The broadnose sevengill shark (Notorynchus cepedianus) is the only common, coastal member of the cowshark family, Hexanchidae – most other members prefer deep-water. Thus, while the basic biology of the whole group remains somewhat mysterious, this large species has been studied more than most hexanchoids (along with the bluntnose sixgill shark). It is distributed widely in the world’s temperate coastal waters.
It has a short, blunt snout, and one dorsal fin far back on the body. It can be distinguished from the only other sevengill shark (the sharpnose sevengill, Heptranchias perlo) by its broad head, larger body, smaller eye, large anal fin, and small black and white spots covering the back and fins. Also, the lower jaw has six rows of large teeth, versus five in H. perlo.
Among its many other common names are sevengill shark, broad snouted sevengill, ground shark, Pacific seven-gill shark, spotted cow shark, and Tasmanian tiger shark.
Broadnose Sevengill Shark Facts
This powerful shark can reach 3 m (almost 10 ft) in total length and up to 107 kg (236 lbs). Its name comes from the presence of seven paired gill openings, compared to the five gills of most other sharks. The lifespan is estimated to last up to 50 years.
The sevengill shark’s body coloring can be described as “counter-shaded”: the brownish-gray back blends in with deeper waters when viewed from above, while the lighter belly makes the shark blend in with surface waters when viewed from below. This adaptation for camouflage is common among predatory sharks.
Even this fierce hunter must be wary of enemies. Potential predators include larger sharks such as the great white, and even other members of its own species.
Habitat and Range
The broadnose sevengill is a coastal shark. It usually stays in water less than 50 m (165 ft) deep, in bays and estuaries, and often less than 1 m (3.2 ft) deep. However it also occurs in deeper waters on continental shelves, with large individuals found offshore as deep as 570 m (1,870 ft). Typically, these sharks swim slowly along the sea floor, with occasional forays to the surface. The preferred habitat is rocky bottom substrates, and also sandy and muddy areas.
This wide-ranging species occurs throughout the temperate regions of the world’s oceans, other than the north Atlantic and the Mediterranean Sea. It exists patchily across its broad range, a distribution pattern that suggests the presence of distinct populations in many areas.
Population structure and movement patterns have been studied with the use of electronic tagging, though there are many gaps in the knowledge of this species’ population ecology. Results reveal consistent seasonal changes in sevengill shark abundance, with an increase from spring to autumn followed by a sharp decline in winter. These movements involve long-distance migration from coastal areas after winter, sometimes out to continental shelves, likely driven by a combination of breeding and foraging dynamics. The destination is known for some populations but not others, and very little is known about the habitat of pregnant females.
Individuals show a high degree of site fidelity, with many sharks returning to the same areas the following spring. Movement and catch data also suggest that individuals of different sizes/life stages may be segregated between different regions. There are reports of cannibalism in this species, mainly by sub-adults, so such spatial separation likely reduces predation on newborn sharks, and/or limits competition for available resources.
The morphology and diet of this species is similar to that of the bluntnose sixgill, which prefers deeper water. Thus, it is thought that the sevengill’s use of coastal habitats may reduce competition for resources and predation by the larger sixgill shark.
Sevengill sharks are skilled, opportunistic predators, capable of feeding on a variety of prey including many kinds of bony fish, dolphins, seals, other sharks, rays, and dead matter. They have sharp, jagged upper teeth useful for grasping moving prey. The large, comb-shaped teeth of the lower jaw are for tearing and cutting.
Diet composition shifts among regions and with maturity. Younger sharks feed mainly on bony fish. Juveniles have a relatively large mouth and will also eat the newborns of other shark species. With age, the diet shifts to include more cartilaginous fish, and marine mammals become increasingly important for larger size classes of sevengill sharks.
Direct, thorough observations of shark feeding behavior are uncommon because they are so difficult to undertake. The foraging behavior of this species has been relatively well documented, probably due to its shallow habitat. A variety of tactics are used, including burst speed attacks on predetermined individuals, ambush attacks in low light conditions (e.g. at night), stealth hunting to sneak up on prey, and scavenging. The approach used depends on the size and type of prey.
Studies of this species in protected areas of Tasmania (Australia) have found that seasonal movements and use of space / habitat by the sevengill are similar to that of other sharks and rays that it eats. Predator and prey are all more abundant in warmer months and absent in the winter. This common pattern suggests that sevengill sharks move into coastal areas to track seasonally food species.
Few other species are known to share resources with sharks in the Order Hexanchiformes like the broadnose sevengill. When Hexanchiforms overlap in space with equivalent-sized sharks of other Orders, the other sharks tend to feed at lower trophic levels (i.e. on smaller prey). Although the broadnose sevengill has a similar size and body form as the bluntnose sixgill, their aforementioned difference in preferred water depths separates the habitat of these two species, likely reducing competition for resources.
Sevengill sharks are thought to sometimes hunt in packs. Their diet includes large animals such as marine mammals and other sharks, so teamwork to capture prey would be a good strategy. Further research is required to verify whether this species uses coordinated social behavior.
Males become sexually mature at a length of about 1.5 m (4.9 feet), and have been known to reach 1.07 m (3.5 ft). Females are much larger, reaching maturity at about 2.2 m (7.2 ft) and growing up to 3 m (9.8 ft) in total length.
After their winter migration, females return to coastal areas in the spring prior to males. They move into shallow bays to give birth in the nursery grounds during spring and early summer, after a gestation period that is estimated to last 12 to 24 months (varying reports).
This is the only Hexanchoid species whose reproductive biology has been studied. The limited available information comes from captive individuals in aquaria. Empty egg cases have been observed inside the uterus or lying on the bottom of water tanks, confirming that the breeding system is ovoviviparity. Eggs hatch inside the female and embryos are nourished via a yolksac within the uterus.
The litter size is large, from 60 to 108 pups. Each neonate is about 40-45 cm (16-18″) long. The young sharks stay in these very shallow waters for the first few years.
Humans and Conservation
The broadnose sevengill shark is considered to be potentially dangerous to humans, as it is aggressive when provoked. This shark thrashes and snaps when caught, and is often shot to avoid human injury. There are reports of five unprovoked attacks since the 16th century, and captive sharks have attacked divers in aquaria.
The sevengill shark is hunted for liver oil and leather, and for human consumption because its meat is considered to be high quality. It is taken commercially and recreationally in many parts of its range, and often as bycatch off the California coast. Intensive fisheries caused a steep local decline in San Francisco Bay during the early 1980s. Catches are generally not recorded, but similarly intense fishing pressure is expected across most of the range, especially off Argentina, California, China, southeast Australia, and southern Africa.
The IUCN Red List assessed the broadnose sevengill as “Data Deficient” in 2005. Data for most regions are sparse, so there is not enough information to determine its vulnerability to extinction. However, it is assessed as “Near Threatened” in the eastern Pacific Ocean. Although it is a moderately common shark with a wide geographic range, the preference for shallow water makes it more susceptible to human pressures. Coastal areas are generally more heavily fished than are deeper waters. People can even fish for it from shore. Development, pollution, and resulting habitat degradation are also potential threats to the nursery areas in coastal bays.
The broadnose sevengill is generally unprotected by legislative or management approaches – just another piece of evidence illustrating that the world’s shark populations need more attention.
Written By: Kara Lefevre
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Ebert DA (2002). Ontogenetic changes in the diet of the sevengill shark (Notorynchus cepedianus). Marine and Freshwater Research 53(2) 517-523.