Manta Ray - Australian Geographic Research
Article from Australian Geographic April-June 2002 Issue 66, text by Liz Ginis
see (1 of 3) MANTA RAY - Demystifying this enigmatic creature of the deep for full article and all photos and illustrations.
STAINED COBALT BLUE, the back of a manta ray is difficult to distinguish from the ocean in which it lives. Its underbelly, however, is a stark white canvas splashed with grey - a dash here, a dot there - and I'm staring up at one, just an arm's length away.
Together we hang, as though suspended on puppeteer's strings, before glassy bubbles from my regulator float upwards, tickling the animal's belly, making it squirm and then slowly move away on silent, beating wings
Manta rays are often encountered around the coral bomrnies, or outcrops, of Lady Elliot Island, a compact coral cay at the southern extent of the Great Barrier Reef Marine Park. They congregate here to be preened by cleaner wrasse - small fish that feed on skin parasites and pieces of loose flesh.
And my aquatic mate hadn't finished with the service. In two wing-beats it moves to the opposite side of the bommie, and suspended once again, is now eyeball to eyeball with me. The experience is pure, unadulterated magic
IDENTIFYING A MANTA
ONE OF THE LARGEST, yet least-known of fishes, the manta ray is found mainly in tropical waters throughout the world. In Australia, they are encountered along the reef edges of Queensland and Western Australia, where tidal action pumps rivers of eggs, larvae and tiny crustaceans the manta's preferred diet - into the open sea; but they've also been spotted as far south as Montague Island in southern New South Wales. Like other members of the shark and ray family, the manta's internal structure is'based on tough but flexible cartilage, not bone. And it's cartilage that helps this graceful animal "fly" through the water (see Under the skin).While sharks are a manta's distant
cousins, eagle and cownose rays, which are closely related, look so similar that people often mistake them for juvenile mantas.
But once you've identified a manta's distinctive features, there's little confusion. A manta has cephalic lobes -large, fleshy protrusions that sprout forward like horns trom its snout; a very broad, rectangular mouth at the front of the head; and, a short, barbless tail. Mantas also grow larger than other rays - wingspans stretch to 9 metres - and top the scales at 1500 kilograms. Finally, they move swiftly in open water, at up to 24 km/h, but are more often seen by humans being preened by wrasse at "cleaning stations" or while feeding.
Primarily solitary animals, manta rays congregate in shallow water to breed, resembling squadrons of stealth bombers when they do.
SO MUCH TO LEARN
DESPITE THE GROWING fascination with manta rays, little is known of their biology, behaviour or movements. Australian Geographic Society-sponsored marine biologist Mark Simmons is committed to learning more.
"Next to nothing is known about these enigmatic creatures," Mark told me as we floated over The Coral Gardens, a confetti of tropical corals and shimmying fishes on the north-western side of Lady Elliot.
Mark has radio-tagged three manta rays believed to be part of a 40-strong population permanently living in the waters surrounding the island.
"I've also placed a data logger in the water near one of the coral bommies where [scuba] divers commonly encounter mantas," Mark said. "When a tagged manta swims within a 500 m radius of the data logger, it records the time, position and identification number of the animal. After six months, the information on the data logger has confirmed that these animals appear to be permanent residents. Continued monitoring, and the placement of a few more data loggers around the island, will hopefully reveal more."
Mark's work also includes a photo-identification program, which involves photographing the ventral surface, or underside, of individual rays. "The markings on the belly of a manta are akin to a human fingerprint," Mark said. "So by recording and creating a database of images of different mantas, we're again learning more about the resident population as well as about migratory animals that may stop off here to feed or breed."
Why is such information important?
"We've no idea what the standing Australian or global population of mantas is," Mark said. "Nor what impact humans have on these amazing animals. But given they're an unprotected species, and mankind's track record in driving wildlife to the brink, it's time we knew more."
While research can be carried out on dead manta rays, it's far more difficult to study them while they're living. They usually die in captivity, refusing to eat and apparently experiencing huge stress when confined. Three mature mantas living in an aquarium in Okinawa, Japan, are the only ones known to have survived for any length of time in a tank, but the size of the tank is massive 200 tonnes of seawater kept at 24.7°C - and they've been there sinc~ just after birth
A HEARTY APPETITE
MOUTH AGAPE and flapping slowly in swirling loops towards the sun dappled surface, a manta feeds. Nearby, I watch as the gigantic filter-feeder unfurls its cephalic lobes and uses them like slender, artistic hands, sweeping water into a cavernous mouth.
On scuba at 10 m near Lady Elliot's Lighthouse Bommie, I'm a guest at the manta's dining table, and as I settle down to watch, I'm mesmerised.
Nearing the surface, and having finished the first course, the manta points its lobes to the sea floor and plunges down, stopping a metre from me. Then, it begins its spiralling ascent - believed to concentrate or herd the plankton - once again.
The manta ray, like the whale shark, feeds on plankton - a mix of tiny plant organisms, crustaceans and small, schooling bony fishes - by filtering plankton-rich water through its gills) which are composed of pinkish-brown spongy tissue arranged in flattened, horizontal plates.
But the manta hasn't always been a filterfeeder. Nine to 12 rows of grinding teeth in its bottom jaw attest to this. Now covered by a thin layer of soft skin, the teeth are remnants of an evolutionary era when mantas hunted larger prey.
Today, the only biting a manta does is during mating.
IT'S ALL IN THE BREEDING
BELIEVED TO BE sexually active from about five years of age, manta rays mate in the summertime in depths of 1-10 m when water temperatures are 26-29°C.
Playing a game of cat and mouse just before copulation, several males court a female, swimming closely behind her at faster-than-usual speeds (9-12 kmfh). This courtship lasts for 20-30 minutes before the female decreases her speed, allowing a male to nip the tip of one of her pectoral fins (wings), severely hampering her swimming ability. The male then positions himself on the underside of the female, abdomen to abdomen, and inserts one of his claspers (paired sex organs developed along the inner margins of the pelvic fin) into her cloaca (vent). This usually lasts 90-120 seconds. Once the sperm is released, the male continues biting the female's fin for a minute or so before swimming off. Another male, which circles the couple during mating, then moves in and repeats the process. After the second encounter, the female swims away, leaving other courting males behind reinforcing the belief that the most dominant males mate, ensuring the continuity of the strongest gene pool.
Gestation takes about 13 months, during which time the one or two developing pups are wrapped in a thin-shelled "egg" that hatches inside the mother. Soon afterwards, the female gives birth in relatively shallow water, where the pups remain for several years before expanding their range further offshore. When born, they measure 1-1.5 m in width and weigh up to 11 kg, while their fins are wrapped in an s-shape one above and one below the body. They double their size in the first year of life.
RECENT RESEARCH into the anatomy of the manta ray has concluded that this intriguing animal has one of the largest brains of all cartilaginous fishes, and that the portion of it that deals with reasoning is highly developed.
I pondered this when I met Superman - a 4 m wide manta named for the white, diamond-shaped blaze on his otherwise blueblack belly - while snorkelling off Lady Elliot. He'd circled me a few times, an inquisitive eye following my movements, before coming in close enough to touch. And then we were swimming together wing-beat and fin-kick - on the surface of the ocean; just Superman and me.
Had he sized me up and reasoned that, given his size and stature, I posed no threat? Or was he coming in closer to inspect an oceanic curiosity?
I'll never know the answer - but like any good comic-book heroine, I fell for my superhero that day: hook, line and sinker
TO RIDE; OR NOT?
AUSTRALIAN GEOGRAPHIC photographer, Mark Spencer,tackles the sensitive issue of riding manta rays after doing just that off San Benedicto Island, Mexico.
Some people may disapprove of my actions and I fully concur that wild animals generally shouldn't be touched, but when the 4-metre-wide manta positioned itself directly beneath me, barely moving, I knew it was inviting me to "hop on". Taking a few deep breaths through my snorkel, I dived down and gently placed the flat of my hand on its back, which caused a muscle to quiver in what seemed to be a sensuous response. Then, with both hands holding on to the hard, rounded edge of the manta's upper lip (rostral edge), and with knees resting on either side of its spine, I was taken for the ride of a lifetime.
As it beat its "wings", I felt the manta's muscles bunch then stretch, propelling us through the Pacific Ocean with the grace of a bird in flight. The skin, stretched taut from the base of the wing to the spine, was rough and sandpaper-like when I ran my hand forward, but smoother and almost polished in the reverse' All too soon, my lungs cried out for air and reluctantly I returned to the surface.
It'd taken nearly two days of diving at a small seamount beside the island of San Benedicto - where I'd heard a group of about 12 large mantas allowed people to ride them - before one allowed me this intimacy. The seamount is a submerged volcanic plug rising almost vertically from a depth of some 60 metres to within 6 m of the surface. The mantas inhabit the waters around the seamount because of a cleaning service provided by clarion angelfish - a bright orange fish found only in a small area of the eastern Pacific.
Why did this manta allow me to ride it? It's possible it saw me as a big cleaner fish. But it seemed to me that the manta simply enjoyed the interaction.
Later that day while diving with the mantas, I saw how they sometimes discourage people from hitching a ride. One of them inverted itself, and, suspended upside down, stared at me as though asking: "How do you intend getting on my back now?"
I've dived with mantas throughout the world, and have concluded that without doubt, the behaviour of the San Benedicto mantas is unique - it's highly unlikely that even a large manta in Australian waters would want to be ridden. When a wild animal seeks out human interaction, however, I believe it's up to the individual to either deter the animal or indulge it. Regarding the possibility that touching these mantas might unwittingly cause skin irritations or infections, I can only say that I saw no evidence of this where divers regularly hold them around the snout area.
Manta rays and human beings are obviously very different from one another - in appearance and the environment they inhabit - yet the mutual curiosity and trest I shared with some of these creatures off San Benedicto Island convinced me that we have more in common than people generally assume.
DEVIL MAY CARE
Derived from the Spanish word for cape, the word "manta" is now synonymous with this intriguing marine creature. In centuries past, however, it was known as the "devil fish" for its cephalic lobes or "horns", and was believed to be aggressive and harmful to humans. Stories told by sailors and fishermen had the manta capsizing small fishing boats by leaping out of the water and crashing down upon them. This could have a basis in fact as mantas do launch themselves out of the water from time to time, but their motivation is hardly malicious: scientists believe they're simply trying to remove parasites that feed on their flesh.
Another misconception is that mantas drown swimmers by wrapping their pectoral fins around them. It's more likely that mantas have become entangled in boat mooring lines and, given their weight and strength, have nearly pulled boats under - the boats' passengers then wrongfully fearing the manta's retribution.
Throughout the 1800s and until the mid-1900s, manta rays were commercially hunted by being harpooned from small fishing boats in tropical waters throughout the world. As with sharks, the skin on the dorsal (upper) side of a manta ray is covered with denticles - raised, tooth-like scales - which made it useful as sandpaper. The animal's liver, rich in oil, was also deemed valuable. Today, mantas are rarely hunted - they're more valuable to the tourist trade as people flock to coastal destinations off Hawaii, Japan and northern Australia to swim with them in the wild.
Caption 1:Like a languid bird in a cloudless summer sky, a graceful manta ray wings through the waters of the Great Barrier Reef pursued by fastidious cleaner wrasse. Cloaked in mystery, the manta is the largest of the rays and is found throughout the world's tropical waters.
Caption 2:A)While people do ride manta rays, touching them can be detrimental. They're coated with slime, a sugar-based substance known as glycoprotein, which reduces surface tension or drag. It also plays a role in preventing infection. Once the slime is removed by parasites, remoras or in some cases, human contact, the manta's skin can develop pink lesions.
A)The diminutive size of the blue-streak cleaner wrasse belies its position as one of the ocean's most effective cleaners. Many marine species, including the 30 cm long chameleon parrotfish, make good use of its services.(left :Scarus chameleon.Right Labroides dimidatus
B)Teira batfish fin against the current under the still-attached mast of THE SEVERANCE, which sank off Lady Elliot Island in 1999. Found singly or in small schools, the teira batfish is commonly encountered in the tropical waters of Australia.(Platax teira)
C)Lady Elliot Island dive instructor Jason Taylor watches the wriggling antics of a zebra shark at Lighthouse Bommie. This shark is most often seen on or near the sea floor, where it uses flat, grinding teeth to feed on molluscs. (Stegastoma fasciatum)
D)Green turtles enchant snorkellers and divers off Lady Elliot year-round. Their presence in the water is supplemented by land sightings during October and November when they come ashore in their hundreds to lay.(Chelonia mydas)
E)ALL IN THE FAMILY - While few may know that sharks and rays are distant cousins, examining their physical structures reveals more than a passing family resemblance.
E1)GREY REEF SHARK (Carcharhinus amblyrhynchos)A medium-sized whaler shark, the streamlined grey reef shark inhabits coral outcrops throughout Australia. It has the classic silhouette that fits the image most people have of sharks.
E2)ZEBRA SHARK(Stegastoma fasciatum)Unaggressive and with a rounder profile than the grey reef shark, the zebra shark rests on the ocean flooras do rays - on its enlarged pectoral fins.
E3)BANDED WOBBEGONG (Orectolobus ornatus)A bottom dwelling shark, the banded wobbegong has a flattened head and body, similar to that of a ray.
E4)WHITE-SPOTTED GUITARFISH (Rhynehobatus djiddensis) Also known as a sandshark - a misnomer as it's actually a ray - the white-spotted guitarfish has the body of a shark and the head of a ray.
E5)BLUE SPOOTED MASKRAY (Dasyatis kuhlii) The blue-spotted maskray's pectoral fins have taken on a wing-like appearance, while two spines on its tail take the place of a dorsal fin.
E6)MANTA RAY(Manta birostris)The shape of a manta's pectoralfin tips bears close resemblance to those of sharks. Given its habit of swimming with fin tips poking out of the water, a manta ray is often mistaken for two sharks swimming close together
A)Lolly-coloured fish rush an approaching manta that's come to be cleaned at Anchor Bommie off Lady Elliot Island. Once above the coral outcrop, the manta will open its mouth and gills to allow the small fish to feed on parasites and pieces of loose flesh inside.
B)Australian-first research at Lady Elliot Island. Using a hand-held spear gun, Clint Hempsall (bottom) implants' a lipstick-sized radio tag (below left) into the coarse skin of the ray's shoulder. But finding a suitable candidate for tagging isn't always easy. While snorkelling (below right), Clint searches, pulled along by a boat to cover maximum territory.
A)Uncannily resembling a squadron of stealth bombers (below), manta rays swim off WA'S central coast near Ningaloo Reef. Mostly solitary animals (right) that roam the seas propelled by their undulating "wings", they congregate to breed during summer.
B)UNDER THE SKIN The skeletal structure of a manta ray, including its skull and spine, is based on cartilage not bone. This adaptation, which is almost exclusive to sharks and other members of the ray family, reduces body weight and helps with buoyancy. Movement is achieved by muscle contractions that cause wave-like ripples along its pectoral fins, or wings.
C)PIPES AND ORGANS Covered in thick, sandpaperlike skin, a manta's internal organs are well protected by its cartilaginous skeleton. Its liver, which is at least six times larger than its heart, is rich in oil. This aids the animal's buoyancy, allowing it to hang almost motionless in the water.