MANTA RAYS
The manta ray a complete delight to encounter and they are so unlike anything else we see underwater.
With their ability to glide effortlessly through the waters where we humans struggle so hard to swim – they have a feel of something otherworldly. No matter how many dives one has done with a manta, there always seems to be very strong sense of awe and the heart always seem to skip a beat.
Wicked Diving assisted in a survey of the manta numbers in both Thailand and Indonesia. This was partly to determine the actual numbers (no one knows how many manta rays there are!). But we also helped with this data collection to determine the economic value of mantas in the tourism sector. If we could show the significant impact they have on tourism, we could help establish more protection for them! We supported this process through our monitoring, through our manta ray educational tours and through financial support.
Anatomy
Manta rays are the largest of all rays. The Spanish word for blanket is ‘manta’ and aptly describes the unique body shape of this animal. With graceful pectoral ‘wings’, manta rays are easily recognised by their paddle-like cephalic lobes projecting forward from the front of the head (actually extensions of the pectoral fins, supported by radial cartilages). They also have a very broad, rectangular terminal mouth. The cephalic lobes help funnel food inside their mouths as mantas are filter feeders who feast on plankton (more information on feeding behaviour below).
Mantas vary in colour. Most are chevron, black on the upper surface of their cartilage body, with white shoulder patches and blotches, and almost pure white on the lower surface of their pectoral fins and body disc. Their body patterns show individual variation and helps identify individuals, as well as species. Some manta rays are melanistic, which means they are completely black with white blotches around the belly area. The rarest manta rays are leucistic, all white mantas.
Distinguishing Features
Although it’s difficult not to recognise an adult manta ray, juveniles can be confounded with mobula rays that grow to 3 metres and share cephalic lobes and gracefully curved pectoral wings. However, mobulas don’t have paddle shaped lobes, they have shorter pointed ones which look a little like horns, earning them the common name ‘devil rays’. Mobula rays have sub-terminal mouths (located underneath the head, similar to many sharks), whilst mantas have terminal mouths (located at the front of the head).
Until 2009, the world knew manta rays as a single species. Dr. Andrea Marshall of Marine Megafauna Foundation re-evaluated the genus into two distinct species. Both have worldwide distribution and sometimes exist in the same region. DNA testing confirmed the genetic divergence. While reef mantas (M. Alfredi) can grow up to approximately 5m, the oceanic or giant mantas (M. Birostris) can grow to over 7m in wingspan. They certainly are impressive but completely harmless to humans as manta rays have long lost the ability to sting. Manta Birostris are pelagic and spend more time off shore. Scientists believe them to be more migratory. Manta Alfredi remains closer to shore. Dr. Marshall is investigating a possible 3rd species around the Atlantic coast of the Americas.
Both identified species differ in multiple ways. These include ventral markings which occur higher than the lowest gill openings only on Manta Alfredi. There are also markings on the upper body in a T shape for M. Birostris and Y shape for M. Alfredi. Lastly, as pictured, oceanic manta have a distinctive remnant spine at the base of their tail.
Behaviour
Manta rays frequently visit reef-side “cleaning stations” to let cleaner wrasse remove small parasites from skin and gill cavities, sometimes several lining up to wait their turn. Mantas sometimes breech, landing with a loud slap, sometimes performing 2 or 3 of these jumps in succession. The act seems to be playing or social behaviour. Getting rid of parasites may also play a role, much like breeching whales.
Seemingly inquisitive, manta rays may sometimes approach and even solicit attention from divers, apparently enjoying the tactile stimulation provided by human contact as well as the bubbles from scuba units. In areas frequented by divers, however, they often become very wary and cease to approach.
When approached rapidly or grasped, they roll onto their backs, dive, or swim away rapidly, righting themselves only when some distance away. Entering the water carefully so as to not scare mantas away will greatly increase the enjoyment of your encounter and protect them from injury. Hovering and staying still will eventually allow the manta ray to approach you. It’s best to position yourself a long the bottom or near a cleaning station to observe them close up.
Touching a manta ray, even if they present their bellies for a rub, will remove some of the mucus that protects them against marine infections. Never touch a manta ray. Stay within their vision and let them decide if they want to approach you. Although some manta rays seem to enjoy the bubbles from the SCUBA unit on their bellies, avoid exhaling bubbles into their face, as it may scare them off.
Flash photography and video doesn’t seem to bother the manta rays of Indonesia, but do not disturb them if they are engage in feeding, cleaning or mating. Mantas occasionally seem to enjoy direct eye contact with divers. Remember to never touch or attempt to ride on one.
Feeding Habits
Even though mantas have up to 300 rows of small peg shaped teeth (the size of pin heads) only on the lower jaw, they really are gigantic filter-feeders, preying on planktonic crustaceans and small schooling bony fishes.
The 2 fleshy lobes of cephalic fins unroll at a downward angle to create a funnel guiding prey into their enormous mouth. Feeding often occurs near or at the surface where plankton accumulates. They may simply swim along allowing plankton to pass into their mouths. Mantas may practise “barrel swimming” when plankton concentration is dense to make the most of the localised bounty. They occasionally feed by swimming along the seabed when plankton is near the floor.
Reproduction
Manta rays are ovoviviparous – the pup develops in a thin-shell that hatches inside the mother, later to be born alive, wrapped around itself. The female gives birth to 1 pup, or 2 in rare occurrences. Birth occurs in relatively shallow water, where the young remain for several years before expanding their range further offshore. Like sharks and other rays, mantas are fertilised internally.
Male manta rays have a pair of penis-like organs called claspers, along the inner part of their pelvic fins. During courtship, males chase the female. Eventually, one will grasp the tip of her pectoral wing between his teeth, and press his belly against hers. Then, the male flexes one of his claspers and inserts it into her vent. Copulation must happen near the surface as manta rays sink when they stop swimming and lasts about 90 seconds. The fertilised eggs develop inside a mother manta’s body for an unknown length of time that may exceed 12 months.
Life Cycle
A newborn manta ray is about 125 cm wide. However, growth is rapid and mantas double in size during the first year of life. Males mature when they reach a size of about 4 metres, females at about 5 metres; it is unknown what age this is. Although, research suggest females are 8-10 years old when they reach sexual maturity. Likewise, the life span of manta rays remains a mystery, but best guesses are about 25 years.
Predation
Although, they often slowly cruise past divers or seemingly hover on to of cleaning stations, manta rays are capable of rapid speed. Only large warm water sharks, such as the tiger shark prey naturally upon manta rays. But their speed is their only defence mechanism against these attacks.
Distribution
Manta ray distribution is circumtropical, around the globe, generally between 35 degrees north and south latitude.
This area includes South Africa, Madagascar, Mozambique to Somalia; in the Gulf of Aden, Red Sea, Arabian Sea, the Bay of Bengal, Burma (Myanmar), Thailand, Malaysia, Indonesia, Philippines, Cambodia to southern Japan, northern Australia, Micronesia, New Caledonia, Fiji, New Guinea, Solomon Islands, Hawaii, southern California to northern Peru, North Carolina to southern Brazil, the Azores, and Senegal to Liberia.
Manta Birostris appears to be more widely distributed, reaching as far north as California and Rhode Island in the USA, North Africa and Japan. They also reach as far south as South Africa, New Zealand and Uruguay. Manta Alfredi is widespread throughout the Asia Pacific region as well as the Indian Ocean.
While there is considerable overlap, it is worth noting that the coastlines of the Americas seem to be the preserve of Manta Birostris. While sightings in the central Pacific region are overwhelmingly of Manta Alfredi.
In Indonesia, we are lucky enough to spot both M. Birostris and M. Alfredi: BOOK NOW
Ecological Considerations
Found in every tropical ocean, mantas stand apart as the largest of all rays; but just as mantas reached star status with scuba divers, promising increased eco-tourism revenues, their populations around the globe plummeted.
The harvest of manta rays in eastern Indonesia has increased exponentially in just a few years. In the Philippines, increasing pressure on local fisheries has forced fishermen to look to these giant rays as an alternative meat source.
Another factor leading to increased manta ray harvests is the new demand for brachial gill plates for traditional Chinese medicine. Skin is also exploited for such as wallets and handbags. Overall this represents a 10-fold increase in farming over historic levels. It is feared that this increased harvest will spread to the Western Pacific.
In the Philippines fishermen are licensed to catch mantas using 1 km-long drift nets about 30m in height. These nets also catch dolphins and endangered turtles, which are being marketed as shark meat. This overexploitation of species population, together with low birth rates and small litters, leaves manta rays highly vulnerable. Since manta rays are filter feeders, they are also affected by (micro)plastics in the ocean.
When it was discovered that entire populations of whale shark and manta ray had been decimated in the Philippines, a ban prohibiting the harvesting of mantas was imposed in 1998. But this was short-lived. 4 years later, due to the lack of resources to implement a sustainable management system, and political pressures from fisherman, the ban was lifted.
A delicate balance exists between the economic well being of small fishing villages, demand for Chinese medicine and the protection of manta rays. Helping local groups protect ecosystems that are frequented by these rays for tourism may be the only viable solution. Conservation efforts need to work internationally and with local communities to protect these majestic creatures.
