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Conservation through Research, Awareness and Education

UK Registered Charity Number: 1145387

Gentle Giants

A feeding reef manta ray drives itself through the water with powerful beats of its 'wing-like' pectoral fins.

Manta rays are fascinating and captivating creatures. Giants of their kind, they range throughout the tropical and sub-tropical oceans of the world. Born into a life of perpetual motion, they must keep swimming to survive. Driven forwards by powerful beats of their wing-like pectoral fins they search the ocean currents for concentrated patches of the tiny planktonic food upon which they feed.

Despite the fact that these creatures are of huge fascination to divers and snorkelers around the world, it has only been in the last 10 years (with a few exceptions) that dedicated research into the mysteries and complexities of these animal’s lives has begun. This page details the fundamentals of what we do know about these fascinating creatures…

 

 

Family History

Taxonomically manta rays are classified as Cartilaginous Elasmobranch Fishes which means they are very closely related to all other rays as well as sharks. The sub-class Elasmobranchii currently contains around 1,100 extant species which can be further classified into around 500 species of sharks and 600 species of rays. Sharks first appeared in the fossil record over 400 million years ago, nearly 300 million years before the dinosaurs! Resembling modern guitarfish and skates, 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’s from these bottom dwelling rays that mantas and mobulas (the Mobulidae Family) evolved around 20 million years ago, with mantas evolving even more recently, most likely only around 5 million years ago.

Unlike the large filter feeding mouths of their close relatives, the spotted eagle ray's pig-like snouts have evolved for digging through the seabed in search of buried molluscs and crustaceans.

 

The defining feature of the Mobulidae family is that they are all filter feeders, using their mouths and modified gill plates to strain plankton and small fishes from the water. In general however, 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 manta rays jaws 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 a small flap, 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 mantas mouth. These fins are used by the mantas when feeding to help them channel their planktonic prey into their mouths.

Manta Rays

Mantas were re-classified as two separate species in 2009. This separation was based on a number of morphological features as well as differences in habitats and behaviour. Genetic work is continuing to further define the true nature of this separation…

A reef manta ray with typical dorsal patterning, including the 'Y' shaped shoulder stripe.

Resident Reef Manta Ray (Manta alfredi)

The first morphological feature which marks the two species of manta apart is their size. The reef manta is smaller than the oceanic manta ray, with an average disc-width of 3-3.5 metres (9-11.5ft) and a possible maximum of 4.5 metres (15ft). It’s likely at such sizes reef mantas would weigh in at around 1.4 tonnes (3,100 lbs).

All mantas start off small, so size is not necessarily a good identification feature. The dorsal shading and ventral spot patterning however can be used to discern between these two species. All chevron colour morphed mantas (predominantly black colouration dorsally, with white undersides) have white patches on their backs. On reef mantas these ‘shoulder’ patches form a ‘Y’ pattern just on top of the mantas head, fading into the black colouration on their backs. The ventral (underside) surface is also different between the species. See our Spot the Difference page for more information.

Reef mantas also live quite different lives to their giant counterparts. They occur 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 resident to a specific home range, migrating around this area as they follow changes in the seasonal abundances of their planktonic food source, or the urge to reproduce. Reef mantas 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.

An oceanic manta ray with typical dorsal 'T' shaped shoulder stripe.

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 4-5 metres (13-16.5 ft) and in extremely large specimens exceeding 7 metres (23 ft). Large oceanic mantas might weigh up to 2 tonnes (4,440lbs).

Dorsal (topside) patterning on oceanic chevron morph mantas forms a ‘T’ pattern and there is a distinctive black/white divide, as opposed to a gradient. Again the spot patterning on the ventral (underside) surface is also different between the species. Please check our Spot the Difference page for more information. The literature on these species also cites the presence of a non-functional spine at the base of the tail, which is not present in their reef counterparts.

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 upwellings, at oceanic island groups and offshore pinnacles or 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.

Mobula Rays

Like the oceanic mantas, spine-tail devil rays are thought to spend much of their time out in the open ocean.

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 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 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 from those 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.

Life in Motion

Like all elasmobranchs mantas have a skeleton comprised of flexible, fibrous and light cartilage as opposed to the dense bony skeletons of the vast majority of all other fish (approximately 29,000 species) and terrestrial vertebrates. This light weight skeleton is about half the density of bone, therefore saving the manta rays valuable energy.  In addition, unlike most free swimming bony fishes, the mantas and their relatives have not needed to evolve a gas filled swim bladder to compensate for increased negative buoyancy that a heavier skeleton causes, instead an enlarged and extra oily liver helps compensate for their density. However, overall mantas are still negatively buoyant and will begin to slowly sink through the water column if they stop swimming. This means for mantas, as it does for many other open water species, a life of perpetual forward motion.

A reef manta ray glides through a school of surface feeding rudderfish.

Big & Brainy

With brains which are disproportionately large when compared to their body size, the weight of a manta’s brain is more comparable to that of a similar sized mammal and is the largest brain of all fish by absolute weight. Not only do manta rays have big brains, but the regions of the brain that account for this enlargement are the telencephalon (the anterior most area of the brain) and the cerebellum (the first area of the hindbrain). These brain regions in mammals, most notably the cerebrum, are known to be responsible for many higher functions, including increased sensory functions.

Manta rays also possess a network of blood vessels that surround their braincase, presumed to help keep this organ warmer than the surrounding tissue. Mantas have been recorded to make very deep dives where sea temperatures decrease rapidly. This system of blood vessels is therefore likely to be important in helping to keep the manta’s brain functioning effectively even in these colder temperatures. Mammals are also able to thermo-regulate, indeed it is one of the major reasons for their success as a group. Whether or not these similarities between the brain physiology of manta rays and mammals is an indication of similar function and intelligence is not known, but certainly manta’s high degree of social interactions and curiosity towards humans would suggest there is a lot more going on behind those captivating eyes than is currently presumed.

© 2014 Manta Trust