated and nearly equal in size. The second dorsal fin is slightly smaller than the first and twice as large as the anal fin. The caudal fin has two asymmetrical lobes, the lower is smaller and rounder and the upper has a deep notch. They do not have any fin spines, unlike the Spiny dogfish. They have a tapering, blunt snout. They can be gray to brown and their underside can be white to yellowish gray. Smooth dogfish have the ability to change colors using melanophores to help them camouflage. Newborns have lighter gray edges on their fins and have tail fins edged in white. On average, smooth dogfish are about 48 inches long, but can reach up to 5 feet. They have an inter dorsal ridge.



Dentition
One main characteristic of elasmobranch fishes is their ability to continually replace the teeth in their upper and lower jaw. Smooth dogfish differ from other sharks because of their 10 rows of flat, blunt teeth. The teeth in the upper and lower jaw are similar and size and are asymmetrical with a rounded cusp. These teeth are used to crush and grind food, rather than bite it. Their unique dentition is important for their diet of crustaceans.

In smooth dogfish, tooth replacement is related to body growth. Smooth dogfish grow about 10 cm per every six rows of teeth replaced. That is an increase of 0.03 nm per replaced tooth. Teeth are replaced at a rate of one row per 10 to 12 days.

Diet
M. canis feed mostly on crustaceans, polychaete s, and mollusks. They are nocturnal scavengers and opportunistic predators. Their nocturnal activity helps them to take advantage of the concentration of crustacean prey.

Reproduction
The mating season of smooth dogfish is from May to June. M. canis are viviparous and have a yolk-sac placenta. Females can store sperm up to a year, but it not known how long it can be utilized. Their gestation period is 10 to 11 months and litters can be between 4 and 20 pups. The pups are 13 to 15 inches (34 to 39 cm) when born. Males reach sexual maturity between 2 and 3 years or 68 to 93 cm in length. Females reach sexual maturity between 4 and 5 years or 70 to 130 cm in length. Smooth dogfish have a relatively low population doubling time of 4.5 o 14 years. Because of their late maturation, low fecundity, and restricted distributions, they are still more vulnerable than teleost fishes.

Juvenile females have filiform uteri, small ovaries with undifferentiated oocyctes, egg cells, and narrow thread-like oviducts with undeveloped oviducal glands. Adolescents have enlarged oviducal glands with distinguishable oocytes and no or few corporalutea. Adults have large ovaries and vitellogenic oocytes. Spermatozoa has been observed in pre-ovulatory females.

Juvenile males have soft, small claspers and undeveloped testes with straight, thread-like ampullae ductus deferens. Adolescents testes have increased weight and claspers are extended and calcified, but were still flexible. Adults have fully formed and calcified claspers and large and developed testes.

Relationship to humans
Commercial fisheries have had an increased interest in smooth dogfish since the 1900s. They are caught using longlines are bottom trawls primarily off of Massachusetts, New Jersey, Maryland, Virginia, and North Carolina. Mostly, they are considered by-catch when fishing for other things, which has put them in the near-threatened category by the IUCN Red List of Threatened Species. They are no danger to humans because of their blunt teeth.



Tonic Immobility
Tonic immobility has been reported in several cartilaginous fishes, one of which being M. canis. Tonic immobility is induced by grasping the first dorsal fin with one hand and the body immediately anterior to the anal fin with the other, inverting the shark and holding it rigidly. The mean time to induce tonic immobility in smooth dogfish was 32.5 seconds. The mean duration of the tonic immobility was 61.9 seconds. Sharks that had the “limp” response also exhibited tonic immobility. The “limp” response is a criterion for the onset of tonic immobility.