Scientists Caught a Suckerfish Using a Manta Ray Exit For Entry and Now Nothing Is the Same

Scientists always assumed the remora was one of nature's good guys.

Then a University of Miami researcher reviewed 15 years of underwater footage and saw something that changed the story.

What she found on camera will make you rethink every "mutually beneficial relationship" you ever learned about in biology class.

Suckerfish and Manta Rays: A Relationship Scientists Got Wrong

For decades, the remora – the small, sucker-headed fish that hitches rides on sharks, whales, and manta rays – was held up as a marine success story.

The arrangement looked fair: the remora cleaned parasites and dead skin off its host, and in return got free transportation, protection, and leftover scraps.

Biologists called it mutualism – the good kind of symbiosis where everyone benefits.

That narrative is crumbling.

Emily Yeager, a marine researcher at the University of Miami, spent 15 years combing through survey footage collected across three ocean sites – the Maldives, Mozambique, and South Florida.

What she documented, across seven separate observations, was remoras doing something textbooks never described.

They were diving headfirst into manta rays' cloacas.

What Is a Cloaca and What the Remora Was Doing Inside One

A cloaca is a multipurpose rear opening that manta rays – and many other animals – use for both waste elimination and reproduction.

It is among the most sensitive and biologically critical parts of the animal.

The behavior Yeager documented, now formally termed "cloacal diving," involves a remora forcing its way into the opening – in some cases disappearing entirely inside, in others hanging half out with only its tail visible.

In one particularly revealing piece of footage, a remora dove into a manta ray's cloaca after being startled by a nearby diver.

The manta ray shuddered.

Then it swam away with the remora still inside.

"That manta ray then shudders pretty violently before continuing on its way," Yeager told CBC Radio's As It Happens.

Scientists believe the rays signal their discomfort clearly – some have been observed leaping entirely out of the water or scraping themselves against the seafloor in apparent attempts to dislodge their uninvited passengers.

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What Cloacal Diving Could Mean for an Endangered Species

Brooke Flammang, a biology professor at the New Jersey Institute of Technology, offered a blunt explanation for what the remoras might be after.

The behavior, she told CBC, could be coprophagy – feeding on fecal matter – with individual remoras competing aggressively for position near the host's waste source.

"They can be territorial about sharing space on the same host," Flammang said.

But Yeager's concerns extend further than the immediate discomfort.

Repeated intrusions into the cloaca – the exact organ manta rays use to reproduce – could cause lasting physical damage over time.

"If they do that in the cloaca opening, which is likely much more sensitive than other parts of the manta ray's body, it could cause really severe damage and influence reproduction and also excretion of waste over time," Yeager said.

For a species already listed as Endangered under the IUCN Red List – one that produces only a single pup every two to three years and faces heavy pressure from commercial fishing – anything that compromises reproduction is not a minor detail.

The behavior was also documented in manta ray gill slits, and researchers noted several animals showing injuries consistent with remora intrusion.

The study, published in the journal Ecology and Evolution, concludes that these interactions represent evidence that the remora's relationship with its hosts "may be more parasitic, at least in some contexts, than previously understood."

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What This Actually Tells Us About the Ocean

The deeper lesson here isn't about one weird fish.

It's about how often science declares something settled before the footage tells a different story.

The remora-as-cleaner narrative persisted for decades partly because the fish are small, hard to tag, and their most invasive behaviors happen out of sight.

Yeager's research suggests remoras stay with individual hosts far longer than previously known, and that the same fish documented cleaning parasites off a host's exterior can, under the right circumstances, turn invasive.

"The narrative is shifting," Yeager said.

That shift has real stakes.

Manta rays face mounting pressure from fisheries that kill an estimated 259,000 manta and devil rays globally each year.

Understanding every threat to their reproductive health – including the ones hiding on their own bodies – matters more now than when scientists thought the remora was simply along for the ride.

"Remoras are just that weird," said Flammang.

They are.

And the ocean is weirder than the textbooks suggested.

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Sources:

• Emily Yeager et al., "Hiding in Plain Sight: Evidence of Echeneidae Cloacal and Gill Diving Behavior in Manta Ray Hosts," Ecology and Evolution, May 2026.
• Catherine Macdonald, University of Miami Rosenstiel School, quoted in university press release, May 2026.
• Brooke Flammang, CBC Radio As It Happens, May 2026.
• Betty Laglbauer et al., "Global manta and devil ray population declines," Biological Conservation, 2026.
• NOAA Fisheries, Giant Manta Ray Conservation Management overview.