Horn and Angel Sharks: Unusual Predators on the Seabed
Many people think of sharks as ferocious killing machines that swim at high speed as they hunt for prey. This image is often inaccurate, however. First, research is showing that far from being "machines", at least some sharks are surprisingly intelligent. Secondly, not all sharks move fast or swim through the open ocean. Some, including the horn shark and the angel shark, live and feed on the seabed. Neither fish is a typical shark.
Horn sharks have a raised ridge above each eye. The ridges look like little horns and may have given the animals their name. The fish also have a blunt snout that resembles the snout of a bull. Horn sharks often hunt for food by "walking" over objects with their strong pectoral fins. They swim quite slowly compared to most other sharks.
Angel sharks have a wide, flattened body and wing-like pectoral fins. They hide in the sediments on the seabed with just their eyes above the ground, bursting out of their hiding place in order to capture passing prey.
Heterodontus, the genus name of bullhead sharks, is derived from two Greek words: heteros, which means "different", and odont, which means "teeth". The name refers to the two types of teeth in the shark's mouth.
Features of Bullhead Sharks
Horn sharks are members of an order of fish known as bullhead sharks (Heterodontiformes). There are nine known species in this order, all belonging to the genus Heterodontus. The term "horn shark" is applied to three of the species—the California horn shark, the Mexican horn shark, and the crested horn shark.
The snout of a bullhead shark is blunt. There is a round opening on each side of the snout known as an oronasal groove. Each of these openings is surrounded by a fleshy ring of tissue.
The shark's teeth are adapted for grasping and grinding hard foods gathered from the seabed, such as sea urchins and crabs. The front teeth are pointed to enable efficient biting and the back teeth are flattened for crushing,
Bullhead sharks have a crest over each eye and a spine in front of each of their two dorsal fins (the fins on the shark's back). Some reports say that the fin spines gave the horn sharks their name rather than the crest over their eyes. The pectoral fin on each side of the body is large and muscular and enables the fish to move with a walking motion over the ocean bottom.
Bullhead sharks generally aren't dangerous to humans, although they may bite or chase when threatened. They shouldn't be confused with bull sharks, which are aggressive.
The California Horn Shark
The California horn shark (Heterodontus francisci) lives on the west coast of North America from the state of California to the Gulf of California. It's an attractive fish with a brown or grey surface that has darker spots. It reaches a maximum length of four feet. Most individuals are shorter than this, however.
The fish is usually a nocturnal animal. During the day it hides in caves or crevices, under ledges, or in thick beds of seaweed. Starting at dusk, it hunts for animals on or close to the seabed, including crabs, snails, squid, shrimp, sea urchins, sea stars (starfish), and occasionally fish.
The shark is able to remove animals attached to a substrate by using its teeth like a chisel. Suction helps draw the prey into the shark's mouth. The shark may also pry animals off a surface with its mouth by bracing itself with its pectoral fins, moving into a vertical position and then rapidly moving its body downwards, in effect using its body as a lever.
About 43 percent of sharks and rays—including skates, most cat sharks, and the nine species of horn shark—lay eggs rather than give birth to live young.— Julie Leibach, Science Friday (A public radio station show)
Fertilization is internal in all sharks. Fish have a pelvic fin on each side of their body behind the pectoral fins. A male shark has a tubular organ called a clasper on the inner side of each of his pelvic fins. Claspers insert sperm into the female's reproductive tract.
After fertilization, the female horn shark reproduces by laying eggs. She produces two eggs at a time, starting in the late winter or early spring. There is a gap of eleven to fourteen days between egg laying sessions. Eggs are produced for as long as four months.
The egg case has a spiral shape. It's soft when it's laid and gradually hardens over time. The female wedges each egg into a rock crevice to protect it from predators. The embryo takes a long time to develop. Pups aren't born until six to eight months after the eggs are laid. The typical lifespan of California horn sharks isn't known for certain, but they have lived for twelve years in captivity. Some reports say that they can live for as long as twenty-five years.
Heterodontus galeatus lives off the coast of Australia. It's known as the crested horn shark or the crested bullhead shark.
Population Status of the California Horn Shark
The IUCN (International Union for Conservation of Nature) classifies animal species according to their nearness to extinction. The California horn shark has been placed in the Data Deficient category. This means that the size of its population isn't known.
The shark generally isn't caught for food, but it is sometimes caught for its long fin spines, which are used in jewelry. Today the main problem for the horn shark seems to be that it forms part of the bycatch in the fishing industry. "Bycatch" is defined as a fish or animal that is accidentally trapped when another type of animal is being caught. Luckily, the horn shark seems to be a hardy creature. When it's returned to the ocean after being trapped in fishing gear it often survives, though sometimes it's injured and dies.
Angel sharks belong to the order Squatiniformes and the genus Squatina. Many species have been recognized. They live in oceans around the world. The front section of an angel shark's body is flattened, including its head. The fish has greatly enlarged pectoral fins that are flat and extend outwards. They often look like wings and make the fish look quite like a ray. In skates and rays, however, the pectoral fins are attached to the side of the head. In angel sharks, they aren't. The shark's smaller pelvic fins also extend out from its body.
The rear section of an angel shark's body isn't flattened and looks more like that of a typical shark. The lower lobe of the tail is longer than the upper lobe, however. In other sharks, the upper lobe is longer than the lower lobe.
Like horn sharks, angel sharks are bottom feeders. Instead of actively patrolling the ocean bottom, they use an ambush technique to catch their prey. They cover themselves with a thin layer of sediment on the seabed so that just their eyes show and then patiently wait. When they detect a suitable prey animal they pounce. The sharks have powerful jaws and sharp teeth, which make them very successful predators.
Angel sharks are generally not dangerous to humans. If a person gets too close to the jaws, though, they may get a nasty wound. It should never be assumed that a totally still angel shark is dead because it almost certainly isn't.
Unlike some sharks that must swim continually to force water over their gills, angel sharks (and horn sharks) have muscles that pump water over their gills. Therefore they can remain motionless and still survive.
The Pacific Angel Shark
The Pacific angel shark (Squatina californica) lives in the eastern part of the Pacific Ocean. Its range extends from Alaska down into South America. The fish reaches a maximum length of about five feet but is usually shorter than this. In California it's sometimes known as the California angel shark.
The shark is brown or grey with dark spots. The mottled surface appearance is very useful for camouflaging the fish against the ocean bottom. The pectoral and pelvic fins are noticeably angular and pointed. The shark also has barbels around its mouth. Barbels are slender sensory organs that are sensitive to both taste and touch.
Like the California horn shark, the Pacific angel shark is more active during the night than the day. It feeds mainly on fish and squid. Research suggests that sight is the most important factor in the shark's detection of food. Scientists say that when the sharks hunt at night, bioluminescence created by planktonic organisms may help them to see their prey.
Another very interesting discovery has been made by means of high speed videography. The researchers who conducted this experiment found that the sharks caught their prey in about one tenth of a second.
Egg Production and Lifespan
Angel sharks have internal fertilization and give birth to live young. Eggs are produced, but they hatch inside their mother's body. The pups feed on yolk stored in a yolk sac until they are born. The shark is said to be ovoviviparous due to this method of producing offspring. The "ovo" part of the description means that the fish produces eggs and the "viviparous" part means that it gives birth to live young.
The Pacific angel shark produces about six pups at a time. The gestation period may vary, but it seems to be about ten months. There are reports that the shark can live for as long as thirty-five years. This fact needs to be confirmed, however.
Population Status of the Pacific Angel Shark
The Pacific angel shark is classified in the Near Threatened category. There was a period in the 1970s and 1980s when it was caught for food in large numbers in the United States, which had a very serious effect on the shark's population.
New fishing regulations seem to have helped in some parts of the animal's United States range, but not everywhere. The effect of the Mexican shark fishery on the species is unknown. Unfortunately, the IUCN says that the overall population of the fish is decreasing, which is not a good sign.
Sharks form a large and diverse group of organisms. The big sharks that are dangerous to humans are generally better known by the public than the smaller, bottom-dwelling fish. This is a shame because the sharks that roam the seabed or hide in its sediments are interesting and sometimes unusual animals. They are definitely worthy of our attention.
- Horn shark facts from the Monterey Bay Aquarium
- Home for a horn shark embryo from Science Friday
- The Heterodontus francisci entry in the Red List of the International Union for Conservation of Nature
- Information about Squatina californica from the University of Florida
- The Squatina californica entry in the IUCN's Red List
Questions & Answers
© 2014 Linda Crampton