The bullhead sharks are a small order (Heterodontiformes) of basal modern sharks (Neoselachii). The nine living species are placed in a single genus, Heterodontus, in the family Heterodontidae. All are relatively small, with the largest species reaching just 1.65 metres (5.5 ft) in maximum length. They are bottom feeders in tropical and subtropical waters.
The Heterodontiforms appear in the fossil record in the Early Jurassic, well before any of the other Galeomorphii, a group that includes all modern sharks except the dogfish and its relatives. However, they have never been common, and their origin probably lies even further back.
|Horn shark, Heterodontus francisci|
L. S. Berg, 1940
J. E. Gray, 1851
The bullhead sharks are morphologically rather distinctive. The mouth is located entirely anterior to the orbits. Labial cartilages are found in the most anterior part of the mouth. Nasoral grooves are present, connecting the external nares to the mouth. The nasal capsules are trumpet-shaped and well-separated from orbits. Circumnarial skin folds are present, but the rostral process of the neurocranium (braincase) is absent, although a precerebral fossa is present. Finally, the braincase bears a supraorbital crest.
The eyes lack a nictitating membrane. A spiracle is present, but small. The dorsal ends of the fourth and fifth branchial arches are attached, but not fused into a "pickaxe" as in lamniform sharks. Heterodontiforms have two dorsal fins, with fin spines, as well as an anal fin. The dorsal and anal fins also contain basal cartilages, not just fin rays.
Bullhead Sharks have distinctive small spikes on the front of their dorsal fins. These are rumoured to be poisonous, but no further scientific tests have been done to prove this rumor true or false.
Nine living species of bullhead shark are described, with another potential undescribed species in Baja California:
Centroscymnus is a genus of squaliform sleeper sharks in the family Somniosidae.Cirrhoscyllium
Cirrhoscyllium is a genus of carpetsharks in the family Parascylliidae.Cow shark
Cow sharks are a family, the Hexanchidae, of sharks 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.Cow sharks are considered the most primitive of all the sharks, as their skeletons resemble those of ancient extinct forms, with few modern adaptations. Their excretory and digestive systems are also unspecialised, suggesting they may resemble those of primitive shark ancestors. A possible hexanchid tooth is known from the Permian of Japan, making the family a possible extant survivor of the Permian-Triassic extinction.Their most distinctive feature, however, is the presence of a sixth, and, in two genera, a seventh, gill slit, in contrast to the five found in all other sharks. They range from 1.4 to 5.5 m (4.6 to 18.0 ft) in adult body length.
Cow sharks are ovoviviparous, with the mother retaining the egg cases in her body until they hatch. They feed on relatively large fish of all kinds, including other sharks, as well as on crustaceans and carrion.Crested bullhead shark
The crested bullhead shark (Heterodontus galeatus) is an uncommon species of bullhead shark, in the family Heterodontidae. It lives off the coast of eastern Australia from the coast to a depth of 93 m (305 ft). This shark can be distinguished from other members of its family by the large size of the ridges above its eyes and by its color pattern of large dark blotches. It typically attains a length of 1.2 m (3.9 ft).
Nocturnal and bottom-dwelling, the crested bullhead shark favors rocky reefs and vegetated areas, where it hunts for sea urchins and other small organisms. It is oviparous, with females producing auger-shaped egg capsules that are secured to seaweed or sponges with long tendrils. Sexual maturation is slow, with one female in captivity not laying eggs until almost 12 years of age. The International Union for Conservation of Nature has assessed this harmless shark as of Least Concern; it is of no economic interest and suffers minimal mortality from bycatch, recreational fishing, and shark nets.Galapagos bullhead shark
The Galapagos bullhead shark, Heterodontus quoyi, is a bullhead shark of the family Heterodontidae found in the tropical eastern Pacific Ocean between latitudes 0° to 10°S, at depths between 3 and 40 m. It can reach a length of 1.07 m.
The reproduction of this bullhead shark is oviparous.Hexanchus
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. They have been found at depths to 2,500 ft (760 m). Though only two extant species (the bluntnose sixgill shark and the bigeyed sixgill shark) were originally known, a third, the Atlantic sixgill shark, was found to exist.Horn shark
The horn shark (Heterodontus francisci) is a species of bullhead shark, in the family Heterodontidae. It is endemic to the coastal waters off the western coast of North America, from California to the Gulf of California. Young sharks are segregated spatially from the adults, with the former preferring deeper sandy flats and the latter preferring shallower rocky reefs or algal beds. A small species typically measuring 1 m (3.3 ft) in length, the horn shark can be recognized by a short, blunt head with ridges over its eyes, two high dorsal fins with large spines, and a brown or gray coloration with many small dark spots.
Slow-moving, generally solitary predators, horn sharks hunt at night inside small home ranges and retreat to a favored shelter during the day. Their daily activity cycles are controlled by environmental light levels. Adult sharks prey mainly on hard-shelled molluscs, echinoderms, and crustaceans, which they crush between powerful jaws and molar-like teeth, while also feeding opportunistically on a wide variety of other invertebrates and small bony fishes. Juveniles prefer softer-bodied prey such as polychaete worms and sea anemones. The shark extracts its prey from the substrate using suction and, if necessary, levering motions with its body. Reproduction is oviparous, with females laying up to 24 eggs from February to April. After laying, the female picks up the auger-shaped egg cases and wedges them into crevices to protect them from predators.
Horn sharks are harmless unless harassed, and are readily maintained in captivity. They are not targeted by either commercial or recreational fisheries, though small numbers are caught as bycatch. In Mexico this species is used for food and fishmeal, and in California its spines are made into jewelry. The International Union for Conservation of Nature (IUCN) does not yet have enough information to determine the horn shark's conservation status. It faces few threats off the coast of the United States.Japanese bullhead shark
The Japanese bullhead shark (Heterodontus japonicus) is a species of bullhead shark in the family Heterodontidae found in the northwestern Pacific Ocean off the coasts of Japan, Korea, and China. This benthic shark occurs at depths of 6–37 m (20–121 ft) over rocky bottoms or kelp beds. Measuring up to 1.2 m (3.9 ft) long, it can be identified by its short, blunt head, two high dorsal fins with anterior spines, and pattern of irregularly shaped, vertical brown bands and stripes. The Japanese bullhead shark is a docile, slow-swimming species that feeds mainly on shelled invertebrates and small bony fishes. Reproduction is oviparous, with females laying spiral-flanged eggs in communal "nests". This species is of little interest to fisheries.Mexican hornshark
The Mexican hornshark (Heterodontus mexicanus) is a bullhead shark of the family Heterodontidae. This shark is grey-brown in color, with black spots scattered on the fins and body. It has a cylindrical trunk, conical head, and small spiracles behind the eyes. The snout of the Mexican hornshark is very round and blunt. Like all members of the order Heterodontiformes, this shark has fin spines in front of both of its dorsal fins. The first dorsal fin originates before the pectoral fins, while the second dorsal fin originates behind the pelvic fins. The Mexican hornshark reaches a maximum length around 70 cm, but usually reaches between 50 and 60 cm on average. Young hornsharks hatch around 14 cm.Oman bullhead shark
The Oman bullhead shark, Heterodontus omanensis, is a bullhead shark of the family Heterodontidae found in the tropical western Indian Ocean around central Oman, from the surface to a depth of 72 m on the continental shelf. This species can reach a maximum length of 61 cm, but on average, reaches 56 cm. This shark was described in 2005, making it one of the most recently described of its genus. The Oman bullhead shark likely is accidentally caught as bycatch, putting the species at risk.Port Jackson shark
The Port Jackson shark (Heterodontus portusjacksoni) is a nocturnal, oviparous (egg laying) type of bullhead shark of the family Heterodontidae, found in the coastal region of southern Australia, including the waters off Port Jackson. It has a large, blunt head with prominent forehead ridges and dark brown harness-like markings on a lighter grey-brown body, and can grow up to 1.65 metres (5.5 ft) long.The Port Jackson shark is a migratory species, traveling south in the summer and returning north to breed in the winter. It feeds on hard-shelled mollusks, crustaceans, sea urchins, and fish. Identification of this species is very easy due to the pattern of harness-like markings that cross the eyes, run along the back to the first dorsal fin, then cross the side of the body, in addition to the spine in front of both dorsal fins.Proscyllium venustum
The spotted smooth dogfish (Proscyllium venustum) is a finback catshark of the family Proscylliidae, found in the temperate northwest Pacific Ocean, in the Okinawa Trough. Little else is known about this harmless oviparous species.Sandbar shark
The sandbar shark (Carcharhinus plumbeus) is a species of requiem shark, and part of the family Carcharhinidae, native to the Atlantic Ocean and the Indo-Pacific. It is distinguishable by its very high first dorsal fin and interdorsal ridge. It is not to be confused with its similarly named shark, the sand tiger shark, Carcharias taurus.Scymnodalatias
Scymnodalatias is a genus of squaliform sharks in the family Somniosidae.Squalidae
The Squalidae, also called dogfish sharks or spiny dogfishes, are a family of sharks in the order Squaliformes. They have two dorsal fins, each with smooth spines, but no anal fin, and their skin is generally rough to the touch. Unlike virtually all other shark species, dogfish sharks possess venom which coats their dorsal spines – this venom is mildly toxic to humans.
These sharks are characterized by teeth in upper and lower jaws similar in size; a caudal peduncle with lateral keels; the upper precaudal pit usually is present; and the caudal fin is without a subterminal notch.
They are carnivorous, principally preying upon organisms smaller than themselves.
The livers and stomachs of the Squalidae contain the compound squalamine, which possesses the property of reduction of small blood vessel growth in humans.Two genera are known: Squalus, which contains numerous species, and Cirrhigaleus, which has three species.Squaliolus
Squaliolus is a genus of deep-sea squaliform sharks in the family Dalatiidae.Whitespotted bullhead shark
The whitespotted bullhead shark, Heterodontus ramalheira, is a bullhead shark of the family Heterodontidae found in the western and northern Indian Ocean between latitudes 22°N to 26°S, at depths between 40 and 305 m. It can grow up to a length of 83 cm.
Little is known about the whitespotted bullhead shark. It is found on the outer continental shelf and is thought to feed on crabs, based on the gut contents of two specimen. As a member of the genus Heterodontus, it is thought to be oviparous, but egg case of this species have never been seen.Wobbegong
Wobbegong is the common name given to the 12 species of carpet sharks in the family Orectolobidae. They are found in shallow temperate and tropical waters of the western Pacific Ocean and eastern Indian Ocean, chiefly around Australia and Indonesia, although one species (the Japanese wobbegong, Orectolobus japonicus) occurs as far north as Japan. The word wobbegong is believed to come from an Australian Aboriginal language, meaning "shaggy beard", referring to the growths around the mouth of the shark of the western Pacific.Zebra bullhead shark
The zebra bullhead shark (Heterodontus zebra) is a bullhead shark of the family Heterodontidae found in the central Indo-Pacific between latitudes 40°N and 20°S, from Japan and Korea to Australia. It is typically found at relatively shallow depths down to 50 m (160 ft), but off Western Australia, it occurs between 150 and 200 m (490 and 660 ft). It can reach a length of 1.25 m (4.1 ft). The reproduction of this bullhead shark is oviparous.