Broadnose sevengill shark (Notorynchus cepedianus)

KingdomAnimalia
PhylumChordata
ClassChondrichthyes
OrderHexanchiformes
FamilyHexanchidae
GenusNotorynchus (1)
SizeLength: up to 3 m (2)

Classified as Data Deficient (DD) on the IUCN Red List. East Pacific subpopulation classified as Lower Risk / Near Threatened (LR/nt) on the IUCN Red List (1).

A large-bodied, powerful species, the broadnose sevengill shark can be distinguished from most other sharks by the fact that it possesses seven conspicuous gill slits on each side of the head, whereas most sharks possess only five (2) (3). The overall colouration is typical of predatory sharks, with silver-grey or brownish upperparts that blend in with the dark marine waters when seen from above, and paler underparts helping to disguise the shark against the lighter surface waters when viewed from below. Adults also have small black and white speckles on the body and fins, and the young may possess white trailing edges on the fins and a black stripe running along the caudal fin (2). The dorsal fin is relatively small and set quite far back on the body, while the caudal fin has a distinctly elongated upper lobe (2) (3). The teeth in the lower jaw are large, wide and serrated and help to tear and cut into prey, while the teeth in the upper jaw have more elongated, jagged points which help to hold onto thrashing prey (2).

The broadnose sevengill shark is widely distributed throughout the world’s major oceans, occurring in coastal regions and along the continental shelf to depths of 570 metres (1) (2). It can be found in the south-western Atlantic from southern Brazil to northern Argentina; and in the south-eastern Atlantic from Namibia to South Africa. It also occurs in the western Pacific, from southern Japan, south to southern China, and from Australia, south to New Zealand; and in the eastern Pacific from British Columbia, south as far as central Chile (1) (2).

Living close to the seabed, the broadnose sevengill shark generally prefers rocky bottom habitats, but is also commonly found over sandy and muddy substrates (2). Its movements appear to be coordinated with tidal cycles, moving into shallow bay areas during the rising tides and out to deeper offshore areas as the tide falls (1).

A powerful and versatile predator, the broadnose sevengill shark takes a wide variety of prey, including marine mammals such as dolphins and seals, and fish including sharks, salmon, sturgeon and herring (2). It has also been known to feed upon shark egg cases, sea snails, and the dead bodies of mammals it find in the water, such as rats and humans (3). Interestingly, this species will sometimes hunt in groups, with the individual sharks working together to capture large, agile prey such as marine mammals (2). At other times, it hunts stealthily, making very little body movement except small undulations of the caudal fin, before making a rapid dash when within striking distance (2) (4). While regarded as potentially dangerous to humans, attacks are very rare, usually only occurring when the shark is provoked (2) (3). Although a formidable predator, the broadnose sevengill shark sometimes falls prey to larger sharks such as the great white (Carcharodon carcharias), and has been known to be cannibalistic (2).

Like many shark species, the broadnose sevengill shark employs a mode of reproduction termed ovoviviparity. This involves the fertilised eggs hatching within the female’s uterus, where the embryos then develop, receiving nourishment from a yolk sac. Once the yolk sac nutrients are exhausted the embryos absorb nutrients from secretions within the uterus, until birth takes place. In this species, the gestation period—from egg fertilisation to birth—takes around one year, with the young born during spring and early summer, usually in a shallow bay area (2). Litters may number between 82 and 95 pups, with each measuring around 40 to 45 centimetres in length (2) (3). The young remain in shallow water nurseries for several years before moving to deeper, offshore regions (2).

Owing to a lack of fisheries data for the broadnose sevengill shark, little is known about its global population size and trends (1). Nevertheless, as a largely coastal and nearshore species, it is believed to be heavily fished, and could be under threat (1) (2). The impact of historical overexploitation is evident in the east Pacific subpopulation, where heavy fishing along the coast of California during the 1930s and 1940s substantially reduced this species’ numbers. While the fishery soon collapsed, it was replaced by recreational fishing of the broadnose sevengill shark, which remained popular up until the late 1980s and 1990s, when alternative species became preferentially targeted. The Californian population is now concentrated within the Humboldt and San Francisco Bays, which provide safe havens for the smaller, juvenile sharks, and are therefore vital for this population’s survival and recovery (2). Elsewhere within its range, the broadnose sevengill shark is targeted by sport and commercial fishers, being generally exploited for food, but also for the production of leather from its skin and for its liver oil in China (1) (2).

There are currently no specific conservation measures in place for the broadnose sevengill shark. Further monitoring is necessary to determine whether this species requires protective measures, or if its population is large and robust enough to withstand current levels of exploitation (1).

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  1. IUCN Red List (June, 2009)
    http://www.iucnredlist.org
  2. Florida Museum of Natural History (June, 2009)
    http://www.flmnh.ufl.edu/fish/Gallery/Descript/Sevengill/Sevengill.html
  3. FishBase (June, 2009)
    http://fishbase.sinica.edu.tw/Summary/SpeciesSummary.php?id=2531
  4. Carrier, J.C., Musick, J.A. and and Heithaus, M.R. (2004) Biology of Sharks and their Relatives. CRC Press, Routledge.