Manta Rays Have the Biggest Brain of Any Fish, They Also Recognize Mirrors.

In 2016, a paper published in the Journal of Ethology by researcher Csilla Ari at the University of South Florida documented something that had not previously been recorded in any fish. Giant manta rays, Mobula birostris, placed in front of mirrors did not behave as fish typically do when confronted with their own reflection. They did not attack the mirror. They did not flee. They moved in front of it repeatedly, made unusual swimming movements, and blew bubbles, behaviors that Ari interpreted as self-directed, suggesting the rays were examining their own reflection rather than treating it as another animal.

The mirror self-recognition test is one of the most debated tools in animal cognition research. Passing it definitively requires demonstrating that an animal responds to a mark placed on its body that is only visible in the mirror, a test that has not been conducted on manta rays. But the behavioral responses Ari documented were sufficiently unusual to place manta rays in a small group of animals, including great apes, elephants, dolphins, and magpies, that show responses to mirrors consistent with some form of self-awareness.

The brain that produces this behavior is the largest of any fish, both in absolute terms and relative to body size.

The Brain and the Cephalic Fins

Mobula birostris can reach a wingspan of up to 7 meters and weigh up to 3,000 kilograms, making it the largest ray species on Earth. The brain is proportionally large enough that early researchers examining manta ray anatomy assumed they had made a dissection error. The encephalization quotient, the ratio of brain size to body size, is higher in manta rays than in many sharks and comparable to some marine mammals.

The cephalic fins, the two horn-like projections on either side of the head that give manta rays their devil ray nickname, are highly mobile sensory and feeding structures. During feeding, they unfurl and funnel plankton-rich water into the mouth. During social interactions and apparent play behavior, they move in ways that suggest active sensory engagement with the environment.

Researcher Andrea Marshall at the Marine Megafauna Foundation, who has studied manta ray behavior and ecology for over two decades and described several new manta ray species, has documented what appears to be play behavior in wild manta rays, including repeated breaching, where rays launch themselves completely out of the water and land with a crash audible from considerable distance. The function of breaching is not fully understood. Parasite removal has been proposed, as has communication, but the frequency and apparent enthusiasm of the behavior in some individuals has led researchers to consider play as a contributing motivation.

Migration and the Open Ocean

Manta rays undertake long-distance migrations that have been documented through satellite tagging studies. Individual rays tagged by Marshall’s team and by researcher Robert Braun at the Scripps Institution of Oceanography have been tracked crossing ocean basins, traveling thousands of kilometers between feeding aggregation sites and areas that may serve reproductive functions.

The navigation mechanism is not understood. Manta rays have no obvious magnetic sensing structures of the kind found in some sharks and migratory fish, but their ability to return to specific sites across open ocean suggests some form of long-distance orientation that researchers have not yet identified.

Feeding aggregations, where hundreds of manta rays gather in areas of dense plankton concentration, are among the most spectacular wildlife events in the ocean. Sites in the Maldives, the Azores, and off the coast of Ecuador reliably attract large numbers of rays during specific seasons, and these aggregations have become significant ecotourism destinations. The economic value of live manta rays to tourism has been estimated at over USD 100 million annually, compared to a one-time value of a few hundred dollars if killed for their gill plates.

The Gill Plate Trade

Manta ray gill plates, the filtering structures that extract plankton from water, are sold in traditional Chinese medicine markets as a treatment for various conditions. There is no scientific evidence that they have any pharmacological effect. The trade has driven significant declines in manta ray populations across the Indo-Pacific, with some regional populations reduced by over 80% in recent decades.

Both giant manta ray species are listed as Vulnerable on the IUCN Red List. The oceanic manta ray, Mobula birostris, and the reef manta ray, Mobula alfredi, reproduce extremely slowly, with females giving birth to a single pup every two to three years after a gestation period of approximately 12 months. A population that loses adults faster than it can replace them collapses slowly and recovers even more slowly.

An animal with the largest brain of any fish, that may recognize its own reflection, that breaches for reasons that include what looks like enjoyment, is being killed for gill plates that do nothing. The gap between what the animal is and how it is treated by the trade that targets it is one of the wider ones in wildlife conservation.