What’s that…? A bird? A dolphin? No it’s a ray! A mobula ray can be seen leaping over one metre out of the water and making an impressive splash for reasons only known to itself.
Even with its large size the mobula ray is an elusive animal with the largest brain to body ratio of any fish. It has a complicated classification record and life history, making it not only a mystery to divers and snorkelers, but also researchers. It is from the family called Mobulidae, which also includes oceanic and reef manta rays. They can be found worldwide in temperate and tropical waters. Different species prefer different oceans; for example the giant mobula ray can be found relatively commonly in the Mediterranean and Eastern Atlantic, whereas the short-fin pygmy mobula ray can be found in the Indian Ocean and Western Pacific. Large aggregations can be encountered in Hawaii, Republic of Maldives and Mexico, but recently due to population decline these aggregations are getting smaller and less frequent.
Mobula rays are often referred to as devil rays, due to their horned appearance which results from their cephalic fins (fins on either side of their mouth) being rolled up. Despite their name devil rays are considered harmless and shy. Originally there were thought to be 12 distinct mobula ray species, but due to advances in molecular biology and genetic studies it has been concluded that there are only nine species and that manta rays are included in the mobula ray family. Currently two separate species of manta rays are recognised, but there could be a third: the black morph manta ray (Manta birostris sensu). This species is currently undergoing DNA examination by Dr. Andrea Marshall of the Marine Megafauna Foundation.
From fossil records it has been concluded that mobula rays first appeared 25 million years ago with other species evolving over time. For example, the manta ray species appeared in fossil records five million years ago. Mobula rays originally evolved from stingrays, which is why some still possess the stinging spine at the base of their tail. Unlike their predecessor who have spiracles to aid their breathing mobula rays must constantly stay mobile to oxygenate their blood.
Mobula rays are the only species of vertebrate that have three working limbs (pectoral, pelvic and cephalic fins). The smallest species of mobula ray is around one metre in wingspan whereas the largest, the oceanic manta ray has an impressive eight metre wingspan. Mobula rays are known to perform amazing aerial displays, including high jumps, twists and belly flops. There is debate over the reasons behind this; theories include communication, courtship displays, escaping predation threats and removing parasites.
Mobula rays are ovoviviparous. This means that females produce eggs which are hatched internally so that they give birth to live young. Normally a single pup is delivered, but occasionally two can be born. Mobula rays have long gestation periods; for example the giant devil ray has a pregnancy period of two years. All species of pups are born relatively large; for example manta ray pups are around one metre in wingspan at birth. This is because there is no maternal bond between mother and pup, and so after birth the pup is left to fend for itself, usually its only defense against predation is its size. Some species, however do have the additional defense of a stinger.
It is estimated that mobula rays live between 40 – 50 years, with females reaching sexual maturity between eight to ten years and males at six years. There is a period of two – five years between each birth and females can have offspring for around 30 years. The mating seasons for these rays depends on the species and location. In Japan oceanic manta rays have been seen to mate in summer, whereas in the Maldives higher sexual encounters are seen in October, November, March and April. Mating occurs in warm water and generally around cleaning stations. Males will venture to cleaning stations in search of a receptive female. These females illustrate their reproductive readiness by releasing mating hormones into the water.
Courtship displays are long (sometimes lasting weeks) and very expressive. Up to 30 males surround the receptive female and compete to mate. They form mating trains whereby they follow the female, who performs elaborate acrobatics that the males must follow. The most impressive male will be selected and have mating rights. The male will then bite the left pectoral fin of the female to hold her in place. They will then go belly to belly and the male will insert one of his claspers into the female for fertilisation. This process takes place in a couple of seconds after which the male disappears. Mating brings together large numbers of rays as does feeding.
Mobula rays can be found individually, although they generally form large schools when food is in high concentration. They are considered planktivores, although they can feed on small fish and zooplankton. They consume food by using their cephalic fins to funnel the plankton into their wide mouth. Different feeding methods are used depending on food availability; for example benthic feeding can be seen in low food concentrations, whereas surface feeding using barrel rolls and feeding trains can be seen when concentrations of plankton are higher. Cyclone feeding is the rarest type of feeding and can only be seen when the plankton concentration is 80% or higher. Hanifaru bay in Baa Atoll (Maldives) is a world renowned manta feeding site and one of the few places on Earth where cyclone feeding can be seen. In manta season (June – November) sightings of 200 manta rays and a couple of whalesharks are common.
It has also been found that devil rays can dive to depths of two kilometers for over an hour to find plankton, making them some of the deepest diving animals in the world. As the temperature at this depth is low the rays must come up and bask in the sun to rewarm and oxygenate their blood. Some rays have a dark band between their eyes which helps warm their brains faster. The oceanic manta ray also has a counter-current heat exchange system which allows them more control over their body temperature than other fish, making them effectively warm-blooded and enabling their deep dives.
Mobula ray populations are declining because they are vulnerable to overfishing, boat traffic, habitat decline, pollution, by-catch and entanglement. They also have limited reproductive capacity, limited habitat range and are slow growing. The biggest threat to mobula rays are targeted fisheries. They are hunted for their gill rakers which are used in ‘medicine’. There is NO evidence to suggest that gill rakers help with any ailments, in fact it is suggested that gill raker ‘medicines’ may actually pose a significant health risk to those taking it, especially pregnant mothers. In a study mobula ray gill raker samples were chemically analysed and arsenic, cadmium, mercury and lead were detected in all samples. Arsenic levels where found to be 20 times higher than permissible levels and cadmium triple permissible levels. A study found a 163% increase in profitability in gill raker markets in China over a three year period, highlighting that this trade is getting worse. A mobula ray population reduction of 50% has been observed in some areas.
In the Maldives all mobula ray species are protected. More countries are also now protecting their mobula rays due to the tourism potential. For example, in 2011 in the Maldives mobula rays were worth eight million dollars to the dive tourism industry – rays are certainly worth significantly more alive than dead.
Over the last two months we have had many sightings of the short-fin pygmy devil ray on snorkels, dives, from the jetties and the villas. Although we cannot be sure why we have had a sudden increase in mobula ray sightings we have hypothesised that it could be due to upwelling currents bringing in plankton which the mobula rays are then feeding on. Either way we are very lucky and we hope to share the experience with you!