Manta Ray

Scientific Name: Manta birostris

The Manta Ray, or Devil Ray as it is also known, is the largest of the ray family and is closely related to the shark. Their wingspan (the distance across their pectoral fins) averages 4 meters (and reaches up to 9 meters.) and they can weigh up to 1,350 kg.

Mantas are most commonly black or dark brown on top and usually white on their belly, although some are blue on their backs and specimens from the eastern Pacific often feature dusky to mostly black undersurfaces. A manta's eyes are located at the base of the cephalic fins on each side of the head, and unlike other rays the mouth is found at the anterior edge of its head. To breathe, like other rays, the manta has five pairs of gills on the underside of their bodies.

They possess distinctive "horns",  or cephalic fins, on either side of its broad head. These unique horns are actually structures made by the pectoral fins where a part breaks off during the embryological stage and moves forward and surrounds the mouth. This makes them the only known example where jawed vertebrates have evolved novel limbs. These flexible horns are used to direct plankton and water into their very wide mouth. To make them more streamlined when swimming, they are able to curl them up, which makes them resemble horns, hence the source of the name, 'Devil Ray.'

Because mantas are filter feeders and not bottom feeders, they are able to grow to a size larger than any other species of rays. Their pelagic lifestyle as plankton feeders has also degenerated some other ray characteristics. All that is left of their oral teeth is a small band of vestigial teeth on the lower jaw, almost hidden by the skin. They are closely related to stingrays, but they don't have any stinger. They have a much thicker body mucus coating than most rays and their spiracles have become small and non-functional (water is taken in through their mouth instead).

To better swim through the ocean, they have evolved a diamond shaped body and they use their pectoral fins as graceful 'wings'. They are also one of the few marine animals seen to breech the water.

Encounters

Giant Mantas are found in temperate and tropical waters near continents and islands of all oceans, typically around coral reefs. Usually they migrate around the world in search of plankton-rich waters. Mantas are most commonly sighted in the daylight hours and are often encountered on reef slopes where they come in from the open ocean to feed. Curious, they will often circle right above diver's heads. Often large groups of mantas will congregate at a 'cleaning' station, where small wrasse-like fish swim up to pick parasites off the manta's skin. Mantas can also be seen in large formations in the open ocean, when they migrate in search of food.

The Maldives is home to an abundance of Manta Rays, and from May to November each year, they can be seen in the eastern side of the atolls, while from December to April, they are encountered on the eastern side of the atolls.

Exmouth in Western Australia is home to manta rays between March and July.

Yap is the best known place for Manta encounters, where plankton rich waters are funneled through channels, offering divers amazing encounters with these graceful giants.

Kona, on the The Big Island of Hawaii has a well known late night spot right off the beach where manta rays congregate in the lights from shore.

What's it like to dive with mantas? Click here to find out some of ZeroBar.org's experiences with these majestic creatures.

Diet

Mantas evolved from bottom feeders and adapted to become filter feeders in the open ocean. They are primarily planktivores which means they generally feed on plankton which are filtered out from the water by their gill rakers (a type of filter feeding called ram-jet feeding). They use the unfurled cephalic (head) fins on the head to funnel plankton-rich water into the mouth where gill rakers filter out the plankton. Some small crustaceans and fish might complement their diet, but like other filter feeders, Manta rays have reduced, nonfunctional teeth and are no threat to larger ocean animals. Unlike many other rays, Mantas don't a spine on the tail. If they are threatened their only defense is their size and powerful wings.

Large warm water sharks such as TIger sharks are the only known animal to prey on the manta.

Reproduction

Giant Mantas reproduce via a placental viviparity. This is when animals hatch from eggs, but the eggs hatch and the babies develop inside the female's body. There is no placenta to nourish the pups. Females give birth to 1-2 pups which are about 1.2 m wide and weigh roughly 45kg. They are born rolled up like tubes and become active as soon as they have rolled out their wings. Witnessing the birth of mantas is extremely rare. Once born, young mantas grow very rapidly.

Male Mantas have a pair of penis-like organs (claspers) that are developed along the inner part of their pelvic fins. Each clasper has a groove through which sperm is transferred to a female Manta's body, where fertilization takes place. During courtship, one or more male Mantas chase a female for prolonged periods. Eventually a successful male grasps the tip of one of her pectoral wings between his teeth and presses his belly against hers. Then, the male flexes one of his claspers and inserts it into her vent. Copulation lasts about 90 seconds. The fertilized eggs develop inside a mother Manta's body for a lengthy but unknown period that may be 9 to 12 months or more.
During the mating season the mantas gather in large numbers (they are otherwise solitary creatures) and several males can bee seen courting single females.

Attacks

The manta imposes minimal danger to humans unless attacked (especially harpooned) or otherwise startled. The enormous size and power of this ray should, however, invite respect.

Captivity

Very few aquariums have the enormous tanks and filtering capacity needed to display a full-grown Manta. Although there have been many attempts to display Mantas, most specimens refuse to feed and die within a few hours or days of being deposited in a tank. The Expo Aquarium, in Okinawa, Japan, has successfully maintained Mantas for as long as 36 days. Despite the failures of the past, several large aquaria continue to refine and test capture and transport methods toward the goal of maintaining a healthy Manta in captivity indefinitely.

Conservation

Mantas seem to be fairly abundant in some areas, rare or absent in others. Until we understand the extent and dynamics of Manta stocks, there is no way to assess their conservation status. Based on their low birth rate, Mantas are probably highly vulnerable to sustained fishing pressure and habitat degradation. This likelihood would seem to favor a cautionary approach to Manta exploitation and management until such time as we have the sound scientific data to make a more informed assessment of this species' risk of extinction.

Manta birostris                                                                                             

Giant Manta
Kingdom: Animalia
Phylum: Chordata
Class: Elasmobranchii                                                                          
Order: Myliobatiformes
Family: Mobulidae
Genus: Manta
species: Manta birostris

Manta Ray Conservation and Research projects:

Lady Elliot Island, Australia: www.ladyelliot.com.au/manta

Croll Tershy Laboratory, California http://bio.research.ucsc.edu/people/croll/mantaray.html

Iemanya Oceanica & Manta Mexico www.iemanya.org

The Manta Network http://mantas.somebox.com/html/

 

Diving With Manta Rays: Dave

I have been lucky to dive with manta rays in the Maldives, Western and Eastern Australia, Djibouti, the Similans, and Zanzibar. Because of their large size, people are often afraid of them, but by far they are my favourite encounter in the ocean: graceful, mysterious, majestic, and awe inspiring. A dive where a manta ray is present pre-empts virtually everything else, well, maybe not a whale shark!

Off North Stradboke Island off the coast of Brisbane, Australia, I dived with 7 mantas, who were visiting this desolate part of reef to allow cleaner wrasse to pick off dead skin. They circled for over 50 minutes, and one manta in particular, a young one, was particularly curious, and hovered right over my head for up to a minute at a time, positioning it's head so it could get a better look at me. Then, it was gone, but, less than thirty seconds later, it would return, and hover, motionless, before disappearing.

When I lived in the Maldives, we dived frequently at manta cleaning stations, where we would sit as still as we could on a rubbly coral bottom at 4-5 meters, watching up to 8 mantas circle in, stop, hover, and wait for the tiny blue fish to come out at pick their skin clean of parasites. With some of the clearest waters in the world, visibility was excellent, but with one flap of their wings, these giants of the deep would disappear, while we would wait, breathless, for them to return. This they did, but eventually, they wouldn't return.

The best encounter I ever had was in Rasfari, in the Maldives' North Male atoll, where we snorkeled in 2 meter deep water with 12 mantas, that circled around us, in ever tighter circles, growing more brave as their curiosity got the better of them. I was alone, and there were five mantas, equally spaced in a circle around me, going round and round as I hovered at the surface. When I dived down and finned along the bottom, they would break their formation, and follow me, until I surfaced, and then they would begin circling all over again. 

I felt like these creatures could sense my love of the sea, and were protecting me, and it was with terrible hesitation when I had to board the boat back home, leaving these friends behind. When I went diving at Rasfari again, the following week, the manta rays were gone; the monsoon season had shifted, and they were off to the eastern side of the atolls where food was more abundant.

Questions about diving with Mantas? Email Dave here.