Bigeye sixgill shark (Hexanchus nakamurai) is a slender deepwater shark, with a small dorsal fin that extends along its back. It has large fluorescent green eyes and six gills instead of the usual five gills that most sharks have. More so, it is a cow shark and belongs to the family of Hexanchidae.
Bigeye sixgill shark has a light to the dark gray coloration on top that is fading to white underneath. This shark has other common names such as Seskiefhaai in Afrikaans and griset in French.
It is important to note that the bigeye sixgill shark is deficient in data which makes it impossible for much to be known about it. However, the little information about this Shark species based on observation is what we present on this page.
Bigeyed Sixgill shark size, age, and Growth
These sharks have been found to grow up to about 6 feet (1.8 m) in length, although their average growth size is about 4 feet (1.2 m). Bigeye sixgill sharks weigh about 20kg when they mature. And, they reach sexual maturity at an average length of 4 feet to 5.8 feet (1.2 to 1.78 m). However, their males tend to mature at 4.0 to 5.1 feet in total length while their females at 4.6 – 5.8 feet in length.
How to Identify a Bigeyed Sixgill shark
These sharks are slim and spindle-form in shape. As their name implies, they have sixgill compared to other sharks who would commonly have only five. Their heads are narrow and somewhat flattened.
The Bigeye sixgill shark has 5 rows of big, comb-shaped teeth in their mouth. Its dorsal fin is laid back towards their caudal fin and it is behind the pelvic fins. This shark’s upper caudal fin is longer than the lower with a deep notch close to the tip. Also, all their fins have white margins on the edge. When in a juvenile state, their upper caudal fin has a black tip.
Bigeyed Sixgills Habitat
These sharks are widespread around the world but are mainly found in the Caribbean Sea, the Mediterranean Sea, the west-central Atlantic, the western Pacific Ocean, and the Indian Ocean. Bigeye sixgill sharks live close to the seafloor about 90 and 600 m deep. Sometimes, they may move closer to the surface in the night to close in on prey there. These sharks make their home on continental shelves, upper slopes, and insular shelves. Bigeye sixgill sharks do not inhabit abyssal surroundings.
This shark is a member of the most ancient cow sharks and frill. As such, they have a single dorsal fin and no protective third eyelids. There are generally four known cow sharks and they are Bigeyed sixgill sharks (Hexanchus nakamurai), Sevengill sharks (Notorynchus cepedianus), Sharpnose seven gills (Heptrancias perlo), Bluntnose six gills sharks (Hexanchus griseus)
Differences between the Bigeyed sixgill shark and other sixgill sharks
Although these cow sharks appear similar, they live in different ecological niches. For example, the Broadnose sixgill shark is larger than bigeyed six gills and is primarily a nocturnal deepwater shark most times found in a water depth of about 600 – 3600 feet. Their distribution is worldwide in tropical and temperate waters. And, they feed on wide arrays of fishes and crustaceans.
Bluntnose seven-gill cow sharks from its name are different because of its seven gills and not six compared to the bigeyed six gills shark. And also, they tend to be quite aggressive and prefer shallow waters near the shore. Therefore, they mostly occur in water depths of 12 – 36 feet. Bluntnose seven gills shark feed on varieties of prey ranging from other sharks to other marine animals. More so, on close observation, they tend to hunt in packs or groups to tackle larger prey.
Behaviour of Bigeyed sixgill Sharks
There is only a little knowledge about these sharks feeding habits. However, based on little of their stomach examinations, they majorly feed on small to moderately sized crustaceans and bony fishes. Also, a small tuna was found in a specimen’s stomach which suggests that bigeyed sixgill shark may be feeding on the surface of the water.
Most of the sharks in the order Hexanchiformes are ovoviviparous which means that they retrain their eggs in the female’s body until hatching. This shark with a period of gestation unknown gives birth to litters of up to 12- 14 pups. Because of these sharks’ small size, their usual predators are larger sharks.
Conservation
Just like most of the deepwater animals, the bigeyed sixgills shark (Hexanchus nakamurai) on the IUCN Red list is “DD” i.e Data Deficient based on the little information available on this species. These sharks may not be suffering from population decline, but there is pressure on deepwater fishes.
Do Bigeyed Sixgill sharks attack Humans?
On the International Shark Attack file, Bigeyed sixgill sharks are harmless fishes because their Natural environment is far from humans. As such, there is No record of attacks on humans yet by this species of shark.
Bigeyed sixgills Parasites
Bigeyed sixgill sharks also harbor quite a number of parasites just like other sharks. Monogeneans of genus Protocotyle of the family of hexabothriidae have been found on gills of Hexanchus species.
On the other hand, Protocotyle euzetmaillardi species has also attacked Hexanchus Nakamurai (Bigeyed sixgills shark) as a parasite of the South Pacific Ocean.
Taxonomy
Bigeyed Sixgill was described originally as Hexanchus griseus Nakamurai due to their close similarities to bluntnose sixgill sharks Hexanchus griseus. Nevertheless, the name changed later to Hexanchus Nakamurai, the genus name derived from Greek “Hexa” which means “six” and “ancho” which means “narrow”.
Geographical Distribution
These sharks are extant from morocco to France south that includes the Mediterranean Sea. They also reside off the coasts of Cote d’Ivoire, and in Nigeria. In the Indian ocean, bigeyed sixgills sharks lived off southern coastal and the eastern of the Aldabra Island and African continent. They are also extant in the Philippines, Taiwan, Australia, and New Caledonia.
Divers Reaction: It does not apply to bigeyed sixgills shark. A captured specimen showed aggressive behavior when manhandled during release.
Diving Logistics: It does not apply to Bigeyed sixgill shark
Dentition of Bigeyed Sixgill Sharks
Their centrally located mouth has nine teeth on each side of their upper jaw and five teeth on each side of their lower jaw. The first two teeth of bigeyed sixgill in the upper jaw with narrowed hooked cusps have no cusplets. However, their other remaining teeth grow wider and have more lateral cusplets toward the corners of their upper jaw.
Bigeyed sixgill sharks have a small symphysial tooth that is centrally located on their lower jaw. On each side of their lower jaw, they have five large teeth that are comb-like and wide with several large cusplets and a cusp.
Side-note that you might find interesting
Richard Waller and Stewart Springer identified this shark from specimens caught in the western North Atlantic ocean and named it Hexanchus vitulus. What they failed to realize was that seven years prior to theirs, H.T. Teng already described this same species in his doctoral dissertation.
However, there arose an argument about whether Teng’s Identification was from a formal publication. In the end, there was a conclusion in 1991, that the name for these species would be H.nakamurai, and has since then become a valid name.
There was a later wide distribution of Teng’s dissertation to Japanese ichthyologists after it became written originally. This conclusion by Teng now has wide acceptance and some revisions in upcoming publications.
Importance of Bigeyed sixgills shark to Humans
They do not have much importance for humans. However, their flesh, liver, and fins may be of commercial importance. Fishermen would catch Bigeyed six gills sharks in trawls and on line gear.
Conclusion
This page contains the little information known about the Bigeyed Sixgill Shark. Of course, with the little already explained, you would know that these are lovely creatures, especially with their distinctive features. Thus, may want to learn more.
However, these fishes are Data Deficient (DD) on the IUCN Red List. Therefore, not all of their behaviors are clear. But, be sure that you cannot keep this species as pets.
References
https://en.m.wikipedia.org/wiki/Bigeyed_sixgill_shark
https://sharkattackdawnofthedead.weebly.com/bigeye-sixgill-shark.html
https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/hexanchus-nakamurai/