Bullhead Sharks — 9 species
- snout pig-like, deep nasoral groove connecting nostrils to mouth corners
- prominent ridge above each eye; eyes without nictitating fold
- five pairs of gill slits, the last three above the pectoral fin base; spiracles tiny
- a stout spine on the forward margin of both dorsal fins
- presence of an anal fin (which serves to distinguish them from the spiny dogfishes, order Squaliformes)
- in adults, molar-like crushing teeth in rear of the jaws that are very different from the conical grasping teeth in front (hence the group's scientific name, which means "different teeth")
- oviparous; egg case auger-shaped, conical with two broad flanges spiraling from apex to base
- bottom-dwellers in shallow coastal waters (usually less than 300 feet or 100 metres); exclusively marine, inhabiting warm temperate and tropical parts of the western and eastern Pacific and western Indian oceans
- a single genus (Heterodontus) in the family Heterodontidae
The heterodontoids just don't get the respect they deserve as card-carrying, union-member sharks. It's not their fault — they are victims of their own appearance. Bullhead sharks have a pig-like snout and a small puckered mouth, a surprised expression punctuated by bovine brow-ridges, and a chubby little body sparkling with over-sized scales. Bullheads move with an endearing clumsiness — either clambering over the bottom on large, paddle-like pectoral fins or swimming with an exaggerated wriggle, resembling fat-bellied pollywogs. An embodiment of piscine cuteness, bullheads seem an ideal antidote to the pernicious image with which all sharks have been saddled since JAWS.
Heterodontoids are nocturnally active predators; juveniles feed on buried worms, adults on small fishes, crustaceans, and molluscs. Bullhead shark table manners range from unrefined to downright comical. When consuming hard-shelled invertebrate prey, bullheads typically grind up the food and swallow it — shell and all — regurgitating the hard parts later. Off California, the Horn Shark (Heterodontus francisci) has a definite predilection for red sea urchins, which often results in its teeth being stained reddish-brown - like the tongue of a child that has just finished a grape popsicle. Horn sharks apparently also appreciate interactive novelty snacks, as some have been observed to 'pounce' on sea anemones, nipping off a mouthful of tentacles before the hapless invertebrate could retract them.
Horn Shark (Heterodontus francisci)
Bullheads are smallish sharks, the various species growing to lengths of 4 to 5.6 feet (1.2 to 1.7 metres). Their modest size renders bullheads vulnerable to predation. The characteristic fin spines of bullheads are an effective anti-predator device, discouraging larger fishes from eating these snack-sized sharks. On several occasions, Pacific Angel Sharks (Squatina californica) off California have been filmed swallowing juvenile Horn Sharks, only to disgorge them with much haste and what looks for all the world like abject disgust. There is even a report from Australia of a Port Jackson Shark (Heterodontus portusjacksoni) stuck in the mouth of a dead wobbegong, which was apparently asphyxiated by its prickly last meal.
The egg cases of bullhead sharks are among the most wonderfully bizarre in all of Nature. They are auger-shaped: more-or-less conical and surrounded by two broad spiral flanges. This shape enables a mother bullhead to wedge her eggs (usually laid two at a time) into crevices, preventing removal by most would-be predators without benefit of opposable thumbs. The Crested Bullhead, (Heterodontus galeatus) has egg cases with a pair of tendrils, which readily tangle around kelp structures, anchoring them. The Japanese Bullhead (Heterodontus japonicus) apparently uses communal nesting sites — with as many as 15 eggs deposited in the same patch — using them as a kind of heterodontoid day care. Due to their awkward shape, each bullhead egg case requires several hours to rotate out of the mother shark's cloaca. Even in the very serious matter of perpetuating their lineage, heterodontoids are less-than-dignified.
Bullhead eggs hatch after a gestation period of 5 to 12 months, depending upon species and water temperature. Size at birth varies with species, ranging from 5.5 to 9.5 inches (14 to 24 centimetres) in length. Recent studies by Wesley Strong off California indicate that adult horn sharks utilize a home range of up to 10,760 square feet (1,000 square metres); tagging studies in eastern Australia have revealed that the Port Jackson Shark makes annual migrations of up to 525 miles (850 kilometres) along the coast. Bullheads do well in private and public aquaria, the Horn Shark both breeding and holding the record for longevity in captivity at 25 years.