Manta and mobula rays are fascinating and captivating creatures. Join Guy Stevens as he comes face to face with these gentle giants of the ocean (Text and Photos by Guy Stevens)
Manta and mobula rays are fascinating and captivating creatures. Giants of their kind, they range throughout the tropical and subtropical oceans of the world. Born into a life of perpetual motion they must keep swimming to survive. Driven forward by powerful beats of their wing-like pectoral fins they travel the oceans in search of food and the urge to mate.
Despite the fact that manta rays in particular are hugely fascinating to divers and snorkelers around the world, it has only been in the last 10 years that widespread research into the mysteries and complexities of these animals’ lives has begun in earnest. This article details the fundamentals of what we know today about these fascinating creatures.
Family History and Global Distribution
Like all cartilaginous elasmobranchs, manta and mobula rays have a skeleton comprised of flexible, fibrous and light cartilage as opposed to the dense bony skeletons of the vast majority of all other fishes. A close relative of all sharks and rays, mantas and mobulas must constantly keep swimming forward in order to ensure a steady supply of oxygen rich water flows over their gills; they can never stop to sleep or rest on the seabed.
The first rays appeared in the oceans approximately 170 million years ago. Radiating from shark-like ancestors, they adapted to a more benthic mode of life. The flattened body shape of rays is essentially a squashed version of the archetypal shark, with internal physiology very similar to that of their shark cousins. It was from these bottom dwelling rays that the pelagic mantas and mobulas (Mobulidae family – devil rays) evolved, taking to the open water around 20 million years ago, with mantas evolving even more recently around five million years ago.
The defining feature of the Mobulidae family is that they are all filter feeders, using their mouths and modified gill rakers to strain plankton and small fishes from the water. In general, mobula rays are much smaller than the mantas and can be distinguished by morphological differences in their mouths and cephalic fins (“head fins”). 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 the jaws of manta rays are aligned evenly.
The other differentiating anatomical feature between the two genus is the shape of their cephalic fins, which when rolled up look like horns projecting off their heads, hence the name “devil rays”. The primary function of these fins is to help funnel planktonic food into the gaping mouths of the rays when they are feeding. Unfurled, the mobula ray’s cephalic fins are just small flaps, but in the mantas, these fins unravel to form much larger paddle-like structures, which touch in the centre to form a complete funnel around the manta’s mouth. These fins are used by the mantas when feeding to help them channel their planktonic prey into their mouths.
The devil ray family contains 11 species of plankton and fish eating rays (see table). Within this family, there are two genus; Mobula and Manta. Very little is known about the mobula rays which, unlike the mantas, are generally very shy towards divers making 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.
Within the genus manta, there are currently two recognised species, Manta birostris (the giant oceanic manta) and Manta alfredi (the resident reef manta). These two species have much in common, but a few differences exist. Mantas were re-classified as two separate species in 2009 based on a number of slight morphological features as well as differences in habitat utilisation and behaviour. Genetic work is continuing to further define the true nature of this separation and later this year, a third Caribbean species of manta ray is also likely to be announced.
Resident Reef Manta Ray (Manta Alfredi)
The reef manta is smaller than the oceanic manta ray, with an average disc-width of 3-3.5 metres (9-11.5ft). These reef mantas live quite different lives to their giant counterparts, occurring in the shallow waters along the coastal reefs of continents and around remote oceanic island and archipelagos. These mantas are more commonly encountered by divers and snorkelers and tend to be highly social. They are often resident to a specific home range, migrating around this area as they follow changes in the seasonal abundances of their planktonic food source, or are driven by the urge to find a mate and reproduce.
Reef mantas commonly frequent the same sites year after year for many decades, allowing researchers to gather in-depth data on the population as a whole and follow more closely the lives of specific individuals as they grow, reproduce and migrate.
Giant Oceanic Manta Ray (Manta Birostris)
As its name suggests, the giant oceanic manta ray is generally larger than its smaller reef resident relatives, with average disc widths of around four to five metres (13-16.5 ft) and in extremely large specimens, possibly exceeding seven metres (23 ft). Large oceanic mantas might weigh up to two tonnes (4,440lbs). The literature on these species also cites the presence of a nonfunctional spine at the base of the tail, which is not present in their reef manta counterparts, as a defining anatomical feature.
While the range of the two species overlaps, the giant mantas appear to be much more transitory in nature, wandering large distances across open oceans. The oceanic mantas are most frequently sighted along productive coastlines with regular upwelling, at oceanic island groups, offshore pinnacles and seamounts.
Oceanic mantas also venture into the slightly cooler waters of the higher latitudes, with reported sightings as far afield as 31ºN (South Carolina, USA) and 36ºS (North Island, New Zealand). While divers and snorkelers still encounter this species, their elusive and migratory nature means that we know much less about these giants than their smaller reef dwelling relatives.
Like all elasmobranches, but unlike nearly all the other fish in the sea, manta rays reproduce through internal fertilisation, which means male and female must come together to mate. They give birth to live young that are miniature versions of their parents; ready to fend for themselves, they are completely independent from birth. Courtship rituals and actual mating events are relatively infrequently observed and the only documented accounts of a birthing manta ray come from a single female, which is housed in the Japanese aquarium at Okinawa. Many gaps still remain in our knowledge of the sexual lives of these fantastic creatures, and what little we do know is based on very little hard scientific data.
Mantas are ovoviviparous, meaning that once they have mated, the fertilised egg grows within the female’s uterus until it has developed into a fully functioning manta pup. In aquaria, the time between mating and pupping has been observed to take around one year. Mantas usually give birth to a single pup, but occasionally two developing embryos have been recorded inside a pregnant female during autopsies.
At birth, mantas measure around 1.5-2 metres from wingtip to wingtip. It’s thought that mantas are probably born at night, but to date no wild births have ever been recorded. It is believed that mantas become sexually mature at around 10–15 years of age, possibly even longer for females.
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 wingtip 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.
Currently, it is not known how often females give birth, but the latest research suggest birth rates average around one pup for each mature female every two to five years. This extremely slow rate of reproduction, coupled with the long gestation period, late maturity age and small litter size, makes manta rays very vulnerable to exploitation.
Research would indicate that mantas probably live for around 50 and possibly up to 100 years. However, until scientists have been studying these animals for several more decades, we are unlikely be able to answer this question more accurately.