The defining feature of the manta and mobula family is that of filter feeding – they use their cephalic fins (head fins), huge mouths and modified gill plates to strain plankton and small fishes from the water. The cephalic fins, which when rolled up look like horns projecting from their heads, have given them the name ‘devil rays’. In general mobula rays are much smaller than mantas and can be distinguished by morphological differences in their mouths. Mobula rays have a bottom jaw which is undercut, so that when their mouths are closed the edge of the lower jaw rests much further back than the upper, whereas manta rays jaws are aligned evenly.

The devil ray family contains 11 species of plankton- and fish-eating rays. Within this family there are two genera: Mobula and Manta. Very little is known about the mobula rays which, unlike the mantas, are generally very shy towards divers and make it hard for scientists to observe their natural behaviour in the wild. Like mantas they are found throughout tropical and sub-tropical oceans. Sometimes aggregating in vast shoals of many hundreds they come together possibly to feed or to seek safety in numbers. These aggregations may also occur more frequently at breeding times and it is not uncommon to encounter these large groups attracting attention at the surface as the rays leap several metres out of the water. There are currently nine described species of mobula rays, but the taxonomy of this genus is likely to change in the coming years as advances in genetics allow scientists working on these poorly studied animals to further define the true nature of the species composition within the genus.

Courtship in mantas can last days or even weeks and is an intricate process led by the female. The males compete to mate with the female, following her around the reef in what is known as a mating train; as many as 30 males may line up behind a single female. Watching a mating train of mantas is an enthralling experience, with the female leading the males as she races, twists, turns and dives around the reef with each male following her every move. In doing this the female tests the fitness of the males as they try to keep up with her and gain the first place in the train. At the end of this process the female selects a single male and the two mate as a pair, belly to belly in the mid water, the male biting down on the female’s left wing to secure them together. These bites leave permanent mating scars on the ends of the female’s pectoral fins where the ‘wing-tip’ is gripped by the male’s muscular pharynx at the back of his mouth. Just like humans, manta rays exhibit signs of lateralisation, with nearly all observed mating scars occurring on the left hand side of the females, suggesting the males favour this side of their body.

As well as their physiological feeding adaptations the rays also employ a variety of behavioural feeding strategies and techniques to maximise the amount of food they can funnel into their mouths at each feeding event – manta rays barrel-roll through dense patches of plankton while speedy fish-eating mobula rays lunge at their faster moving prey with powerful beats of their pectoral fins.

Unlike the mobula rays, which often feed more on larger prey items, the plankton-eating mantas spend a lot of time swimming around with their mouths wide open. As a result particles of food and detritus become lodged in their gills creating an unwanted build-up of dental detritus. Manta rays have no hands or appendages with which to clean or groom themselves, so instead they visit specific locations on a reef to have their bodies cleaned of parasites and dead tissue by small fish known as cleaners. These cleaner fish are commonly wrasses although other fish families, such as angelfishes and butterflyfishes, also perform this task. The relationship is mutually beneficial to both the manta rays and the cleaner fish, which receive their dinner served on a giant flapping plate.

Data collected on this fishery in Sri Lanka estimates many hundreds of manta rays and tens of thousands of mobula rays are being killed each year by this country’s fishing fleet, which travels far into the Indian Ocean in search of fresh fishing grounds. When certain aspects of manta and mobula rays’ biology – such as their low reproductive rates, slow growth and late maturity – are taken into account, these high catch rates are likely to have significant impacts on the respective populations in this region. If the fishing of manta and mobula rays carries on at its existing levels globally it is highly likely there will be population crashes of these species within a few years. Regional population crashes have already been documented in the Gulf of Mexico and Indonesia, where boom and bust fisheries for these species have occurred in recent decades.

The future

In the past 10 years scientists have made a good start at uncovering the secret lives of manta rays, however we are only just beginning to glimpse below the surface. Much still remains to be found out about even the basics of their biology, and with the largest brain to body ratio of all the world’s fishes, there is much still to understand about their intelligence, curiosity and social interaction. Learning more about these species is vital to being able to provide them with effective protection and ensuring their persistence in years to come. I hope I never tire of seeking the answers to the unanswered questions, and that by sharing my knowledge and passion I can promote imaginative thought and empathy in others, so that together the science and conservation of manta rays will be driven forward.