Sharpnose Sevengill Shark

The sharpnose sevengill (Heptranchias perlo) is the only shark in its genus. This is the smallest “cow shark” (member of the Hexanchidae family), alternatively known as the Perlon Shark, One-finned Shark, Sevengill Cow Shark, Sharpsnouted Sevengill, or Slender Sevengill. Compared to other sharks, little is known about this wide-ranging family that frequents deep waters. Hexanchoids are considered to be the most primitive of the modern sharks, as they are most similar to sharks from the fossil record. They have relatively simple skeletal, digestive, and excretory systems.

The sharpnose sevengill is a brownish-grey shark with a lighter belly. It has a slender, cigar-shaped body, green eyes, and a long, narrow mouth. It can be distinguished from the only other sevengill (the broadnose sevengill shark, Notorynchus cepedianus) by its noticeably larger eyes, narrow head, smaller body without black spots, and the presence of five rows of large teeth in the lower jaw, compared to six rows in the broadnose. This species has just one small dorsal fin, set far back on the body.

Sharpnose Sevengill Shark Facts

This species reaches a maximum length of 1.4 m (4.6 ft). Its name describes the presence of seven paired gill openings that extend down to the throat, while most sharks have only five.

It is imagined to be a strong swimmer, based on its wide geographic range.

Habitat and Range

The sharpnose sevengill shark occurs widely throughout the world’s temperate and tropical seas, other than the Eastern North Pacific. It is common in only a few areas, with virtually nothing known of its biology and population ecology.

The typical habitat occurs in deep waters of insular or outer continental shelves, usually from 300 to 600 m (984 to 1970 ft), but ranging from 27 to 1000 m (90 to 3,280 feet). Sharpnose sevengills tend to stay on or near the sea floor, occasionally venturing to the surface (although occasional reports from shallow water could be misidentifications of the broadnose sevengill).

Sharpnose Sevengill Shark Habitat Map

Feeding Ecology

The sharpnose sevengill shark has a more specialized diet with lower prey diversity, compared to other cow sharks. This skilled predator feeds mainly on smaller fish of the open sea, but also small sharks and rays, squid, and shellfish. During the night is when the peak feeding activity occurs. Diet composition changes slightly with location, and also over development, with older sharks consuming larger types of fish.

There is potential for resource competition among different cow sharks, especially the deep-water species. The sharpnose sevengill has a similar depth range, diet, body size, and morphology as the bigeye sixgill shark (Hexanchus nakamurai), so there could be resource sharing between them in areas where the species overlap. The sharpnose diet differs from the broadnose sevengill’s, yet these two sharks share similar methods of prey handling. Like the broadnose (refer to our article on that species), different sized/ aged sharpnose individuals use different hunting tactics. Similarly, the sharpnose sevengill also concentrates on prey that migrate from deeper to coastal waters-this suggests that perhaps the sharpnose also undertakes seasonal migrations, like the broadnose sevengill.

Social Behavior and Species Interactions

This species is thought to be more common on outer shelves, slopes, and around seamounts, where individuals may aggregate. Seamounts are uprisings in the ocean bed that don’t quite reach the water’s surface. The effect of these raised marine landscapes on currents seems to be increased local food richness, causing the attraction of plankton, fish, and other organisms. Thus, these areas tend to support increased fishing activity.

Larger sharks are potential predators of the sharpnose sevengill.

Few other species share resources with sharks of the Order Hexanchiformes. When they overlap, similar-sized squaloid, lamnoid and carcharhinoid sharks generally feed on smaller prey. However, being smaller, the sharpnose sevengill may be more likely to experience competition than the other cowsharks. For example, it is similar in size to many gulper sharks (Centrophorus spp.).

Breeding Biology

Males reach sexual maturity at 75 to 85 cm (30 to 34″) in length. As is true for other cow sharks, females are larger, reaching maturity at 90 to 105 cm (35 to 41″, roughly 3 ft).

Breeding is thought to occur year-round, however little is known of the details, including gestation time and breeding interval. The breeding system is ovoviviparous: a litter of 6 to 20 pups is born after hatching and developing within the mother. They are about 25 cm (10″) long at birth.

Humans and Conservation

Sharpnose sevengill sharks are not considered to be a threat to humans. Of course they are strong fish with sharp teeth, so that doesn’t mean they are docile! They have been kept in captivity in Japan.

Apparently the meat is mildly poisonous, so this species is not a target of fisheries. However other reports claim that is used for eating and as fishmeal, when landed asbycatch from efforts to catch other deep-water fish with trawls and longlines.

No clear information exists about the population status of this species. Declines are suspected in regions with active deep-water fisheries, a growing activity worldwide. TheIUCN Red List considers the sharpnose sevengill as “Near Threatened” (2003), because although it has a wide range, it is generally uncommon where it occurs. Further, areas where it may be more common are also the site of greater fishing activity, and hence increased human pressure. The concern is that this species, like most sharks, probably has a low reproductive rate. If so it would not respond quickly to depletion. No known conservation measures are in place to protect the sharpnose sevengill shark.

Written By: Kara Lefevre


 

Sources

Barnett A, Braccini JM, Awruch CA & Ebert DA (2012). An overview on the role of Hexanchiformes in marine ecosystems: biology, ecology and conservation status of a primitive order of modern sharks. Journal of Fish Biology 80(5):966–990.

Braccini JM (2008). Feeding ecology of two high-order predators from south-eastern Australia: the coastal broadnose and the deepwater sharpnose sevengill sharks. Marine Ecology Progress Series 371:273–284.

Florida Museum of Natural History

Paul L & Fowler S (2003). Heptranchias perlo. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2.

Wikipedia: Cow Shark

Wikipedia: Seamount


 

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