Sharks and Rays have been in our ocean for over 450 million years. They are in a family known as elasmobranchs, which have a skeleton made from cartilage (the same as our ears and nose), as opposed to bones like other fish. There are over 440 described species of sharks; from well-known apex predators like Great Whites, Bulls and Tigers; to the largest fish in the ocean (the enigmatic Whale Shark), to timid Whitetip Reef Sharks and Epaulette Sharks. There are even more species of Rays, with over 500 described.
Sharks and Rays can be found within all the world’s oceans in a variety of habitats, from estuaries, to sea grass meadows, mangrove forests, coral reefs and the open ocean. Many species move between these habitats at different life stages.
Australian waters are home to the highest diversity of Sharks, Rays and Chimeras of any continental area. In the Great Barrier Reef there are 134 known species of Sharks and Rays to be discovered. Half of these species are found nowhere else in the world.
Sharks are a keystone species performing an important role. Sharks regulate the populations of prey species, maintaining the balance of the marine environment. In some regions, the depletion of Shark populations led to an increase in other elasmobranch species which preyed upon and decimated Scallops or Lobsters, impacting local commercial fisheries. Sharks also prey upon fish that are sick, ensuring other fish don’t contract the disease or parasites. They prey upon other marine creatures, like Turtles, to ensure sea grass and algae in an area isn’t being over-eaten. Without Sharks, the entire ocean food chain changes.
The Epaulette Shark is a long-tailed carpet shark. Its tail comprises over half the shark’s length. This shark has a slender body with a short head and broad, paddle-shaped paired fins. These paired fins can rotate to be used as limbs for “walking” amongst tidal pools and coral flats. At night, receding tides generally isolate the Epaulette Shark in shallow reef platforms or tidal pools. The combined respiration of all the organisms within a tidal pool can cause the amount of dissolved oxygen to drop by 80%. The Epaulette Shark has evolved to cope with these hypoxic conditions by dropping their blood pressure and respiration rate. In a laboratory they have even survived for an hour without any oxygen. They can be found in shallow waters within the Great Barrier Reef, preferring tidal pools, coral flats and stands of staghorn coral. It is easily identifiable by a white-ringed black spot behind each pectoral fin, which is reminiscent of military epaulettes and is the reason for this sharks’ common name.
The most common species we see on our reefs are Whitetip and Blacktip Reef Sharks. They are easy to identify with a white tip (or black) upon the end of the first dorsal fin and caudal fin (tail).
During the day, the Whitetip is quite docile and is often spotted resting upon sandy bottoms next to coral reefs. Unlike other shark species, they do not need to swim to breathe, as they are able to pump water over their gills instead. At night, the shark can become aggressive, thrashing through coral reefs when hunting for prey. Generally, they are solitary hunters, but have been seen working with other Whitetips in the pursuit of prey. They feed primarily on Octopus, Lobsters and Crabs, chasing them into a crevice where they jam their body in, after sealing the exit and thus, capturing their meal.
The skittish Blacktip Reef Shark is generally found in shallower water (sometimes in only 30 centimetres). When attacking prey, they are able to breach (like Great Whites and Mako Sharks), leaping completely out of the water. The speed and force of an attack of this nature can result in acrobatic spins.
There are two Blue Spotted Rays commonly spotted by our snorkel guests; there is the Blue Spotted Kuhl’s Stingray and the Blue Spotted Ribbontail Ray. Kuhl’s Stingray have a more triangular-diamond shaped body, while the Ribbontail is rounded with very vivid electric blue spots and streaks that run the length of its tail. Behind the eyes of both species are spiracles. As the gills are located on the underside (or ventral side), these spiracles allow water to reach the gills while they rest or feed upon the bottom. Both species rarely bury themselves in the sand, except to hide from predators. Other stingray species bury themselves regularly to hunt. Look under coral ledges to spot these rays.
Over one-quarter of Sharks and Rays are considered vulnerable to endangered on the IUCN Red List of Threatened Species. Elasmobranch species are particularly vulnerable to population declines, as they have slow growth rates, low reproduction rates (sometimes only bearing one or two pups every three to four years), and mature late. As many subpopulations are low in abundance, it takes local populations a long time to recuperate.
© ABC Snorkel Charters 2023