The dusky smooth-hound (''Mustelus canis''), also called the smooth dogfish or the dog shark, is a species of

houndshark Houndsharks, the Triakidae, are a family of ground sharks, consisting of about 40 species in nine genera. In some classifications, the family is split into two subfamilies, with ''Mustelus'', '' Scylliogaleus'', and '' Triakis'' in the subfamily ...

in the family Triakidae. This shark is an olive grey or brown in color, and may have shades of yellow or grayish white. Females live to 16 years and males have a lifespan of 10 years. ''M. canis'' was the first shark recognised to have viral infections.

Taxonomy

''M. canis'' can be known as smooth dogfish, Atlantic smooth dogfish, dusky smooth-hound, grayish, nurse shark, smooth dog, or smooth-hound. It was originally named ''Squalls canis''. In Latin, ''mustelus'' translates to weasel and ''canis'' translates to dog. ''M. canis'' has an

allopatric Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...

relationship with ''M. mustelus'' (

common smooth-hound The common smooth-hound (''Mustelus mustelus'') is a houndshark of the family Triakidae. It is found in the eastern Atlantic Ocean from the British Isles to South Africa, and in the Mediterranean Sea, Madeira, and the Canary Islands at depths r ...

) and a

sympatric In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species sh ...

relationship with ''M. norrisi'' ( narrowfin smooth-hound).

Distribution and habitat

''M. canis'' is found in marine and brackish waters and is demersal (bottom-dwelling) and oceanodromous (migratory in seas). They can be found between 42°N and 44°S and 100 and 46°W. ''M. canis'' in the North Atlantic migrates 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 to 665 ft (200 m) deep.

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 fins 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 The spiny dogfish (''Squalus acanthias''), spurdog, mud shark, or piked dogfish is one of the best known species of the Squalidae (dogfish) family (biology), family of sharks, which is part of the Squaliformes order. While these common names ma ...

. They have a tapering, blunt snout. They can be gray to brown and their undersides 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 in long, but can reach up to 5 ft. They have an inter-dorsal ridge. Mustelus canis SI.jpg Mustelus canis jaws.jpg, Jaws Mustelus canis upper teeth.jpg, Upper teeth

Dentition

One main characteristic of

elasmobranch Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of gil ...

fishes is their ability to continually replace the teeth in their upper and lower jaws. Smooth dogfish differ from other sharks because of their 10 rows of flat, blunt teeth. The teeth in the upper and lower jaws are similar in size and are asymmetrical with rounded cusps. These teeth are used to crush and grind food, rather than bite it. Their unique dentition is important for their crustacean-based diet. In smooth dogfish, tooth replacement is related to body growth. They grow about 10 cm per every six rows of teeth replaced. That is an increase of 0.03 mm per replaced tooth. Teeth are replaced at a rate of one row per 10 to 12 days.

Diet

''M. canis'' feeds mostly on

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

polychaete Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...

s, and

mollusks Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

. Smooth dogfish also eat squid, worms, small fish, razor clams, and occasionally garbage. They are nocturnal

scavenger Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding b ...

s and opportunistic predators. Their

nocturnal Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite. Nocturnal creatures generally have highly developed sens ...

activity helps them to take advantage of concentrations of crustacean prey.

Reproduction

The mating season of smooth dogfish is from May to June. They are

viviparous Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...

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

teleost Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Tel ...

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

corpora lutea The corpus luteum (Latin for "yellow body"; plural corpora lutea) is a temporary endocrine structure in female ovaries involved in the production of relatively high levels of progesterone, and moderate levels of estradiol, and inhibin A. It is ...

. Adults have large ovaries and vitellogenic oocytes. Spermatozoa has been observed in preovulatory females. Juvenile males have soft, small

clasper In biology, a clasper is a male anatomical structure found in some groups of animals, used in mating. Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve to channel semen into the female's c ...

s and undeveloped testes with straight, thread-like ampullae ductus deferens. Adolescents' testes have increased weight and claspers are extended and calcified, but are 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 and bottom trawls primarily off of Massachusetts, New Jersey, Maryland, Virginia, and North Carolina. Mostly, they are considered bycatch when fishing for other species, 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 Apparent death, colloquially known as playing dead, feigning death, or playing possum, is a behavior in which animals take on the appearance of being dead. It is an immobile state most often triggered by a predatory attack and can be found in a ...

has been reported in several cartilaginous fishes, one of which is ''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.

References

External links

* {{Authority control dusky smooth-hound Fauna of the Southeastern United States Fish of the Western Atlantic Fish of the Dominican Republic Southeastern South American coastal fauna dusky smooth-hound