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We assumed fish didn't care about each other. We were wrong.

Researchers have long thought fish were heartless and cold, incapable of the relationships mammals cultivate, but new research among fish in coral reefs suggests fish can work in long-term paired relationships. 

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A diver snorkels in the Great Barrier Reef off Australia's Queensland state. Rabbitfishes from a coral reef have just been found to exhibit reciprocal cooperation, meaning they are the first fish known to take care of each other.

Queensland Tourism/AP

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Fish living in the vast network of coral reefs near Australia are already known to moviegoers for their devotion, thanks to the loving clownfish father-and-son pair in Pixar's "Finding Nemo."

But in reality, marine researchers have long thought fish were a bit cold and self-centered. A recent study published Friday indicates that their temperament is warming by a few degrees. 

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Clownfish like Marlin and Nemo do have a symbiotic relationship with anemones, according to PBS, but another inhabitant of the coral reef – the rabbitfish – shows the first-observed signs of what researchers call reciprocal cooperation. This means one fish helps another, and the effort, no matter how small, is somehow returned. 

Members of this coral reef fish species feed in pairs, according to a study published at ARC Centre of Excellence for Coral Reef Studies at James Cook University in Australia. Rabbitfish couples have a symbiotic relationship, and one member of the pair helps by keeping watch while the other fish eats. 

Such behavior had not been seen or suspected in fish before, though it has been studied with great success in some birds and mammal species, especially primates, according to study authors Simon J. Brandl & David R. Bellwood from the ARC Centre of Excellence for Coral Reef Studies. 

"There has been a long-standing debate about whether reciprocal cooperation can exist in animals that lack the highly developed cognitive and social skills found in humans and a few species of birds and primates," said Dr. Simon Brandl in a press release. 

Rabbitfish have been known to travel in schools, as many fish do, to maintain safety in numbers. But researchers also observed that rabbitfish benefit from the pair relationship – those traveling in pairs are willing to dive deeper into the crevices of the coral reef than those that swim alone.  They also stick around to finish their dinner, taking more bites of food at once than singles do. 

Rabbitfish pairs seemed to have a system for standing – or swimming – guard, and the same two fish stay together for some time. Researchers observed that one rabbitfish would forage head-down for food while the other would watch, head up. The watcher would swim away – probably fleeing from danger – leading the forager to follow, according to the study published in the journal Scientific Reports.

"In other words, one partner stays ‘on guard’ while the other feeds – these fishes literally watch each others’ back," Dr. Brandl said.

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The study's co-author said the study suggests that other fish of the ocean may have greater social depth than had been thought. His hope is that more study will soon follow to plumb the social mechanisms of ocean life.

"Our findings should further ignite efforts to understand fishes as highly developed organisms with complex social behaviors,” Dr. David Bellwood said in a press release. "This may also require a shift in how we study and ethically treat fishes."

These aquatic creatures from Australia "really do look after their mates," the release reads.