Dusky smoothhound  (Mustelus canis)

The dusky smoothhound is classified as Near Threatened (NT) on the IUCN Red List (1).

The dusky smoothhound (Mustelus canis) is a medium-sized, relatively slender shark with an elongate body, large eyes and a fairly long, tapering, blunt-ended snout (2) (3) (4) (5) (6). Its body is uniformly grey above and white to yellowish-grey below (2) (3) (4) (5) (6), with individuals from dark environments often being much darker than those from paler, sandy habitats (3) (4) (5). Young juvenile dusky smoothhounds have dusky grey tips and white rear margins on the dorsal fins and caudal fin (2) (3) (4) (5).

One of the larger Mustelus species (2) (5), the dusky smoothhound is distinguished from sharks in other genera by its small, flattened teeth, which have bluntly rounded cusps and are arranged in rows (3) (4) (5). Mustelus sharks have two large dorsal fins, the second of which is only slightly smaller than the first. The pelvic and pectoral fins are also fairly large. The anal fin of the dusky smoothhound is smaller than, and positioned just behind, the second dorsal fin, while the caudal fin is asymmetrical, with a much smaller lower than upper lobe (2) (3) (4) (5) (6).

In general, Mustelus species are difficult to separate from one another (2), and the dusky smoothhound can easily be confused with the common smoothhound (Mustelus mustelus) and narrowfin smoothhound (Mustelus norrisi) (2) (3) (5). The dusky smoothhound has been divided into two subspecies, Mustelus canis canis and Mustelus canis insularis. Although almost identical in appearance, M. c. insularis has slightly higher dorsal fins and a longer caudal fin, and juveniles have more conspicuous white fin margins than juvenile M. c. canis. However, the two are mainly separated by their differing number of vertebrae (5).

The dusky smoothhound is found in the western Atlantic Ocean, from Massachusetts to Florida in the USA, in the northern Gulf of Mexico, the Caribbean and Bermuda, and along the coast of South America from southern Brazil to northern Argentina (1) (3) (4) (5) (6). It may occur in several quite widely separated populations, with little movement of individuals between them (1) (2) (6).

The subspecies M. c. insularis is found only around a number of Caribbean islands, including Cuba, Jamaica, Puerto Rico and the Bahamas, and east to Bermuda (5). In the northern parts of its range, the dusky smoothhound migrates in response to changes in water temperature, moving south in the autumn and north again in spring (2) (3) (4).

A bottom-dwelling, coastal shark, the dusky smoothhound is typically found in inshore waters to depths of around 200 metres (1) (2) (3) 4) (6). In some areas, it may move into the mouths of rivers, but it is unlikely to be able to survive for long in freshwater (2) (3) (4).

The dusky smoothhound generally prefers areas with muddy or sandy bottoms (2) (4) and it avoids coral reefs (2). The subspecies M. c. insularis is found in deeper water than M. c. canis, having been recorded to depths of 808 metres, and appears to prefer rocky bottoms (5).

The unusual teeth of the dusky smoothhound are adapted to crushing and grinding prey, rather than biting and tearing it like most other shark species (1) (3) (6). It is an active, nocturnal predator (2) (3) (7) which feeds mainly on large crustaceans, such as rock crabs (Cancer irroratus), lady crabs (Ovalipes ocellatus), blue crabs (Callinectes sapidus) and lobsters. The dusky smoothhound also eats squid, small fish, worms, razor clams (Ensis directus) and other molluscs, and occasionally garbage, such as discarded chicken heads (2) (3) (4) (8). Juveniles feed mainly on crabs, small shrimps and worms (3) (9).

In the North Atlantic Ocean, the dusky smoothhound mates from mid to late summer. The young are born the following May to July (2) (3) (9) (10), after a gestation period of 10 to 12 months (2) (3) (5) (10). In other parts of its range, births may be less seasonal (11). The dusky smoothhound is viviparous, giving birth to live young which have been nourished inside the female by a yolk-sac placenta (2) (3) (4) (5). The female may give birth to between 3 and 20 young at a time, with larger females having larger litters. The young measure about 28 to 39 centimetres at birth (2) (3) (4) (5) (9) (10).

The dusky smoothhound has been recorded using shallow estuaries and tidal marshes as nursery grounds, where the females give birth and where the young then remain and develop for several months (9). This species grows relatively quickly for a shark, with males reaching maturity from about 82 to 99 centimetres in length, and females from about 90 to 108 centimetres (2) (4) (5) (11) (12). These lengths correspond to ages of two to three years and four to five years, respectively (3) (10). The female dusky smoothhound may grow larger than the male (11), and maximum body size is reached at around seven to eight years of age (3). Females may live for up to 16 years in the wild, and males for around 10 years (3) (12).

The dusky smoothhound appears to be an abundant shark species in parts of its range (1) (2) (3) (4) (6). However, it is fished off Cuba, Venezuela, Brazil, Mexico and probably also elsewhere in the Caribbean (2) (4), and commercial interest in the species has increased in recent years (1) (3) (6). Rapid increases in directed gillnet fishing has caused a decline in the dusky smoothhound in some areas (1) (6).

Many houndsharks (sharks in the family Triakidae) do well in captivity, and species such as the dusky smoothhound are kept in public aquaria, as well as being used as lab animals (3) (4).

There are currently no management plans or specific protection in place for the dusky smoothhound (1) (6). Although it appears to grow and reproduce relatively fast for a shark species (10) (12), it is still potentially vulnerable to overexploitation (11). More work is needed to predict how the dusky smoothhound population will respond to increased fishing activity, which will help scientists to develop the most appropriate management measures for this species (1) (6) (11).

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Find out more about the dusky smoothhound and its conservation:

• Smooth Dogfish Biological Profile, Ichthyology Department, Florida Museum of Natural History:
  http://www.flmnh.ufl.edu/fish/Gallery/Descript/SmoothDogfish/SmoothDogfish.html

More information about shark conservation:

• Save Our Seas Foundation:
  http://www.saveourseas.com/
• Shark Trust:
  http://www.sharktrust.org/
• Project AWARE:
  http://www.projectaware.org/
• IUCN / SSC Shark Specialist Group:
  http://www.iucnssg.org/