Resilient beautiful little nocturnally active shark with large spines
The Horn shark (Heterodontus francisci) is a species of Bullhead shark, belonging to the family Heterodontidae. 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 preferred shelter during the day. Their daily activity cycles are controlled by environmental light levels. They lay auger-shaped egg cases that are wedged and hidden in rock crevices.
Family: Heterodontidae – Bullhead Sharks
Common Name– Bullhead Sharks
Common Name– Bullhead Sharks
Status: IUCN Red List DATA DEFICIENT
Average Size and Length: The egg case is 10-12 x 3-4 cm. Horn sharks from the Channel Islands produce longer egg cases than those from mainland California, suggesting that they are separate populations. They are born between 5.9-6.3 inches. Mature males measure between around 1.9 feet. Females grow larger. The longest recorded was 122 cm/ 4 feet.
Current Rare Mythical Sightings: 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. It was later placed in the genus Gyropleurodus, which was eventually synonymized with the genus Heterodontus. The specific epithet francisci is a reference to San Francisco, although the range of the Horn shark does not extend that far north. The type specimen from Monterey Bay has since been lost.
Teeth and Jaw: The inflow openings of the mouth 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. They have powerful crushing jaws. 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. The ones at the sides of the jaws are much larger, elongated lengthwise and are molar-like. The Horn shark’s teeth are regularly replaced; it takes 4 weeks for a dropped tooth to be replaced.
Head: 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 nictating 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.
Denticles: The Horn shark’s dermal denticles are small and smooth, numbering some 200/cm2 on the back in adults.
Tail: The caudal fin has a short lower lobe and a long, broad upper lobe with a strong notch near the tip.
Demographic, Distribution, Habitat, Environment and Range: The Horn shark can be found in the east Pacific in the USA, mostly along the southern coast of California. They more than likely are found in Ecuador and Peru. They can be found in the intertidal zone to at least 492 feet, but mainly between 7-36 feet. They live among rocks in deep crevices and caves (possibly as deep as 660 feet), among kelp, sandy gullies and flats. Juvenile sharks will find shelter on the sand. Their home range is small and limited. They may possibly partake in limited winter migrations into deep water in the north. It is possibly that they may 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.
The relative abundances of the Horn shark and the Swellshark (Centroscyllim ventriosum), which shares the same habitat, are negatively correlated because Horn sharks prefer temperatures warmer than 68 °F, while Swellsharks are more cold-tolerant. At Santa Catalina Island, a 20-year warming trend has resulted in an increase in the Horn shark population and a decrease in the Swellshark population. Horn sharks are less common than Swellsharks in the northern Channel Islands, where the water is cooler.
Horn sharks maintain small home ranges of around 1,000 m2 or 11,000 square feet, 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 19.9 miles.
Diet: They feed on benthic invertebrates and very rarely on small fish. Adult sharks prey mainly (95%) on hard-shelled mollusks, echinoderms, and crustaceans, which they crush between powerful jaws and molar-like teeth. Juveniles prefer softer-bodied prey such as polychaete worms and sea anemones. Juveniles have been known to leap on anemones to bite off tentacles before they can be retracted. 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!
Off southern California, Horn sharks are known to take advantage of seasonal opportunities. In the summer, diurnally active fishes, like the Blacksmith (Chromis punctipinnis), are especially abundant and are easily captured at night when they lie dormant. In the winter, the sharks scavenge on Market squid (Loligo opalescens), which die by the tens of thousands after their mass spawning event.
The Horn shark is preyed upon by larger fish and the Northern Elephant seal (Mirounga angustirostris). They will eat adults, juveniles, and egg cases. In addition, there are some accounts of them being captured and eaten by Bald eagles (Haliaeetus leucocephalus) at Catalina Island. Large marine snails are able to drill into their egg cases to extract the yolk. A Pacific angelshark was filmed engulfing a juvenile Horn shark, and immediately spitting it out because of the spines.
Ram-Suction Index: The Horn shark captures prey with 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.
To crack the shells of their prey, the Horn shark generates the highest known bite force relative to its size of any shark. One study found the average bite force for this species in the wild to be 95 N with a maximum of 135 N, while under experimental conditions sharks could be induced to bite with over 200 N of force.
Aesthetic Identification: The body of the Horn shark is cylindrical. The Horn shark is dark to light grey or brown and usually have small dark spots that are less than one-third the diameter of the eye. There is no light bar between the eye ridges, but there is a dark patch of small spots below the eye. In addition, there is no harness pattern. There are no white fin tips. The young sharks are more brightly colored and have much more obvious dark saddles. The ventral side is yellowish. It has two high, falcate dorsal fins with stout spines preceding each fin. 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.
Biology and Reproduction: Reproduction of the Horn shark is oviparous, with females laying up to 24 eggs. The eggs are auger-shaped. The Horn shark mates between December through January (possibly annually). The eggs are deposited between February and April hidden and wedged under rocks or in crevices by the female. They hatch within 7 to 9 months. In captivity, 2 eggs are laid every 14-11 days for the duration of 4 months. Newly hatched sharks are equipped with an internal yolk sac and do not have to feed until they are a month old. The young sharks start feeding 1 month after hatching.
It is suggested that the male chases the female to indicate interest. Then it is thought they settle on the bottom. The male may possibly grip the female’s pectoral fins with his teeth and inserts his claspers into her cloaca, although this isn’t confirmed. Copulation is suggested to last between 30-40 minutes. It is then suggested that the female may spin with her snout in the sand for another 30 minutes.
The egg case has two flanges spiraling around it and may take the female several hours to deposit. At first the case is soft and light brown, and over a few days it hardens and darkens in color.
Horn sharks grow slowly and at a highly variable rate that does not correspond to their size. Therefore, it is difficult to determine an aging process. Males mature at a length of 1.8-2 feet and females at a length of at least 1.9 feet. Individual Horn sharks have lived to over 12 years old in captivity, and there exists an unconfirmed report of a shark reaching 25 years of age.
Known parasites include the tapeworms Acanthobothrium bajaensis and Acanthobothrium puertecitense, the copepod Trebius heterodonti, and the nematode Echinocephalus pseudouncinatus, which spends its larval stage inside potential prey such as scallops and sea urchins.
Behavioral Traits, Sensing and Intelligence: The Horn shark is nocturnal. They rarely move from resting places during the day. Their daily activity cycles are controlled by environmental light levels. After dusk, they actively swim along the reef or bottom searching for prey. 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. In nature, Horn sharks exposed to a bright light at night may stop swimming and sink to the bottom.
They are mainly solitary sharks; however small aggregations have been reported.
Young sharks may be segregated spatially from the adults, with the former preferring deeper sandy flats and the latter preferring shallower rocky reefs or algal beds.
Horn sharks hunt mainly using their sense of smell. Although electroreception certainly plays a role in locating prey, they have only 148 ampullae of Lorenzini. This is much fewer than in most other sharks, which may have over 2,000.
One of the few sharks to exhibit parental care, female Horn sharks in the wild pick up their eggs in their mouths and wedge them into crevices. However, in captivity the eggs are simply dropped on the bottom and may later be cannibalized.
Speed: They are slow, poor, sluggish swimmers. The Horn shark uses its mobile muscular paired fins to crawl over the seabed.
Horn Shark Future and Conservation: Currently, there isn’t enough data to evaluate, however they are of least concern by commercial fisheries. They are occasionally taken as bycatch. In Mexico, they are used for food and fishmeal, and in California its spines are made into jewelry.
The average annual bycatch off California is 4,000 pounds, though historically it has varied from 5.5 pounds in 1976 to 20,900 pounds in 1979.
They can easily be maintained in aquariums. In July 2018, three people were arrested after stealing a juvenile Horn shark from the San Antonio Aquarium. The shark was smuggled out of the aquarium in a stroller under a blanket. It was returned unharmed two days later.
Horn Shark Recorded Attacks on Humans: Horn sharks are harmless unless stressed and are readily maintained in captivity. They can be provoked into biting, and some aggressive individuals have been known to chase and bite divers after being harassed. Their spines can impose a painful wound if not careful.
Edmonds, M.A., Motta, P.J. and Hueter, R.E. (2001). “Food capture kinematics of the suction feeding horn shark, Heterodontus francisci“. Environmental Biology of Fishes. 62 (4): 415–427.
Nelson, D.R.; Johnson, R.H. (December 12, 1970). “Diel Activity Rhythms in the Nocturnal, Bottom-Dwelling Sharks, Heterodontus francisci and Cephaloscyllium ventriosum“. Copeia. 1970 (4): 732–739.
Huber, D.R., Eason, T.G., Hueter, R.E. and Motta, P.J. (2005). “Analysis of the bite force and mechanical design of the feeding mechanism of the durophagous horn shark Heterodontus francisci“. Journal of Experimental Biology. 208 (Pt 18): 3553–3571.