Zebra sharks (Stegostoma fasciatum) are known in Thailand and around the Andaman Sea as Leopard sharks. This is confusing as there is another species of shark called the Leopard shark (Triakis semifasciata) found along the Pacific North American coast (See image below). It is an amazing experience as a diver to get close to these sharks whilst they move slowly across the sea bed. Unfortunately sightings of them are becoming more and more rare. Hopefully with your help through citizen science and investigation into why their population is decreasing, we can play a part in their spectacular come back.
Photo Credit: Zebra Shark by Ivan Hiver
Anatomy and Appearance
There is no size difference between males and females and they can grow to a length of 3.5 meters, with its tail, or lower caudal fin to give it its proper name, making up nearly half of that length! It has small barbels (slender, whisker-like sensory organs) on its snout, a small mouth, and small eyes. Their spiracle is located just behind the eye. Its teeth are pointed, with each tooth having two smaller, lateral, flanking points with prominent ridges. The name 'leopard' comes from its' adult appearance. Their juvenile appearance however is completely different with white and black stripe patterns. This is why they are called Zebra Sharks.
Juvenile Zebra Shark
https://creativecommons.org/licenses/by-sa/3.0), via Wikimedia Commons
A female albino zebra shark, measuring 2 meters, was found with no 'spots' at all but divers were able to identify her due to her five dorsal ridges, one along the dorsal midline and two either side which are quite prominent. The two pectoral fins are broad and they have been known to prop themselves up using these when resting on the sea floor. The dorsal fins are smaller and pelvic and anal fins are even smaller again.
Zebra sharks (Stegostoma fasciatum) - also known as Leopard Sharks in Asia
Photo Credit: Ivan Hiver
The "REAL" Leopard shark (Triakis semifasciata)
By Mfield, Matthew Field, www.photography.mattfield.com
Habitat and Migration
They are found throughout the tropical indo-pacific region on sandy flats and in and around coral reefs at depths of up to 62 meters. In Australia's summer and autumn period, Zebra sharks have been known to migrate to shallow coastal coral areas from deeper waters. Channels between the coral reefs are ideal for them to rest in the day, as it a provides faster, more oxygenated flow of water. In these channels they use their pectoral fins to prop themselves up whilst opening their mouths so that water can enter and respiration is possible with very little effort. In stronger currents they have also been observed 'surfing', using their pectoral fins, not to lean on, but to undulate, keeping themselves steady in the current.
Zebra sharks are generally nocturnal hunters, eating crustaceans, small bony fish, and they even enjoy the odd sea snake. It uses its small mouth and muscular buccal cavity to create large amounts of suction - this in combination with its narrow head means it can wiggle into many crevices other larger predator species can't. The barbels attached to its nostrils are common in benthic sharks and facilitate numerous sensory organs and that locate prey. They also channel water into the olfactory cavity where the shark can detect even more information about its prey.
They are usually solitary creatures although they are known to form large seasonal congregations of between 20 and 50 around the world. Specific to Queensland they appear in the hundreds. The females outnumber the males by three to one and whilst we don't know why they congregate we do know it is not for mating. In order to mate, males attract females by vigorously biting their fins! If he is lucky, the male is then allowed to mate with his chosen ‘victim’. The female can lay up to four eggs at a time which are around 17cm long, brown or purplish black in color and are attached to the substrate. These take between four and six months to hatch. When the young eventually appear, they are about 20-36cm long and are the beautifully striped in thick bands of black and white and can live for up to 30 years.
A mother and daughter Zebra shark in captivity in Australia shocked keepers when they both laid eggs containing live embryos, subsequently hatching five Zebra pups after being separated from the males for two years! Sperm storage has been recorded in shark species but the genetic analysis of the pups suggests that it more likely to be asexual reproduction!
Conservation & Citizen Science Research
Whilst they are preyed upon by larger fish and marine mammals, it is humans that provide the greatest threat to their existence. They are hunted by humans for their meat, fins and liver oil, unsurprisingly, their numbers are dwindling rapidly as they are an easy catch due to their relatively shallow habitat. They are classified as 'endangered' by the IUCN in all of their habitats except one. Australia has enforced a ban on their hunting and as such they are classified there, and only there, as 'least concern' - go Aussies go!
Organizations such as Shark Guardian and Thai researchers Phuket Marine Biological Center have taken inspiration and are working with The University of Queensland to implement and improve the Spot the Leopard Shark project. Members of the diving community and other ocean users are encouraged to log their experiences with the Zebra shark. Shark Guardian is pleased to help promote this project in Thailand. Please get involved, spread the word and contribute your photographs - and of course, if you identify a new shark to the database you get to name it!
Relationship to Humans
Zebra sharks are docile, generally slow-moving and are not dangerous to humans. Only one provoked attack has every been recorded, they have however bitten divers who pull their tails and try to ride them. In fairness, I would get upset if a stranger tried to jump on my back and ride me!
Whilst Zebra sharks in the wild can live up to 30 years, in captivity they only live to about half that. Many public aquariums have them captive due to their docile nature, this may hold out some hope for future population declines. More often than not captive releases don't end well but scientists from all different disciplines are always learning and improving techniques in improving this process. With the work that many organizations are doing, including Shark Guardian and Spot the Leopard Shark, we hope that we won't need to rely on this for the future of the species.