Horn shark
Encyclopedia
The horn shark is a species of bullhead shark
, 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 reef
s 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 fin
s 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 range
s 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, echinoderm
s, and crustacean
s, which they crush between powerful jaws and molar
-like teeth, while also feeding opportunistically on a wide variety of other invertebrate
s and small bony fishes. Juveniles prefer softer-bodied prey such as polychaete worms and sea anemone
s. The shark extracts its prey from the substrate using suction and, if necessary, lever
ing 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
.
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. This species 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. The scientific name for this species has been given erroneously as Heterodontus californicus.
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 (98.4 ft). This species has been found in caves as deep as 200 m (656.2 ft). Juvenile horn sharks between 35–48 cm (1.1–1.6 ft) long prefer sandy flats with low vertical relief, in water 40–150 m (131.2–492.1 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 swellshark
(Centroscyllim ventriosum), which shares the same habitat, are negatively correlated, because horn sharks favor water over 20 °C (70°F) while swellsharks are more tolerant of cold. 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.
Like other bullhead sharks, the horn shark has a short, wide head with a blunt snout and prominent supraorbital ridge
s 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 nostril
s 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 fin
s 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 caudal fin has a short lower lobe and a long, broad upper lobe with a strong notch near the tip. The horn shark's dermal denticles are small and smooth, numbering some 200/cm2 on the back in adults. 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. This species may reach a length of 1.2 m (3.9 ft), though most individuals do not exceed 1 m (3.3 ft).
The horn shark is a clumsy, 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 1000 square metre, 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).
Unlike most fishes, 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.
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 eagle
s (Haliaeetus leucocephalus) at Catalina Island, and large marine snails are able to drill into their egg cases to extract the yolk. The tough skin and spines of this species confer some protection; a Pacific angelshark
(Squatina californica) has been filmed engulfing a juvenile horn shark, only to spit it out due to its spines. Known parasites of this species include the tapeworms Acanthobothrium bajaensis and Acanthobothrium puertecitense, the copepod
Trebius heterodonti, and the nematode
Echinocephalus pseudouncinatus, which spends its larva
l stage inside potential prey such as scallop
s and sea urchin
s.
Some 95% of the adult horn shark's diet consists of hard-shelled molluscs (e.g. bivalves and gastropods), echinoderms (e.g. sea urchins) and crustaceans (e.g. crab
s, 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). 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. 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.
Other prey items of adults include peanut worms, sea star
s, cephalopod
s, and small bony fishes. Juveniles feed primarily on polychaete worms, sea anemones, and small clam
s, and have been known to "pounce" on anemones to bite off tentacles before they can be retracted. Off southern California, horn sharks are known to take advantage of seasonal opportunities. In the summer, diurnally active fishes, in particular 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. Horn sharks hunt mainly using their sense of smell
. Although 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. 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
cartilage
s 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.
. After 30–40 minutes of copulation, the pair disengages and the female spins with her snout in the sand for another 30 minutes. 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. The egg case has two flanges spiraling around it, and thus 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. 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.
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. The eggs hatch in 6–10 months; at emergence the young measure 15–17 cm (5.9–6.7 in) long. 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. Males mature at a length of 56–61 cm (22–24 in) and females at a length of at least 58 cm (22.8 in). 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.
Under normal circumstances, horn sharks are harmless to humans and can readily be approached underwater. However, they can be provoked into biting, and some pugnacious individuals have been known to chase and bite divers after being harassed. These sharks should be handled with care as their fin spines can inflict a painful wound. The horn shark adapts well to captivity and has been maintained and bred in many public aquarium
s across the United States.
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. This species benefits from general restrictions placed on coastal fishing gear by the State of California. The average annual bycatch
off California is 1800 kg (3,968.3 lb), though historically it has varied from 2.5 kg (5.5 lb) in 1976 to 9500 kg (20,943.9 lb) in 1979. 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 gillnet
s, 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
.
Bullhead shark
The bullhead sharks are a small order of basal modern sharks . There are nine living species in a single genus, Heterodontus, in the family Heterodontidae. All are relatively small, with the largest species being just in adult length...
, family Heterodontidae. It is endemic to the coastal waters off the western coast of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, from California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
to the Gulf of California
Gulf of California
The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...
. Young sharks are segregated spatially from the adults, with the former preferring deeper sandy flats and the latter preferring shallower rocky reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
s or algal
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
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 fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
s 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 range
Home range
Home range is the area where an animal lives and travels in. It is closely related to, but not identical with, the concept of "territory".The concept that can be traced back to a publication in 1943 by W. H. Burt, who constructed maps delineating the spatial extent or outside boundary of an...
s 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, echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s, and crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s, which they crush between powerful jaws and molar
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
-like teeth, while also feeding opportunistically on a wide variety of other invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s and small bony fishes. Juveniles prefer softer-bodied prey such as polychaete worms and sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s. The shark extracts its prey from the substrate using suction and, if necessary, lever
Lever
In physics, a lever is a rigid object that is used with an appropriate fulcrum or pivot point to either multiply the mechanical force that can be applied to another object or resistance force , or multiply the distance and speed at which the opposite end of the rigid object travels.This leverage...
ing 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
Auger
An auger is a drilling device, or drill bit, that usually includes a rotating helical screw blade called a "flighting" to act as a screw conveyor to remove the drilled out material...
-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
Commercial fishing
Commercial fishing is the activity of catching fish and other seafood for commercial profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice it as an industry must often pursue fish far into the ocean under adverse conditions...
or recreational fisheries
Recreational fishing
Recreational fishing, also called sport fishing, is fishing for pleasure or competition. It can be contrasted with commercial fishing, which is fishing for profit, or subsistence fishing, which is fishing for survival....
, though small numbers are caught as bycatch
Bycatch
The term “bycatch” is usually used for fish caught unintentionally in a fishery while intending to catch other fish. It may however also indicate untargeted catch in other forms of animal harvesting or collecting...
. In Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
.
Taxonomy
The French biologist Charles Frédéric GirardCharles Frédéric Girard
Charles Frédéric Girard was a French biologist specializing in ichthyology and herpetology.Born in Mulhouse, France, he studied at the College of Neuchâtel, Switzerland as a student of Louis Agassiz. In 1847, he accompanied Agassiz as his assistant to Harvard...
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. This species was later placed in the genus Gyropleurodus, which was eventually synonymized
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
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
Monterey Bay
Monterey Bay is a bay of the Pacific Ocean, along the central coast of California. The bay is south of San Francisco and San Jose, between the cities of Santa Cruz and Monterey....
has since been lost. The scientific name for this species has been given erroneously as Heterodontus californicus.
Distribution and habitat
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
of the eastern Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
, occurring off the coasts of California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
and Baja California
Baja California
Baja California officially Estado Libre y Soberano de Baja California is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is both the northernmost and westernmost state of Mexico. Before becoming a state in 1953, the area was known as the North...
from Monterey Bay
Monterey Bay
Monterey Bay is a bay of the Pacific Ocean, along the central coast of California. The bay is south of San Francisco and San Jose, between the cities of Santa Cruz and Monterey....
southward, and in the Gulf of California
Gulf of California
The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...
. Uncommon influxes of warm water northward may bring it as far as San Francisco Bay
San Francisco Bay
San Francisco Bay is a shallow, productive estuary through which water draining from approximately forty percent of California, flowing in the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, enters the Pacific Ocean...
. There are unconfirmed reports of this species off Ecuador
Ecuador
Ecuador , officially the Republic of Ecuador is a representative democratic republic in South America, bordered by Colombia on the north, Peru on the east and south, and by the Pacific Ocean to the west. It is one of only two countries in South America, along with Chile, that do not have a border...
and Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
, 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 (98.4 ft). This species has been found in caves as deep as 200 m (656.2 ft). Juvenile horn sharks between 35–48 cm (1.1–1.6 ft) long prefer sandy flats with low vertical relief, in water 40–150 m (131.2–492.1 ft) deep. They often take advantage of large feeding pits excavated by the bat ray
Bat ray
The bat ray, Myliobatis californica, is an eagle ray found in muddy or sandy sloughs, estuaries and bays, kelp beds and rocky-bottomed shoreline in the eastern Pacific Ocean, between the Oregon coast and the Gulf of California. It is also found in the area around the Galápagos Islands. The largest...
(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
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
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 swellshark
Swellshark
The swellshark, Cephaloscyllium ventriosum, is a catshark of the family Scyliorhinidae. It is found in the subtropical eastern Pacific Ocean between latitudes 40° N and 37° S, from the surface to 460 m....
(Centroscyllim ventriosum), which shares the same habitat, are negatively correlated, because horn sharks favor water over 20 °C (70°F) while swellsharks are more tolerant of cold. At Santa Catalina Island
Santa Catalina Island, California
Santa Catalina Island, often called Catalina Island, or just Catalina, is a rocky island off the coast of the U.S. state of California. The island is long and across at its greatest width. The island is located about south-southwest of Los Angeles, California. The highest point on the island is...
, 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
Channel Islands of California
The Channel Islands of California are a chain of eight islands located in the Pacific Ocean off the coast of Southern California along the Santa Barbara Channel in the United States of America...
, where the water is cooler.
Description
Supraorbital ridge
The supraorbital ridge, or brow ridge, refer to a bony ridge located above the eye sockets of all primates. In Homo sapiens sapiens the eyebrows are located on their lower margin.Other terms in use are:* supraorbital arch...
s 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
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
. The nostril
Nostril
A nostril is one of the two channels of the nose, from the point where they bifurcate to the external opening. In birds and mammals, they contain branched bones or cartilages called turbinates, whose function is to warm air on inhalation and remove moisture on exhalation...
s 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 fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
s 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 caudal fin has a short lower lobe and a long, broad upper lobe with a strong notch near the tip. The horn shark's dermal denticles are small and smooth, numbering some 200/cm2 on the back in adults. 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. This species may reach a length of 1.2 m (3.9 ft), though most individuals do not exceed 1 m (3.3 ft).
Biology and ecology
Unlike most fishes, the daily activity pattern of the horn shark is under exogenous
Exogenous
Exogenous refers to an action or object coming from outside a system. It is the opposite of endogenous, something generated from within the system....
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.
The horn shark is preyed upon by larger fishes and the northern elephant seal
Northern Elephant Seal
The northern elephant seal is one of two species of elephant seal . It is a member of the family Phocidae . Elephant seals derive their name from their great size and from the male's large proboscis, which is used in making extraordinarily loud roaring noises, especially during the mating...
(Mirounga angustirostris), which consumes adults, juveniles, and egg cases. In addition, they are captured and eaten by bald eagle
Bald Eagle
The Bald Eagle is a bird of prey found in North America. It is the national bird and symbol of the United States of America. This sea eagle has two known sub-species and forms a species pair with the White-tailed Eagle...
s (Haliaeetus leucocephalus) at Catalina Island, and large marine snails are able to drill into their egg cases to extract the yolk. The tough skin and spines of this species confer some protection; a Pacific angelshark
Pacific angelshark
The Pacific angelshark is a species of angel shark, family Squatinidae, found in the eastern Pacific Ocean from Alaska to the Gulf of California, and from Ecuador to Chile, although those in the Gulf of California and southeastern Pacific may in fact be separate species...
(Squatina californica) has been filmed engulfing a juvenile horn shark, only to spit it out due to its spines. Known parasites of this species include the tapeworms Acanthobothrium bajaensis and Acanthobothrium puertecitense, the copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
Trebius heterodonti, and the nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
Echinocephalus pseudouncinatus, which spends its larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
l stage inside potential prey such as scallop
Scallop
A scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...
s and sea urchin
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
s.
Feeding
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...
s, shrimp
Shrimp
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...
, 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
Spiny dogfish
The spiny dogfish, spurdog, mud shark, or piked dogfish, Squalus acanthias, is one of the best known of the dogfish which are members of the family Squalidae in the order Squaliformes. While these common names may apply to several species, Squalus acanthias is distinguished by having two spines ...
(Squalus acanthias) and the blacktip shark
Blacktip shark
The blacktip shark is a species of requiem shark, family Carcharhinidae. It is common to coastal tropical and subtropical waters around the world, including brackish habitats. Genetic analyses have revealed substantial variation within this species, with populations from the western Atlantic Ocean...
(Carcharhinus limbatus). 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. Large horn sharks that feed mainly on sea urchins (particularly the short-spined purple urchin
Strongylocentrotus purpuratus
The purple sea urchin, Strongylocentrotus purpuratus, lives along the eastern edge of the Pacific Ocean extending from Ensenada, Mexico to British Columbia, Canada. This sea urchin species is deep purple in color and lives in lower intertidal and nearshore subtidal communities...
, Strongylocentrotus purpuratus) have their teeth and fin spines stained purple.
Other prey items of adults include peanut worms, sea star
Sea star
Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea...
s, cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s, and small bony fishes. Juveniles feed primarily on polychaete worms, sea anemones, and small clam
Clam
The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...
s, and have been known to "pounce" on anemones to bite off tentacles before they can be retracted. Off southern California, horn sharks are known to take advantage of seasonal opportunities. In the summer, diurnally active fishes, in particular the blacksmith
Chromis punctipinnis
The blacksmith is a fish native to the eastern Pacific Ocean. Its range is from Monterey Bay, California, to central Baja California, Mexico. The blacksmith reaches a maximum size of 25 cm and is dark blue.The blacksmith is symbiotic with the senorita fish...
(Chromis punctipinnis), are especially abundant and are easily captured at night when they lie dormant. In the winter, the sharks scavenge on market squid
Opalescent Inshore Squid
The Opalescent Inshore Squid is a small squid in the family Loliginidae. It is a myopsid squid, which is the near shore group and that means that they have corneas over their eyes...
(Loligo opalescens), which die by the tens of thousands after their mass spawning event. Horn sharks hunt mainly using their sense of smell
Olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
. Although electroreception
Electroreception
Electroreception is the biological ability to perceive natural electrical stimuli. It has been observed only in aquatic or amphibious animals, since water is a much better conductor than air. Electroreception is used in electrolocation and for electrocommunication.- Overview :Electroreception is...
certainly plays a role in locating prey, this species has only 148 ampullae of Lorenzini
Ampullae of Lorenzini
The ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of jelly-filled pores. They are mostly discussed as being found in cartilaginous fishes ; however, they are also reported to be found in Chondrostei such as Reedfish and sturgeon. Lungfish have also been...
. This is much fewer than in most other sharks, which may have over 2,000. 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
Lip
Lips are a visible body part at the mouth of humans and many animals. Lips are soft, movable, and serve as the opening for food intake and in the articulation of sound and speech...
cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
s 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
Chisel
A chisel is a tool with a characteristically shaped cutting edge of blade on its end, for carving or cutting a hard material such as wood, stone, or metal. The handle and blade of some types of chisel are made of metal or wood with a sharp edge in it.In use, the chisel is forced into the material...
to dislodge firmly attached prey.
Life history
Mating in the horn shark occurs in December or January, on a possibly annual reproductive cycle. 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 cloacaCloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
. After 30–40 minutes of copulation, the pair disengages and the female spins with her snout in the sand for another 30 minutes. 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. The egg case has two flanges spiraling around it, and thus 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. 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.
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. The eggs hatch in 6–10 months; at emergence the young measure 15–17 cm (5.9–6.7 in) long. Newly hatched sharks are provisioned with an internal yolk sac
Yolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
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. Males mature at a length of 56–61 cm (22–24 in) and females at a length of at least 58 cm (22.8 in). 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.
Human interactions
Public aquarium
A public aquarium is the aquatic counterpart of a zoo, housing living aquatic species for viewing. Most public aquariums feature tanks larger than those kept by home aquarists, as well as smaller tanks. Since the first public aquariums were built in the mid-19th century, they have become popular...
s across the United States.
The horn shark has no commercial value in California, where it is captured unintentionally in traps and trawls and by recreational anglers
Recreational fishing
Recreational fishing, also called sport fishing, is fishing for pleasure or competition. It can be contrasted with commercial fishing, which is fishing for profit, or subsistence fishing, which is fishing for survival....
. The shark's hardiness ensures that it can often be returned to the water alive. This species benefits from general restrictions placed on coastal fishing gear by the State of California. The average annual bycatch
Bycatch
The term “bycatch” is usually used for fish caught unintentionally in a fishery while intending to catch other fish. It may however also indicate untargeted catch in other forms of animal harvesting or collecting...
off California is 1800 kg (3,968.3 lb), though historically it has varied from 2.5 kg (5.5 lb) in 1976 to 9500 kg (20,943.9 lb) in 1979. 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 gillnet
Gillnet
Gillnetting is a common fishing method used by commercial and artisanal fishermen of all the oceans and in some freshwater and estuary areas. The gillnet also is used by fisheries scientists to monitor fish populations. Because gillnets can be so effective their use is closely monitored and...
s, 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
Least Concern
Least Concern is an IUCN category assigned to extant taxon or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, Near Threatened, or Conservation Dependent...
.
External links
- Heterodontus francisci, Horn shark at FishBase
- Heterodontus francisci (Bullhead Shark, California Horn Shark, Horned Shark, Horn Shark, Port Jackson Shark) at IUCN Red List
- Biological Profiles: Horn Shark at Florida Museum of Natural History Ichthyology Department
- Kelp Forests: Horn Shark at ReefQuest Centre for Shark Research