Horn shark

Species of 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 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.

The French biologist Charles Frédéric Girard published the first scientific description of the horn shark under the name Cestracion francisci in 1855, in the Proceedings of the Academy of Natural Sciences of Philadelphia. The type specimen from Monterey Bay has since been lost. The scientific name for this species has been given erroneously as Heterodontus californicus.

Like other bullhead sharks, the horn shark has a short, wide head with a blunt snout and prominent supraorbital ridges over the eyes. The horn shark's supraorbital ridges are low and terminate abruptly; the space between them on top of the head is deeply concave. Each eye lacks a nictitating membrane and is followed by a tiny spiracle. The nostrils are split into inflow and outflow openings by a long flap that reaches the mouth. The inflow openings are encircled by a groove, while another groove connects the outflow openings to the mouth. The mouth is small and curved, with prominent furrows at the corners. There are 19–26 tooth rows in the upper jaw and 18–29 tooth rows in the lower jaw. The teeth at the front of the jaws are small and pointed, with a central cusp flanked by a pair of lateral cusplets; those at the sides of the jaws are much larger, elongated lengthwise, and molar like. This strongly benthic species seldom ventures more than above the substrate. The horn shark is preyed upon by larger fishes and the northern elephant seal (Mirounga angustirostris), which consumes adults, juveniles, and egg cases. In addition, they are captured and eaten by bald eagles (Haliaeetus leucocephalus) at Catalina Island, and large marine snails are able to drill into their egg cases to extract the yolk.

95% of the adult horn shark's diet consists of hard shelled mollusks (e. g. bivalves and gastropods), echinoderms (e. g. sea urchins) and crustaceans (e. g. crabs, shrimp, and isopods). To crack their shells, the horn shark generates the highest known bite force relative to its size of any shark, well in excess of other measured species such as the spiny dogfish (Squalus acanthias) and the blacktip shark (Carcharhinus limbatus). Large horn sharks that feed mainly on sea urchins (particularly the short spined purple urchin, Strongylocentrotus purpuratus) have their teeth and fin spines stained purple. Horn sharks hunt mainly using their sense of smell. Like other sharks, the horn shark's teeth are regularly replaced; it takes 4 weeks for a dropped tooth to be replaced. The horn shark captures prey via suction, created by expanding its buccal cavity. Its labial cartilages are modified so that the mouth can form a tube, facilitating the suction force. Once the prey is drawn into the mouth, it is secured with the sharp front teeth and then ground into pieces by the flat lateral teeth. To extract buried or affixed prey, the horn shark grips it and adopts a vertical posture with the head and pectoral fins against the substrate and the tail arched above. The shark then acts as a lever with its pectoral fins as the fulcrum: with a downward stroke of the tail, it forces its head upwards and pulls the prey loose; this mode of feeding has not been observed in any other shark. The horn shark is also capable of protruding its upper jaw up to 15% the length of its head; this motion takes only 20 milliseconds to accomplish and allows the shark to use its upper jaw like a chisel to dislodge firmly attached prey.

Mating in the horn shark occurs in December or January, on a possibly annual reproductive cycle. At first the case is soft and light brown, and over a few days it hardens and darkens in color. Not including the flanges, the case measures long and wide; sharks from the Channel Islands produce longer egg cases than those from mainland California, suggesting that they are separate populations. The horn shark has no commercial value in California, where it is captured unintentionally in traps and trawls and by recreational anglers. The shark's hardiness ensures that it can often be returned to the water alive. Divers sometimes kill them for sport or to make jewelry out of their fin spines, which may be the cause of a decline in the numbers of horn sharks in the most intensely dived areas of southern California. Off Mexico, this species is caught incidentally in shrimp trawls and demersal gillnets, and used for human consumption and fishmeal. The expansion of Mexican gillnet fisheries may pose a conservation concern in the future. At present, the International Union for Conservation of Nature (IUCN) does not have sufficient information to assess the overall conservation status of this species; its status in United States waters is likely Least Concern.