Horn Shark

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. The body is cylindrical, with two high, somewhat falcate (sickle-shaped) dorsal fins bearing stout spines at the front. The fin spines of reef-dwelling horn sharks are shorter than those living in algal habitats, as their spines become worn down on rocks from the sharks' movements. The first dorsal fin originates over the bases of the large pectoral fins, while the second dorsal fin originates slightly anterior to the free rear tips of the pelvic fins. …

The horn shark inhabits the continental shelf of the eastern Pacific Ocean, occurring off the coasts of California and Baja California from Monterey Bay southward, and in the Gulf of California. Uncommon influxes of warm water northward may bring it as far as San Francisco Bay. There are unconfirmed reports of this species off Ecuador and Peru, which may be misidentifications of other species. For most of the year, horn sharks are most common at a depth of 2–11 m (6.6–36.1 ft). At the onset of winter, they migrate to water deeper than 30 m (98 ft). This species has been found in caves as deep as 200 m (660 ft). Juvenile horn sharks between 35–48 cm (1.15–1.57 ft) long prefer sandy flats with low vertical relief, in water 40–150 m (130–490 ft) deep. They often take advantage of large feeding pits excavated by the bat ray (Myliobatis californica) for shelter and food. As they mature, horn sharks shift into shallower water and their preferred habitat becomes structurally complex rocky reefs or algae beds. This strongly benthic species seldom ventures more than 2 m (6.6 ft) above the substrate. The relative abundances of the horn shark and the swell shark (Centroscyllim ventriosum), which shares the same habitat, are negatively correlated because horn sharks prefer temperatures warmer than 20 °C (68 °F) while swell sharks are more cold-tolerant. …

The horn shark is a sporadic swimmer that prefers to use its flexible, muscular pectoral fins to push itself along the bottom. It is usually solitary, though small groups have been recorded. During the day, horn sharks rest motionless, hidden inside caves or crevices, or within thick mats of algae, though they remain relatively alert and will swim away quickly if disturbed. After dusk, they roam actively above the reef in search of food. Horn sharks maintain small home ranges of around 1,000 m2 (11,000 ft2), which they may remain faithful to for over a decade, returning to the same shelter every day. The shelter is usually located at the edge of the resident shark's foraging area. The longest documented movement for an individual horn shark is 16 km (9.9 mi). The daily activity pattern of the horn shark is under exogenous control, meaning that it is regulated by environmental factors rather than by an internal physiological cycle. Observations of captive horn sharks show that the relevant cue is light intensity: the sharks become active immediately after the lights are turned off, and stop as soon as they are turned back on. In one experiment where the sharks were kept in darkness, they remained continuously active for 11 days before slowing, possibly from fatigue. …