Manta rays

They have the largest brain relative to their body size of any fish in the ocean and are highly intelligent (some studies suggest that they are self-aware; mirror tests in 2016 showed that they recognise their own reflections). Manta rays are also important to the functioning of many ocean habitats, including coral reefs; research has shown that they play a key role in cycling nutrients across habitats. The conservation status of the two recognised species – the giant (also called the oceanic) manta ray and the reef manta ray – makes it vital that we gather as much information about them as possible in order to best manage and protect their populations. And the more we know, the better we can highlight their interesting lives and their importance to us, hopefully to inspire more of us to raise awareness about them!

How are manta rays related to sharks?

Manta rays are chondrichthyans, or cartilaginous fish, belonging to a group of fish that have skeletons made of cartilage. This key trait sets up a host of differences between sharks, rays, skates and chimaeras (chondrichthyans) and bony fish (teleosts). For instance, their lightweight skeletons mean that sharks and rays can move efficiently and quickly through the water. Both chondrichthyans and teleosts are negatively buoyant; that is, they’ll sink to the seafloor without other physical strategies to compensate. The difference is that, while teleosts have a gas-filled swim bladder to solve this problem, sharks have large, fatty livers. These different solutions keep both fish groups close to (but not perfectly) neutrally buoyant. However, if manta rays don’t keep moving forward through the water, they are still slightly negatively buoyant and will sink!

Rays and sharks also belong to a group within the chondricthyans whose members all have plate-like gills and are called the elasmobranchs. Rays aren’t the same as sharks, however, and with at least 633 confirmed species, they make up the largest subgroup within the chondrichthyans. They differ from sharks because their gill slits are found under their heads, not on the sides of their heads as sharks’ gill slits are. Their bodies are also usually flattened and disc-like, earning them the nickname ‘flat sharks’.

What is the difference between manta and mobulid rays?

Taxonomy is the science that makes sense of the earth’s biodiversity, grouping together animals and plants with similar genetics and physical characteristics (called morphometrics) at different levels and hierarchies. Within the class of animals called rays, manta rays fall under a group called the Myliobatidae. This group comprises four families: the eagle rays (Myliobatidae and Aetobatidae), cownose rays (Rhinopteridae) and devil rays (Mobulidae). Eagle, cownose and devil rays are grouped together because the 40 or so species in the group all have diamond-shaped bodies and pectoral fins that move (and look) a lot like wings.

Manta rays fall under the family Mobulidae and used to be described under their own genus, Manta. That’s because manta rays look different from their relatively shy, lesser-known counterparts, the mobulid rays. A manta ray has a ‘terminal’ rather than a ‘subterminal’ mouth, which means that its mouth faces forward at the front, between its cephalic fins (the ‘horn-like’ fins at the front of the animal that give all devil rays their name). However, taxonomy changes how species are grouped whenever new information comes to light, and genetic analyses have shown that the two species of manta rays (giant and reef) should not form a separate genus from the six species of mobulid rays. So they too now fall under the genus Mobula.

What is the difference between giant and reef manta rays?

Manta rays represent one of the largest and most exciting ‘discoveries’ in recent scientific history. For many decades researchers had classified only one species of manta ray, but in 2008 Dr Andrea Marshall and her colleagues described the giant manta ray as a separate species. The fact that the largest rays in the ocean remained cryptic for so long hints at the complexity of their lives and the challenges involved in learning more about them.

Size is the first and perhaps most obvious physical difference between the two confirmed species of manta ray. The aptly named giant manta ray reaches an impressive seven metres (23 feet) maximum width, compared to the maximum five metres (16.5 feet) of the reef manta ray. Other distinguishing features are their coloration and a calcified lump behind the giant manta ray’s dorsal fin, which is absent in the reef manta ray. Their diet (plankton and small fish) and lifespan (maximum 40 years for the giant and 45 years for the reef manta ray) are similar, but it is how they live that really separates these species from one another.

Giant manta rays can be found in tropical and temperate oceans around the world and may be associated with sea mounts far offshore and with oceanic islands. Reef manta rays tend to favour coral and rocky reefs closer to the coast and in the tropics. Hugging the coastline, they are less widely nomadic than the giant manta rays. Their range is bounded in the west at the Indian Ocean as far south as South Africa’s KwaZulu-Natal coastline, although some occasionally venture even further south. This species prefers these shallower habitats and makes use of cleaning stations on coral reefs where small fishes perform manta-cleaning operations, scouring their skin for food. Manta ray cleaning sites are socially significant and are a key habitat that reef manta rays frequent. By contrast, the giant manta ray is encountered less often because it spends much of its time in the open ocean.

The giant manta ray has been classified as Endangered by the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species, whereas the reef manta ray is classified as Vulnerable.

Is there a third species of manta ray?

Currently, only two species – the reef manta ray and the giant manta ray – are formally recognised, but a third manta ray species has been proposed. Found in the Caribbean, this possible third species overlaps geographically with the giant manta and occupies the same ecological niche (position and role in the ecosystem) as the reef manta ray. Scientists believe that it may have evolved from giant manta rays that were crossing the open ocean to the Caribbean and found good food sources on this sea’s inshore reefs. Over time, these rays may have become another manta ray species in a process that mimicked the evolution of reef manta rays.

Where do manta rays live?

Manta rays are found in tropical oceans around our planet. The reef manta ray calls the Indo-West Pacific its home, with its range (the full extent of where populations of reef manta rays can be found) extending eastward as far as Hawaii and French Polynesia and westward into the Indian Ocean as far as southern Africa. It prefers coral and rocky inshore reefs and seeks out upwelling regions, where seasonal winds drive the abundance and availability of plankton. The giant manta ray is found in open water and prefers upwelling habitats, but is also found near oceanic islands and offshore sea mounts between the latitudes of 40 degrees north and 40 degrees south.

What do manta rays eat?

Manta rays are filter feeders, taking in microscopic plants and animals (called plankton) from the water column and filtering them through their gill plates. They swim with their mouths open to feed and their cephalic fins (the front fins that give the family the name ‘devil rays’) funnel water into their mouth, where the nutritious plankton is sifted through the tiny, rake-like plates. These rays often feed at night, when plankton have moved up into the water column and are more easily available. Manta rays have many different strategies for making the most of each mouthful: some skim just below the sea’s surface, heads tilted to the sky above. Others use the ocean’s currents, chasing their plankton prey along a concentrated channel. One population creates mesmerising chains, groups of mantas swirling in a follow-the-leader cyclone to concentrate the plankton. And yet others swoop and somersault, looping upwards and over as they target the densest plankton patches.

That’s not to say manta rays eat any and all plankton, swimming through the ocean with their mouths agape. In fact, manta rays have something of a refined plankton palate and may favour certain species of plankton over others. They tend to move in search of the best regions for food and aggregate where the plankton concentrations are suitable and high.

What roles do manta rays play in the ecosystem?

It has been demonstrated that reef manta rays play a vital role in keeping coral reefs healthy. Studies have shown that they are real reef ‘homebodies’, often relying on a particular site and returning faithfully to favoured areas. This means that, even though they are filter feeders and are regarded as feeding low down in the marine food web, they play quite an integral role in the coral reef ecosystems in which they spend so much time. Studies in Seychelles have indicated that reef manta rays around D’Arros Island and St Joseph Atoll feed on plankton offshore in deeper waters at night and return during the day to their cleaning sites on the coral reefs. By feeding offshore, they help to promote inshore coral reef growth because they transport novel nutrients via their faeces as they socialise at their cleaning stations. Although they may travel in search of food, they spend most of their time at coral reefs around D’Arros and St Joseph, making them key in the local nutrient cycle.

The social nature of manta rays, particularly reef manta rays, also means that ‘cleaner’ species of fish come to rely on mantas as a food source, since they remove dead skin and parasites from the rays when numbers of them visit coral reef cleaning stations. This makes reef manta rays important players in the coral reef ecosystem, particularly for cleaner fish of the wrasse family. Giant manta rays have a role to play too. In the open ocean ecosystem the serve as ‘taxis’, allowing many species to hitch a ride on them – and thus benefit from an energy-savvy mode of transport – as they roam pelagic (open) waters. Not only can the passengers get a ride with the giant manta rays, but the large animals are a moving source of food, shelter and protection from predators.

What are the threats to manta rays?

Manta rays are threatened primarily by overfishing, the same issue that has been the leading cause of decline in most sharks and ray populations. They evaded being the target species of dedicated fisheries for many centuries because their flesh wasn’t considered as tasty and edible as bony fish species (like salmon or cod), but more recently many countries have initiated manta ray fisheries. Some of these, such as the fishery in Mexico’s Sea of Cortez, have already collapsed.

A key concern, however, is the growing market for manta ray products in international trade, with demand having risen in recent years across South-East Asia. As well as being classified as Vulnerable and Endangered respectively on the IUCN Red List of Threatened Species, reef manta and giant manta rays are listed on the Convention for International Trade in Endangered Species (CITES) Appendix II, which regulates the international trade in their products. These protection levels are necessary because both manta ray species are targeted for their gill plates, which are used in Chinese traditional medicine – despite having no proven efficacy. The fishing of manta rays has increased hugely in response to this newer and growing trade. Manta rays are also caught incidentally in other fisheries.

The reason for manta rays being so susceptible to declines by overfishing is linked to their life history. Like many other sharks and rays, they grow slowly, live long and have low reproduction rates, giving birth to one pup after a full year’s gestation period. This makes their populations quick to crash and very slow to recover.

Manta rays also face threats in the form of microplastic pollution and climate change. They are huge filter feeders in the ocean and studies have cautioned that tiny particles of plastic mixed into the plankton soup they feed on may accumulate in their bodies. As for climate change, the implications of rising sea surface temperatures, changing ocean pH levels and shifts in seasonal upwelling (and plankton concentrations and bloom frequency) for manta rays are still being deciphered. What we do know is that the many impacts of climate change on the oceans will have a range of different, and often interacting, outcomes for manta rays – and their primary food source. Other threats to the rays include boat strikes, entanglement in discarded fishing gear, unsustainable tourism and habitat loss. Studies have shown that certain key habitats, such as nursery sites or feeding grounds, can be impacted or lost as human beings develop and urbanise our coastlines.

How are we protecting manta rays?

Scientists and conservationists around the world have worked hard to achieve two major wins to combat the overfishing of manta rays. Firstly, manta rays have been listed on CITES Appendix II, which means that any country trading manta gill plates needs to prove that doing so is sustainable. Secondly, manta rays have been listed on the Convention on Migratory Species, which aims to protect them across their range. Indonesia, Peru and the Philippines have put laws in place to protect manta rays, but many other countries that still have manta ray fisheries have yet to make this effort.

New science pours in all the time to improve our understanding of these intelligent and important rays. Studies that identify critical habitats for manta rays will inform the Important Shark and Ray Areas process that aims to map essential habitats for sharks and rays and amalgamate all the information needed to protect these places. The effort to reach 30% protection of the oceans in marine protected areas by 2030 also holds a glimmer of hope for manta rays in that fishing for them should be reduced or managed and their key habitats prioritised.

But everyone can help the ongoing efforts to protect manta rays, by adopting a plant-based diet, for example, or choosing sustainable seafood. By reducing our consumption of fish, meat and dairy, we can help combat overfishing, especially where fish are caught to grind into fishmeal for livestock agriculture. Lowering your carbon footprint and advocating for climate solutions from world leaders will help to ensure that coral reefs and the other habitats that manta rays rely on will stay intact.