Horn shark

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Horn shark

Data Deficient (IUCN 3.1)[1]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Subdivision: Selachimorpha
Order: Heterodontiformes
Family: Heterodontidae
Genus: Heterodontus
Species:
H. francisci
Binomial name
Heterodontus francisci
(Girard, 1855)
Range of the horn shark
Synonyms

Cestracion francisci Girard, 1855

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

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

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
.

Taxonomy

The French biologist

type specimen from Monterey Bay has since been lost. The scientific name for this species has been given erroneously as Heterodontus californicus.[2]

Description

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.[2][3]

The body is cylindrical, with two high, somewhat falcate (sickle-shaped)

dermal denticles are small and smooth, numbering some 200/cm2 on the back in adults.[2] The dorsal coloration consists of various shades of gray or brown with many small dark spots, though these may be absent in older sharks; the underside is yellowish. There is a dark patch of small spots below the eye.[2][3] This species may reach a length of 1.2 m (3.9 ft), though most individuals do not exceed 1 m (3.3 ft).[4]

  • The horn shark has a distinctively shaped head with prominent ridges above its eyes.
    The horn shark has a distinctively shaped head with prominent ridges above its eyes.
  • Jaws
    Jaws
  • Lower teeth
    Lower teeth

Distribution and habitat

A horn shark off Santa Catalina, California.
Unlike the adults, juvenile horn sharks prefer a flat, sandy habitat.

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.[2] There are unconfirmed reports of this species off Ecuador and Peru, which may be misidentifications of other species.[1]

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).

benthic species seldom ventures more than 2 m (6.6 ft) above the substrate.[3]

The relative abundances of the horn shark and the

Channel Islands, where the water is cooler.[3]

Biology and ecology

Horn sharks rest during the day and only become active at night.

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.[2] 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.[5] Horn sharks maintain small home ranges of around 1,000 m2 (11,000 sq ft), 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.[3] The longest documented movement for an individual horn shark is 16 km (9.9 mi).[4]

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.[5]

The horn shark is preyed upon by larger fishes and 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.[7][8][9][10]

Feeding

Sea urchins are a favored prey of the horn shark.

95% of the adult horn shark's diet consists of hard-shelled mollusks (e.g.

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).[11] 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.[11] 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.[3]

Other prey items of adults include

electroreception certainly plays a role in locating prey, this species has only 148 ampullae of Lorenzini. This is much fewer than in most other sharks, which may have over 2,000.[12] Like other sharks, the horn shark's teeth are regularly replaced; it takes 4 weeks for a dropped tooth to be replaced.[13]

The horn shark captures prey via suction, created by expanding its

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.[14]

Life history

The spiral-flanged egg case of a horn shark; the shape allows the egg to be secured within crevices.

Mating in the horn shark occurs in December or January, on a possibly annual reproductive cycle.[15] The male chases the female to indicate interest; once she is ready both sharks settle on the bottom, where the male grips the female's pectoral fin in his teeth and inserts one of his claspers into her cloaca. After 30–40 minutes of copulation, the pair disengages and the female spins with her snout in the sand for another 30 minutes.[6] From February to April, the females lay a maximum of 24 eggs two at a time once every 11–14 days, in water 2–13 m (6.6–42.7 ft) deep.[1] The egg case has two flanges spiraling around it, and thus may take the female several hours to deposit.[16] 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 10–12 cm (3.9–4.7 in) long and 3–4 cm (1.2–1.6 in) wide; sharks from the Channel Islands produce longer egg cases than those from mainland California, suggesting that they are separate populations.[2][3]

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.[3] However, in captivity the eggs are simply dropped on the bottom and may later be cannibalized.[2] The eggs hatch in 6–10 months; at emergence the young measure 15–17 cm (5.9–6.7 in) long.[1] Newly hatched sharks are provisioned with an internal yolk sac and do not have to feed until they are a month old, though they are capable of feeding and will accept food during this period. Horn sharks grow slowly and at a highly variable rate that does not correspond to their size; this has frustrated attempts to determine their aging process.[3] Males mature at a length of 56–61 cm (22–24 in) and females at a length of at least 58 cm (23 in).[1] Individual sharks have lived to over 12 years old in captivity, and there exists an unconfirmed report of a shark reaching 25 years of age.[3]

Human interactions

Horn sharks are innocuous towards humans.

Under normal circumstances, horn sharks are harmless to humans and can readily be approached underwater.[3] However, they can be provoked into biting, and some pugnacious individuals have been known to chase and bite divers after being harassed.[6] These sharks should be handled with care as their fin spines can inflict a painful wound.[3] The horn shark adapts well to captivity and has been maintained and bred in many public aquariums across the United States.[2] 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.[17]

The horn shark has no commercial value in California, where it is captured unintentionally in traps and trawls and by

Least Concern.[1]

References

  1. ^ . Retrieved 19 November 2021.
  2. ^ .
  3. ^ .
  4. ^ a b c d Buch, R. Biological Profiles: Horn Shark. Florida Museum of Natural History Ichthyology Department. Retrieved on June 18, 2009.
  5. ^
    JSTOR 1442315
    .
  6. ^ a b c d Martin, R.A. Kelp Forests: Horn Shark. ReefQuest Centre for Shark Research. Retrieved on June 19, 2009.
  7. JSTOR 3278414
    .
  8. .
  9. .
  10. .
  11. ^
    PMID 16155227.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  12. ISBN 978-0-387-23192-1.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  13. .
  14. S2CID 21860117.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  15. ^
    ISBN 978-2-8317-0700-6.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  16. ^ Martin, R.A. Heterodontiformes: Bullhead Sharks. ReefQuest Centre for Shark Research. Retrieved on June 19, 2009.
  17. ^ "Shark returned to San Antonio aquarium after being stolen in baby stroller". ABC News.

External links