The Hexanchiformes are a primitive order of sharks, that numbering just seven extant species in two families. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens.
A sawshark or saw shark is a member of a shark order bearing a unique long, saw-like rostrum edged with sharp teeth, which they use to slash and disable their prey. There are eight species within the Pristiophoriformes, including the longnose or common sawshark, shortnose sawshark, Japanese sawshark, Bahamas sawshark, sixgill sawshark, African dwarf sawshark, Lana's sawshark and the tropical sawshark.
Thresher sharks are large mackerel sharks of the family Alopiidae found in all temperate and tropical oceans of the world; the family contains three extant species, all within the genus Alopias.
Cow sharks are a shark family, the Hexanchidae, characterized by an additional pair or pairs of gill slits. Its 37 species are placed within the 10 genera: Gladioserratus, Heptranchias, Hexanchus, Notidanodon, Notorynchus, Pachyhexanchus, Paraheptranchias, Pseudonotidanus, Welcommia, and Weltonia.
The lemon shark is a species of shark from the family Carcharhinidae and is classified as a Vulnerable species by the International Union for the Conservation of Nature. Lemon sharks can grow to 3.4 metres (11 ft) in length. They are often found in shallow subtropical waters and are known to inhabit and return to specific nursery sites for breeding. Often feeding at night, these sharks use electroreceptors to find their main source of prey, fish. Lemon sharks enjoy the many benefits of group living such as enhanced communication, courtship, predatory behavior, and protection. This species of shark gives birth to live young, and the females are polyandrous and have a biennial reproductive cycle. Lemon sharks are not thought to be a large threat to humans; there have been 10 recorded bites, none of which were life-threatening. The lemon shark's life span is unknown, but the average shark is 25 to 30 years old.
The Atlantic blue marlin is a species of marlin endemic to the Atlantic Ocean. It is closely related to, and usually considered conspecific with, the Indo-Pacific blue marlin, then simply called blue marlin. Some authorities consider both species distinct.
The prickly shark is one of the two species of sharks in the family Echinorhinidae, found in the Pacific Ocean over continental and insular shelves and slopes, and in submarine canyons. Bottom-dwelling in nature, it generally inhabits cool waters 100–650 m (330–2,130 ft) deep, but it also frequently enters shallower water in areas such as Monterey Bay off California. This stocky, dark-colored shark grows up to 4.0 m (13.1 ft) long, with two small dorsal fins positioned far back on its body and no anal fin. It is characterized by a dense covering of thorn-like dermal denticles, hence its common name.
Pelagic fish live in the pelagic zone of ocean or lake waters—being neither close to the bottom nor near the shore—in contrast with demersal fish that live on or near the bottom, and reef fish that are associated with coral reefs.
The bluntnose sixgill shark, often simply called the cow shark, is the largest hexanchoid shark, growing to 20 ft (6.1 m) in length. It is found in tropical and temperate waters worldwide and its diet is widely varied by region.
The sixgill sharks are a genus, Hexanchus, of deepwater sharks in the family Hexanchidae. These sharks are characterized by a broad, pointed head, six pairs of gill slits, comb-like, yellow lower teeth, and a long tail. The largest species can grow up to 8 m long and weigh over 600 kg (1320 lb). They are continental shelf-dwelling and abyssal plain scavengers with a keen sense of smell and are among the first to arrive at carrion, together with hagfish and rattails. They show a characteristic rolling motion of the head when feeding.
The bigeyed sixgill shark is a cow shark of the family Hexanchidae. Its dorsal surface has a brownish-gray color, and is sharply separated from the light coloring of its ventral surface. The eyes are a fluorescent green while the shark is alive. The body of this shark is small, slim, and fusiform in shape. As the name suggests, this shark has six gill slits, unusual among most shark species. The head is narrow and somewhat flattened, and the mouth contains 5 rows of large, comb-shaped teeth. This shark's single dorsal fin is pushed back towards the caudal fin, and is behind the pelvic fins. The upper caudal fin is much longer than the lower, with a deep notch near the tip. All fins have thin white margins on the edge. In juveniles, the upper caudal fin has a black tip.
The bigeye thresher is a species of thresher shark, family Alopiidae, found in temperate and tropical oceans worldwide. Like the other thresher sharks, nearly half its total length consists of the elongated upper lobe of the tail fin. Its common name comes from its enormous eyes, which are placed in keyhole-shaped sockets that allow them to be rotated upward. This species can also be distinguished by a pair of deep grooves on the top of its head, from which its scientific name is derived.
The sixgill stingray is a species of stingray and the only extant member of the family Hexatrygonidae. Although several species of sixgill stingrays have been described historically, they may represent variations in a single, widespread species. This flabby, heavy-bodied fish, described only in 1980, is unique among rays in having six pairs of gill slits rather than five. Growing up to 1.7 m (5.6 ft) long, it has a rounded pectoral fin disc and a long, triangular, and flexible snout filled with a gelatinous substance. It is brownish above and white below, and lacks dermal denticles.
The bigeye sand tiger is an extremely rare species of mackerel shark in the family Odontaspididae, with a possible worldwide distribution. A large, bulky species reaching at least 3.6 m (12 ft) in length, the bigeye sand tiger has a long bulbous snout, large orange eyes without nictitating membranes, and a capacious mouth with the narrow teeth prominently exposed. It can be distinguished from the similar smalltooth sand tiger by its teeth, which have only one lateral cusplet on each side, and by its uniformly dark brown color.
The sixgill sawshark, Pliotrema warreni is a sawshark of the family Pristiophoridae. Presence of 6 pairs of gill slits highlights this genus among sharks; outside Hexanchiformes order, Pliotrema is the only shark with more than 5 gill slits. Unlike other sawsharks, the barbs on this shark's rostrum continue onto the sides of the head. Its barbels are also closer to its mouth than in other species. At maximum, females can reach over 136 cm long, and males can reach over 112 cm long.
Polyopisthocotylea is a subclass of parasitic flatworms in the class Monogenea.
Protocotyle euzetmaillardi is a species of monogenean of the family Hexabothriidae.
Protocotyle is a genus of monogeneans in the family Hexabothriidae. The genus was created by Louis Euzet and Claude Maillard in 1974.
The Sahara Seamounts are a group of seamounts which are located in the Atlantic Ocean between 140 and 190 mi southwest of the island of El Hierro, part of the Canary Islands. A number of these seamounts are placed on the sea bed at depths of almost 4,000 m (13,000 ft) and some summits are just 200 m (660 ft) below sea level. An example of this is the seamount called Echo, which is also called the Endeavour Bank, where the summit is made up of a large plateau covering an area of 350 km2 (140 sq mi), at depths of between 230 and 350 m and with the slopes dropping down to 3,800 m (12,500 ft) into the deep. The presence of seismic lines around these seamounts show local unconformity or even sills which are quite high up among the sedimentary series and these features suggest that there was volcanic activity as late as at least the Cenozoic.
