For most of human history, fish were considered simple creatures — driven by instinct, incapable of learning, without memory, without social bonds, without anything resembling the inner life that we associate with intelligent animals. The phrase “a memory like a goldfish” entered common usage as shorthand for the most basic, unreflective kind of cognition imaginable.
The manta ray has been quietly disproving this assumption for millions of years. We just weren’t paying attention.
The oceanic manta ray (Mobula birostris) is the largest ray on Earth — reaching up to 7 metres in wingspan and weighing up to 3 tonnes. It is found in tropical and subtropical oceans worldwide, cruising the open ocean and visiting coral reefs and seamounts with a slow, effortless grace that has made it one of the most sought-after encounters in recreational diving. It feeds on zooplankton — tiny crustaceans and fish larvae — filtering them from the water through its open mouth as it glides through plankton-rich currents.
It also has the largest brain of any fish on Earth, relative to its body size. And what researchers have discovered about how it uses that brain is forcing a fundamental revision of our understanding of fish cognition — and raising urgent questions about what it means to kill an intelligent animal for a product that has no proven medical value.
The Brain That Shouldn’t Exist
Fish brains are generally small relative to body size — a reflection of the relatively simple behavioural repertoire that most fish species require. The manta ray is a dramatic exception. Its brain-to-body ratio is comparable to that of some mammals and birds — animals that we readily accept as intelligent — and its brain has a structure that reflects this complexity, with enlarged regions associated with learning, memory, and social behaviour.
This large brain is not an accident. It is an adaptation to a lifestyle that requires sophisticated cognitive abilities — navigating across vast ocean distances, remembering the locations of productive feeding sites, recognising and maintaining relationships with other individuals, and solving the complex problems posed by life in a dynamic, three-dimensional ocean environment.
The question of what manta rays are actually doing with their large brains has been the focus of a growing body of research over the past two decades — and the answers have been consistently surprising.
The Mirror Test
In 2016, a study published in the Journal of Ethology reported something that had never been documented in any fish species: manta rays appeared to pass the mirror self-recognition test.
The mirror test — developed by psychologist Gordon Gallup Jr. in 1970 — is one of the most widely used measures of self-awareness in animals. An animal is marked with a spot that it can only see in a mirror, and its behaviour in front of the mirror is observed. Animals that recognise themselves — that understand that the reflection is an image of their own body — typically investigate the mark on their body after seeing it in the mirror. Animals that do not recognise themselves typically treat the reflection as another individual, displaying or fleeing.
Self-recognition in the mirror test has been documented in great apes, dolphins, elephants, and some corvid birds — all animals with large, complex brains and sophisticated social lives. It has never been convincingly demonstrated in any fish species.
The 2016 study found that manta rays exposed to mirrors showed behaviour consistent with self-directed responses — investigating their own bodies, making repetitive movements while watching their reflection, and showing none of the aggressive or affiliative responses that would be expected if they were treating the reflection as another individual. The researchers were cautious in their interpretation — the mirror test has limitations, and fish cognition is difficult to assess using methods developed for mammals — but the findings were striking enough to prompt significant scientific interest.
Whether manta rays are truly self-aware in the way that great apes are self-aware remains an open question. What is clear is that their behaviour in front of mirrors is unlike that of any other fish species tested — and that it suggests a level of cognitive sophistication that demands serious scientific attention.
Memory, Navigation, and Social Intelligence
Beyond the mirror test, research on manta ray cognition has revealed a range of abilities that challenge the conventional view of fish as simple, instinct-driven animals.
Long-term memory appears to be well-developed in manta rays. Individual rays have been documented returning to the same cleaning stations — specific reef locations where small fish remove parasites from the ray’s skin — year after year, sometimes after absences of many months. This requires not just memory of the location, but memory of its value — an assessment that involves comparing the cleaning station to other possible destinations and choosing to return based on past experience.
Individual recognition has been documented in manta ray populations. Rays at cleaning stations show differential responses to different individuals — approaching some fish cleaners preferentially and avoiding others — suggesting that they recognise and remember specific individuals based on past interactions. This kind of individual recognition is a prerequisite for the complex social relationships that characterise intelligent social animals.
Problem-solving has been observed in captive manta rays, which have been documented finding novel solutions to challenges posed by researchers — navigating mazes, manipulating objects, and responding to changes in their environment in ways that suggest flexible, experience-based learning rather than fixed instinctual responses.
Social learning — the ability to learn from watching other individuals — has been suggested by observations of manta rays at feeding aggregations, where individuals appear to adjust their feeding behaviour based on the behaviour of others in the group.
The Gill Plate Trade
The manta ray’s intelligence, its longevity — individuals can live for up to 50 years — and its slow reproductive rate make it acutely vulnerable to exploitation. A female manta ray produces only one pup every two to five years — one of the lowest reproductive rates of any marine animal. Populations that are heavily exploited recover extremely slowly, if at all.
And manta rays are being heavily exploited — for a product that has no proven medical value.
Manta ray gill plates — the comb-like structures that filter plankton from the water — are sold in traditional Chinese medicine markets as a supposed treatment for a range of conditions including chickenpox, cancer, and fertility problems. There is no scientific evidence that manta ray gill plates have any medicinal properties. The active ingredient claimed by proponents — a substance called “peng yu sai” — does not exist as a distinct compound; the gill plates are composed of the same proteins found in any other animal tissue.
Despite the lack of evidence for any medical benefit, the gill plate trade has grown significantly since the 1990s, driven by increasing wealth in China and Vietnam and the marketing of gill plates as a luxury health product. Manta rays are targeted by fisheries in Indonesia, Sri Lanka, India, Peru, and other countries, and their gill plates are exported to markets in China and Hong Kong.
The scale of the trade is difficult to quantify — manta ray fisheries are poorly monitored in many countries — but estimates suggest that tens of thousands of manta rays are killed annually for their gill plates. Given the species’ slow reproductive rate, this level of exploitation is unsustainable.
The Cleaning Station
One of the most extraordinary aspects of manta ray behaviour — and one of the most important for understanding their intelligence — is their use of cleaning stations.
Cleaning stations are specific locations on coral reefs where small fish — particularly wrasse species — set up territories and offer cleaning services to larger fish and rays. The cleaner fish remove parasites, dead skin, and food debris from their clients, receiving a meal in return. It is one of the most remarkable examples of interspecies cooperation in the ocean.
Manta rays visit cleaning stations regularly — sometimes daily — and their behaviour at these stations reveals a level of social sophistication that is striking. They queue — waiting their turn when the station is busy, sometimes hovering in line for extended periods. They adopt specific postures that signal their readiness to be cleaned. They remain still while the cleaner fish work, even allowing them to enter their mouths and gill slits. And they return to the same stations, with the same cleaner fish, over periods of years.
This behaviour requires memory, social awareness, impulse control, and the ability to engage in cooperative interactions with members of a different species. It is, in short, the behaviour of an intelligent animal.
Conservation: The Race Against the Trade
The oceanic manta ray was listed as Endangered on the IUCN Red List in 2019, and both manta ray species were listed on CITES Appendix II in 2013 — providing some regulation of international trade. Several countries, including Indonesia, the Maldives, the Philippines, and Ecuador, have declared their waters manta ray sanctuaries, prohibiting fishing and trade.
These protections have had some effect — in areas where manta rays are protected and ecotourism has developed, local attitudes toward the animals have shifted from viewing them as a fishing resource to viewing them as a tourism asset. A living manta ray, attracting divers to a reef over decades, generates far more economic value than a dead one sold for its gill plates.
But the gill plate trade continues in countries where enforcement is weak, and the demand from Chinese and Vietnamese markets has not declined significantly despite growing awareness of the lack of medical evidence for gill plate efficacy. Demand-reduction campaigns — similar to those that have had some success in reducing shark fin consumption — are underway, but changing deeply embedded cultural practices takes time.
The Intelligence We Almost Missed
The manta ray’s large brain, its apparent self-awareness, its long memory, its social sophistication — all of these qualities were unknown to science until very recently. For most of human history, the manta ray was simply a large, graceful fish — impressive to look at, but not an animal whose inner life warranted serious consideration.
The research of the past two decades has revealed something very different: an animal of genuine cognitive complexity, whose intelligence has been shaped by millions of years of life in one of the most demanding environments on Earth. An animal that remembers, that recognises, that learns, that may — in some meaningful sense — know itself.
We are killing it for a medicine that doesn’t work.
The manta ray has the largest brain of any fish on Earth. It deserves the chance to use it.
Go Deeper
The complete story of the manta ray — its biology, intelligence, the gill plate trade, and its conservation status — is covered in our premium Manta Ray Wildlife Guide.