Walking and Epaulette Sharks: Intriguing Fish With Muscular Fins
Unusual and Interesting Fish
Walking sharks are unusual fish with an elongated shape and muscular fins on the lower part of their body. The fins enable the fish to "walk" along the ocean floor. In the case of at least one species, they enable the animal to move over land as well. The body of walking sharks is decorated with spots, blotches, or stripes. The animals belong to the genus Hemiscyllium. Researchers have recently announced the existence of four new species of walking sharks, bringing the total number to nine.
Though most people probably think of sharks as fierce and fast-moving predators that are occasionally dangerous for humans, many more benign species (from our point of view) exist. Walking sharks are one example. When the fish are "walking", the front part of their body resembles that of a moving salamander. They aren't believed to be closely related to the amphibian, however. In this article, I highlight the epaulette shark, which is the best known species in its genus and the one known to leave the water and travel over land.
The genus Hemiscyllium belongs to the family Hemiscylliidae. This family is also known as the longtail carpet shark family. Carpet sharks (or carpetsharks) are named for the fact that the pattern on their body surface resembles that of an ornate carpet.
Fins and Features of a Walking Shark
The illustration above shows the type of shark that is probably most familiar for many people. The fish has:
- two dorsal fins on its back
- a pectoral fin on each side near the gills
- a pelvic fin on each side underneath the body and towards the rear end
- an unpaired anal fin behind the pelvic fins
- a caudal fin, which forms the tail; the upper lobe of this fin is larger than the bottom one
The body shape of a walking shark is different from that of the torpedo-shaped animal shown above. A walking shark is long and slender. It has a long "tail" or caudal peduncle behind the last dorsal fin. The caudal peduncle is the narrow area in front of the caudal fin of a fish.
Walking sharks have the same fins as the classical shark. The pectoral and pelvic fins are muscular and paddle-shaped, however. In addition, the caudal fin has only one lobe. The dorsal fins are located far back on the body and the anal fin is located near the caudal one.
The new announcement that nine species of walking sharks exist is based on a twelve-year study. Researchers suspect that more fish belonging to the genus Hemiscyllium will be discovered. The known species live in warm water on the coast of Australia, New Guinea, and Indonesia.
The Epaulette Shark
The epaulette shark lives in water around Australia and New Guinea. Unconfirmed reports suggest that it also lives in other areas nearby. The fish is light grey or brown in colour and has darker spots. Its name comes from a large black spot encircled with white that is located on each side of its body just behind the pectoral fin. The spot reminded earlier biologists of military epaulettes. It's thought that it may act as a distraction for predators. It looks like a large eye from a distance. It's easy to imagine that the "eye" belongs to a bigger animal than the shark. The animal's real eye is much smaller.
The adult fish is generally no longer than a metre. It has the long caudal peduncle that is typical of its genus. Its snout is noticeably rounded and has barbels at its tip. Barbels are fleshy extensions that are believed to act as sense organs and to play a role in detecting food.
An opening called a spiracle is located under and behind each eye. The spiracle absorbs water and sends it to the gills. The blood vessels in the gills absorb oxygen from the water. They also send carbon dioxide waste made by the shark's body into the water. The water then returns to the ocean through the gill slits on the side of the animal.
Surviving on Land
The epaulette shark is usually found in shallow water. It swims through the water and walks on the ocean floor and on land. Though walking sharks don't walk in the way that we do, their muscular fins have a wide range of motion and are a useful aid to propulsion on a solid surface. The walking movement of the epaulette shark is thought to resemble that of the first four-legged animals that appeared on land.
The shark can survive for an amazingly long time with a low oxygen level in its body. This means that it can exploit habitats that other fish are unable to reach. It can reportedly stay out of water for up to an hour. Some researchers claim that the time is even longer. The features that enable the fish to survive under these conditions are still being studied. It seems to have multiple adaptations that enable it to live on land, including the ones listed below.
- The breathing rate decreases.
- The heart rate also decreases.
- Blood flow to the heart increases.
- Blood vessels going to some parts of the brain dilate, allowing more oxygen to reach the organ. Blood flow to less vital areas of the brain is decreased, however.
- Nerves continue to function in a low-oxygen environment.
On land, the fish sometimes dampens its body as it travels through tide pools and into puddles in the coral reef. It also moves over dry sand, however, which would seem to be a less hospitable environment. Its survival abilities are impressive.
Diet of an Epaulette Shark
Epaulette sharks are mainly bottom feeders and feed mostly on invertebrates, such as crabs, shrimp, and polychaete worms. They also eat small fish, including ones trapped in tide pools. By coming onto land as well as feeding in the ocean, epaulette sharks are able to find food items that are unavailable to most other fish. They sometimes chew their prey before swallowing it, which is an unusual behaviour for a shark.
Researchers have discovered that the sharks are active at any time during the day or night but are most active during crepuscular (dawn or dusk) conditions. They detect their prey by using their sense of smell or by detecting the weak electric currents produced by the muscles of the animals. Sharks have electroreceptors that contain tubes filled with jelly and connect to the outer world via pores in the animal's skin. The receptors are known as the ampullae of Lorenzini.
Behaviour on Land
It might be thought that by coming onto land the epaulette sharks expose themselves to terrestrial predators. If this is the case, it doesn't seem to be hurting their population size. It's known that the sharks sometimes come into conflict with other fish when they are on land. The epaulette shark is not the only species to leave the ocean and survive. Some species of moray eels occasionally come onto land, for example.
Epaulette sharks sometimes travel over sand to reach desirable feeding areas. They may pause and become stationary on the sand for a while. This appears to be the case for the animal in the photo above. The fish face the prevailing air current when they pause. The behaviour is known as rheotaxis. It's not known for certain why epaulette sharks perform this behaviour. Suggestions include improvement of respiration or a way to detect predators.
The beautiful photo above shows a typical egg case of a shark and the developing animal inside it. Epaulette shark egg cases and the moving youngsters inside them are shown in the video below.
In their natural environment, epaulette sharks breed from July or August to December. Fertilization is internal, as in other sharks. The male inserts sperm into the female's body with his claspers. A clasper is an elongated structure under each pelvic fin. It can be seen in the illustration of shark fins shown above. Sperm leaves the male's body, travels along a groove in a clasper, and enters the female's cloaca.
Epaulette sharks lay eggs and are therefore said to be oviparous. Two eggs are generally laid for every mating incident, which sometimes happens as often as every two weeks. The female ignores the eggs after they are released. She may lay as many as fifty eggs in total during the breeding season, though there is some debate about this number.
Each egg is enclosed in a case that is sometimes known as a mermaid's purse. Fibrous extensions on the case help to attach it to the surroundings. Gestation lasts for around 120 days. The young sharks that emerge from the cases are decorated with black and white bands. These break up into spots as the fish mature. The animals may live for twenty years or more.
Thankfully, the epaulette shark population appears to be doing well. The fish is not of much interest to commercial fisheries. It's captured for the aquarium trade, however, and is sometimes collected by local people who need food.
Some people pick up the fish for amusement when they find it. This is potentially harmful for two reasons. The fish is sometimes injured when it's handled. In addition, it's not advisable to pick up any shark, no matter how docile it seems to be. Although the epaulette shark is often said to be harmless to humans, it may bite when it's frightened. In Australia, some of the animal's habitat is located in marine reserves, which is probably helpful for its numbers.
The IUCN (International Union for Conservation of Nature) classifies the shark in its Least Concern category and says that its population is stable. The assessment is based on data obtained in 2015. It's good to know that the fish seems to be safe at the moment.
The epaulette shark has some fascinating features. More research is needed to clarify some of the facts about the fish and to solve the puzzles linked to its life. Hopefully more information about the animal and more discoveries about walking sharks in general will soon appear.
- New species of walking sharks discovered from the phys.org news service
- Information about the epaulette shark from Aquarium of the Pacific
- Facts about a shark that can walk on land from the Oceanic Society
- Information about epaulette sharks from the ReefQuest Centre for Shark Research
- Hemiscyllium ocellatum discoveries from the Florida Museum of Natural History
- Population status of the shark from the IUCN
© 2020 Linda Crampton