The Horn shark (Heterodontus francisci) is a small-sized common shark species clearly distinguished by the horn-like ridges over its eyes. With the broad, flathead and the high ridges, this shark is one of the hardiest among all sharks.
This shark species is small and measures just about 3.3 feet (1 m) in length. It is from the family Heterodontidae and a species of bullhead shark. The horn shark is native to the coastal waters off the pacific coast of North America from central California through the Gulf of California.
Horn sharks are generally slow with the juvenile showing spatial segregation from the adult. That is to say, the young ones prefer deeper sandy flats while the adults mostly stay at the shallower algal beds or rocky reefs. Popular for its notable nocturnal hunting behavior and resting during the day, this shark has lots of interesting characteristics worth learning about.
Naming and Scientific Classification
It is obvious this shark got its common name from the appearance of its head. That is the short, blunt head with pronounced ridges over the eyes looking just like horns.
In the first scientific description of the horn shark, Charles Frédéric Girard referred to it as Cestracion francisci. Later on, this species was reclassified into a new genus Gyropleurodus which was synonymized eventually with the genus Heterodontus. The specific name of this shark francisci is a reference to San Fransisco, even while the range of this shark may not extend that far north.
Habitat and Distribution Range of Horn Sharks
This species of shark lives in the continental shelf of the eastern Pacific Ocean. They do not have a wide distribution. As such, they occur mostly off the coast of California and Baja California ranging from Monterey Bay southward, and in the Gulf of California.
Following an unusual inflow of water toward the northside, horn sharks may move as far as San Francisco Bay. There are reports of this shark species in places such as Peru and off Ecuador, however, these are not verified and could be a misidentification of other species.
Horn sharks are common at depths ranging from 6.6 to 36.1 feet (2 – 11 m). Although, at the beginning of winter, they tend to migrate to deeper waters below 98 feet (30 m). The maximum depth where this species has been found is about 660 feet (200 m) in deepwater caves.
Juvenile horn sharks which are usually between 1.2 and 1.7 feet long tend to inhabit waters of about 130 – 40 feet deep. Also, they prefer sandy flats with low vertical reliefs. However, as they grow older, they move into shallower water inhabiting rocky reefs or algae beds with structural complexities.
The Negative Correlation of Horn Sharks and Swell Sharks
The horn shark and the swell shark (Centroscyllim ventriosum) share the same habitat, however, their relative population negatively correlates. This results from the horn shark’s preference for water temperatures warmer than 68 °F (20 °C). While on the other hand, the swell shark prefers colder waters.
As a result of this, warming trends will cause the increase of the horn shark’s population in a habitat while the swell shark’s population will tend to decrease. This is the case at Santa Catalina Island. But, where the water is colder, the population of horn sharks decreases as against that of swell sharks. An example is the northern Channel Island with cooler waters.
Horn Shark Description
Head and Snout
The distinctive features of horn sharks include a short, wide head with a blunt snout and pronounced ridges positioned just above the orbit of the eyes. These ridges are not so high and terminate abruptly while creating a deeply concave space between them on top of the shark’s head making this species appear to have a horn.
Horn sharks are among the sharks lacking nictitating membrane in their eyes. And, after the eyes are very tiny spiracles. The nostrils have a long flap splitting it into the inflow and outflow openings while extending toward the mouth. There is a groove encircling the inflow opening, while another groove connects the outflow opening to the mouth. Horn sharks have a quite small and curved mouth with pronounced furrows by the corners.
The upper jaw of this shark contains about 19 – 26 tooth rows, while the lower jaw has 18 – 29 tooth rows. While teeth positioned at the front part of the jaws are pointed and small bearing a central cusp with two lateral cusplets on each side; those at the side of the jaws are larger, longer, and looking more like a molar adapted for crushing.
The horn shark has two high dorsal fins with the shape of a sickle and bearing bold spines at the front. Horn sharks living in reefs tend to have shorter fin spines than those living in an algal environment. This is because rocks wear their fins down as the sharks move.
The first dorsal fin has its origin over the bases of the large pectoral fins, while the second dorsal fin originates almost in the front of the free rear tips of the pelvic fins. This shark has a caudal fin with a broad, long upper lobe that has solid notch near the tip, and a short lower lobe.
The horn shark has a cylindrical body covered by small and smooth dermal denticles with about 200 per square centimeter on an adult’s back.
In the dorsal side of this shark are various shades of gray or brown colors with many small dark blotches. These blotches may be absent in adult sharks. The underside of this shark is yellowish. Also, mostly noticed is a dark patch of small spots situated just below the eyes.
Horn sharks will mostly grow to a length of about 3.3 feet (1 m). However, some may grow as long as 3.9 feet (1.2 m).
Typical Behavior of the Horn Shark
The horn shark is among the solitary shark species, even while there are records of small groups. This shark is an occasional swimmer that prefers to push itself along the bottom of the water using its flexible, muscular pectoral fins. As a species that is most active at night, the horn shark rests motionless during the day. It would likely hide inside caves, crevices or thick algae mats while remaining relatively alert to quickly swim away in case of any disturbance.
At night, horn sharks actively swim above the reef and roam around in search of food. The foraging area of these sharks is usually at the edge of its shelter. Thus, they are unlikely to migrate to far distances. Moreover, horn sharks will always return to the same resting spot each day and can occupy there for more than a decade. This home range will not usually be more than 11,000 square feet (1000 square meters). The farthest distance in record traveled by a horn shark is 9.9 miles (16 km).
The pattern of activity of the horn shark is not a result of internal physiological factors, instead, this shark responds more to environmental demands. Therefore, they act under exogenous control. And, their most relevant cue is lighting, that is, they respond quickly to varying light intensity. While in nature, active horn sharks swimming at night are likely to stop swimming and sink to the bottom of the water if exposed to bright light.
To What Extent Does Horn Sharks Respond to Light Intensity?
Observation of captive specimens shows that immediately the lights go off they become active. However, once the light comes back on, they stop every activity. In an experiment, horn sharks were kept in a dark environment and they remained active for about eleven days before slowing down probably due to exhaustion. This shows that the activeness of this shark highly depends upon light intensity.
Adaptations for Hunting
While hunting for prey, horn sharks make the most use of their sense of smell. Although they have the ampullae of Lorenzini for electroreception which also plays a role, these are in few numbers of 148. Hence, cannot compare to most other sharks with over 2000.
As the food of horn sharks consists mainly of hard-shelled prey, they generate a heavy bite force to crack the shells of their prey. This shark species has an average bite force of 95N and a maximum of 135N as recorded by a study. When induced, this shark can generate a bite force of up to 200N. With this, horn sharks have the highest bite force compared to other measured species if its sizes such as the blacktip shark (Carcharhinus limbatus) and the spiny dogfish (Squalus acanthias).
This shark just like other sharks also replaces their teeth regularly and each shed tooth takes up to a month (4weeks) to replace.
To capture their prey, horn sharks use the process of suction. And, they create the pressure by expanding their buccal cavity, facilitating the process with their labial cartilages modified in a way to enable the mouth form a tube.
The horn shark is capable of protruding its upper jaw up to 15% of the length of its head. This motion takes the shark less than a second, about 20 milliseconds to achieve. With this, the horn shark can use its upper jaw like a chisel dislodging firmly attached preys.
Capturing of Preys
Once prey is drawn into the mouth of this shark through suction, the sharp front teeth secure it, while the flat lateral teeth grind the prey into pieces. If the prey buries or attaches itself to something, the horn shark grips it while adopting a vertical posture with the head and pectoral fins against the substrate and the tail stay above forming an arch. The shark then acts in the form of a lever with the pectoral fin serving as the fulcrum, as effort is applied through the downward stroking of the tail. Eventually, this shark pulls the prey loose by forcing its head upwards. This particular mode of capturing prey is unique to this species.
Horn Sharks Diet
Adult horn sharks feed mainly on hard-shelled mollusks (gastropods and bivalves), crustaceans (shrimps, crabs, and isopods), and echinoderms (sea urchins). Large individuals would prefer mainly a diet of short-spined purple urchins (Strongylocentrotus purpuratus), thus having their teeth and fins stained purple.
Adult horn sharks also prey on small bony fishes, cephalopods, sea stars, and peanut worms. While juveniles feed mainly on small clams, polychaete worms, and sea anemones. They have a known habit of pouncing on sea anemones to bite off tentacles before they could be retracted.
While being observed off southern California, this shark species appears to take advantage of seasonal opportunities. Thus, during summer, they feed on the abundant diurnally active fishes with particular reference to the blacksmith (Chromis punctipinnis). Which is very much abundant and only active during the day, as such easy for the horn sharks to capture at night while they sleep.
However, during winter, horn sharks scavenge on market squid (Loligo opalescens) which after their mass spawning die in great numbers.
Mating and Reproduction
Horn sharks mate around December or January. To initiate mating, the male tends to chase the female to show interest. If the female is ready, both sharks settle at the bottom as the male grabs the pectoral fin of the female with its teeth while inserting one of his claspers into the cloaca of the female. Copulation lasts about 30 to 40 minutes which after the female takes about another 30 minutes spinning in the sand with her snout.
The female lays eggs from February to April in batches of two eggs per every interval of 11 to 14 days. Egg-laying occurs in the shallow water of about 6.6 – 42.7 feet (2 – 13 m) deep. The egg case has two flanges spiraling around it, thus, it may take several hours for the females to deposit. Upon delivery, the egg case is usually soft and light brown. But as the day passes it hardens and the color darkens as well.
The egg case measures 3.9 – 4.7 inches (10 – 12 cm) in length and 1.2 – 1.6 inches (3 – 4 cm) in width, without including the flanges. The size of the egg case can suggest separate populations, for instance, horn sharks from the Channel Islands produce longer cases than the ones from mainland California.
After about 6 – 10 months, the eggs hatch producing young horn sharks measuring 5.9 – 6.7 inches (15 – 17 cm). Newly hatched sharks may not need to feed until after a month. This is because of the provisioned internal yolk sac. Although, they will accept food and are capable of feeding at this period.
Horn sharks grow quite slowly and at a varying rate that does not tally with their size. This makes it very difficult to ascertain the aging process. At a length of 22 – 24 inches (56 – 61 cm), the male horn shark matures. While the female grows at least 23 inches (58 cm) before maturity. In captivity, this shark species recorded a lifespan of over 12 years. But there is a report of this shark living up to 25 years, although not confirmed.
The horn shark is among the few shark species to show parental care. This is observed as the female shark picks up its eggs in its mouth and wedge them into crevices. Behavior associated with horn sharks in the wild. Of course, in captivity the just drop the egg on the bottom which they may later cannibalize.
Horn Sharks Predators
In the wild, larger fish species such as northern elephant seal (Mirounga angustirostris) prey upon horn sharks. They feed on both the adult and juvenile horn sharks, even egg cases. Also, at Catalina Islands bald eagles (Haliaeetus leucocephalus) hunt and feed on this species. Large marine snails can drill into this shark’s egg case extracting the yolk.
This shark species could have more predators; however, they get some protection from their spines and tough skin. There is a filmed record of the Pacific Angelshark (Squatina californica) engulfing a horn shark only to spit it out due to the spines.
Parasites known to attack the horn shark include the copepod (Trebius heterodonti), the tapeworm (Acanthobothrium bajaensis and Acanthobothrium puertecitense), and the nematode (Echinocephalus pseudouncinatus). The nematode spends its larval stage inside sea urchins and scallops which are potential prey of the horn shark.
Horn Sharks and Humans
Attacks on humans
Horn sharks normally are harmless to humans when not provoked. As such, human divers can readily approach this shark underwater without getting into trouble. But, as peaceful as they may seem, they can still attack humans on provocation. There are still quarrelsome individuals known to have chased and bitten divers who harassed them. While handling this shark species care must be taken as the spines can inflict a painful injury.
Humans capture horn sharks and keep them in public aquaria. This shark species tend to adapt well to captivity and has been bred in many of these aquaria across the United States.
Can Horn Sharks be Kept in Home Aquariums?
The answer is no! Of course, these sharks adapt to public aquariums because of the advanced level of care given to them. So, keeping a horn shark in your home can cost you so much. Even while this shark is small compared to its marine counterparts, the size is still too large and unsuitable for a home aquarium.
In all, the keeping of sharks in the home is not advisable, as such, prohibited in some countries.
The horn shark has no value commercially in California. Most of the time, this shark comes as bycatch in deepwater traps and trawls. Also, recreational anglers may occasionally catch this shark.
Some human activities threaten the population of horn sharks in nature. For example, divers kill this species sometimes for sports or make jewelry from the spines of their fins. This particular activity may be the cause of the horn shark’s population decline in areas most heavily dived in southern California. The use of deepwater gillnets and shrimp trawls off Mexico leads to incidental catching of horn sharks. These bycatches may be used for human consumption and/or fishmeal.
Horn Shark Conservation Status
The IUCN (International Union for Conservation of Nature) do not have enough data on this species to assess its conservation status. As a result, they classified it as Data Deficient (DD). However, this shark still benefits from the general restriction by the government of California placed on coastal fishing gear.
10 Interesting Horn Shark Facts at a Glance
1. The Name is a Result of the Horn-like Ridges on the Shark’s Head
This shark species has high ridges above the eyes’ orbit, projecting from its short, blunt head giving rise to its common name “Horn Shark”.
2. Horn Sharks Actively Respond to Light Intensity
Once in darkness, this shark will be active till exhaustion. However, when a bright light comes on, they stop all activities and remain dormant. In the wild, they are only active at night and rest all through the day.
3. They are mostly Solitary
Horn sharks stay by themselves and hunt alone, however, there are records of few groups.
4. The Bite Force is Highest of Similar-sized Species
The horn shark exhibits a bite force which is far higher than other species of similar size already measured. Thus, they have a maximum induced bite force of 200 Newtons.
5. Exhibits Parental Care
The female horn shark has the capacity to pick up its eggs and wedge them in crevices for safety. This shark and very few others can do this.
6. The Teeth of Large Adults are Purple
By feeding on a diet of short-spined purple urchins (Strongylocentrotus purpuratus), large adult horn sharks have their teeth stained purple.
7. Juvenile and Adult Sharks Show Habitat Segregation
Juveniles live in deeper water while adults prefer shallower waters.
8. Horn Sharks Always Return to The Same Resting Spot
A horn shark is unlikely to change a resting spot, and, will always come back to its resting place usually within 11,000 square feet after each hunting. They can remain faithful to this are for a very long time.
9. They are Harmless to Human but Have the Tendency to Bite
Horn sharks would not naturally attack humans, but, if provoked or harassed, they would eventually retaliate with a bite.
10. The Spines of this Shark may Serve as Protective Adaptation from Predators
With the sharp spines of this shark, it can inflict a painful wound on predators attempting to swallow it, thus leading the predator to let the horn shark be.
- “Horn Shark (Heterodontus francisci)“, IUCN Red List of Threatened Species.
- “Heterodontus francisci, Horn Shark”, FishBase.
- “Kelp Forests: Horn Shark”, ReefQuest Centre for Shark Research.
- “Biological Profiles: Horn Shark”, Florida Museum of Natural History