Giant Rays of the Deep
The Manta Ray
The manta ray was once believed to be a monotypic species – the only species in the genus Manta. They belong to the family Myliobatidae
However this taxonomy is likely to change as some authorities are pushing for a second species to be recognised. These two are called the giant manta (Manta birostris) and the reef manta (Manta alfredi). Other common names include Atlantic manta, Pacific manta and devilfish. The word 'manta' derives from the Spanish word for blanket or cloak.
It is the largest species of rays. The largest known representative had a width in excess of 7.6 metres and a weight of 1,300 kg. It has the largest brain-to-body ratio of the sharks, skates and rays and will dive as deep as 500 metres.
The manta ray is a most graceful swimmer and has a gentle nature. Any snorkeler or diver fortunate enough to have an encounter with one of these giants is awed by its size, often describing the sensation of a dark cloud moving overhead and taking forever to go on its way.
The manta ray is found throughout all oceans but typically around coral reefs and over continental shelves in tropical and subtropical regions.
The manta ray has a skeleton composed of cartilage rather than bone.
The body shape of the manta ray is very distinctive with its triangular ‘wings’. Paddle-like lobes or feeding flaps extend in front of the mouth. These are fleshy extensions of the pectoral fins. At the top of the flaps are the eyes. On the underside of the eyes and to each side are five gill slits.
The upper surface may range from black to greyish-blue to brown and the undersides are pale. Each ray has a unique pattern of splotches and scars which can be used for identification purposes. The lower jaw has 18 rows of teeth but these are vestigial and almost hidden by skin. The cavernous mouth scoops up planktron and krill.
The spiracles are non-functional as all water is taken in through the mouth. Spiracles are small holes behind each eye. These open to the mouth but, as stated, are non-functional in the manta ray.
The body is covered with a thick mucous, much thicker than that found on other rays. Although the whip-like tail is similar to that of a stingray, there is no stinger and thus no danger to divers.
Manta rays are often seen in groups and spend a lot of time near the surface. Reef mantas have a stable population of 6,000 to 7,000 around the Maldives where the water is rich in plankton. Reef mantas may form a feeding vortex with whale sharks.
Mantas commonly visit cleaning stations, making slow circuits round an area while small fish ‘clean’ the manta of parasites and dead cells. Remoras, angelfish and wrasse swim in the gills and over the body, feeding off the dead tissue. Sucker fish have a large sucking disc on top of the head and may attach themselves to larger fish, picking up dropped prey .
Mantas will sometimes launch themselves from the ocean, hitting the water with a slap as they land again. This may be a way of communicating with others or just a play activity. Although they seem graceful and slow, they can move at fast speeds if the need arises. Groups of males sometimes perform acrobatic stunts for a female’s attention,
The manta ray is a filter feeder. It scoops in water and food through the mouth. Modified denticles on each gill arch filter plankton, fish and larvae from the water. They are bottom feeders, catching their prey on gill rakers. These rusty-coloured, spongy, flat plates of tissue fill the spaces between the gill bars. Around 20 to 30 kg (44 to 66 pound) of plankton may be ingested daily.
On either side of the mouth, fleshy projections filter prey into the opening. These projections or lobes are closed or furled in front of the mouth when not feeding. Slow vertical loops when feeding are believed to concentrate prey.
Males may follow a female for up to half an hour. Mating patterns seem to be triggered by a full moon. Other males follow the first male in a 'mating train'. Mating takes place just below the surface. The male bites the pectoral fin, inserts the claspers into the cloaca and stays in place for a minute or two. The eggs may stay in the female for as long as 12 months. The eggs hatch internally and eventually the female has two young (on average). There may be a two year gap before she gives birth again. This process of hatching eggs inside the mother’s body is known as ovoviviparity.
The main natural threats to the manta ray are large sharks and, occasionally, orcas. Fishing and use of body parts, especially gill rakers, in Chinese medicines, has also resulted in a decline in manta ray populations. There is no data on manta rays being a significant form of by-catch from commercial fishing operations, certainly not in Australia. As they are more likely to be found near shallower waters, there doesn’t seem to be too many rays lost through misadventures with fishing nets and trawlers.
Threats are most likely to take the form of activities or processes which impact on water quality or which disturb the habitat in which they live.
Because of their size, manta rays are rarely kept in captivity. However there are a few. Nandi now resides in a 6.2 million gallon exhibit, the Ocean Voyager, at Georgia Aquarium after outgrowing her previous home at uShaka Marine World, Durban, South Africa.
In Okinawa Churaumi Aquarium, several manta rays have been born in captivity.
In 2011, the manta ray was included in the Convention of Migratory Species of Wild Animals (CMS) and is now strictly protected on a global scale. The International Union for Conservation of Nature (IUCN) has listed the giant manta ray as ‘vulnerable with an elevated risk of extinction’. This happened in October 2011.
The manta ray can be seen in the wild at the world heritage listed Ningaloo Reef off Western Australia's coast.