New observations of remoras diving into the cloacas and gill slits of manta rays suggest these relationships may be far more invasive, and complicated, than scientists once thought.

Provided by Catherine Mcdonald

The first time most people notice a remora, it is usually attached to something much bigger. Maybe it is riding along the belly of a shark in a nature documentary or stuck to the underside of a manta ray gliding through the azure water like some sort of ethereal, living spaceship. There is sometimes one, sometimes many. And looking quite unassuming, they are often treated as comic relief in the ocean world, lovingly known as “little hitchhikers” for their tendency to catch a free ride on whatever (un)willing bigger animal. But the deeper scientists look at these fish, the stranger the story becomes.

A new study documenting remora behavior in manta rays reveals interactions that are far more intimate, and potentially harmful, than many researchers expected! Led by PhD student Emily A. Yeager of the Shark Research and Conservation Program at the University of Miami, a team of scientists recorded seven observations of remoras engaging in what is known as “cloacal diving behavior” in manta rays across all three known manta species (the reef manta ray, the giant oceanic manta ray and the Atlantic manta ray) and across multiple ocean basins. In simple terms, the remoras were entering the cloaca of the ray, an opening used for digestion and reproduction.

Yes, really. That hole. The “butt hole.”

And as if that wasn’t invasive enough, researchers also documented remoras attaching beneath manta ray gill slits as well as injuries consistent with them even intruding on into the gills of both juveniles and adults alike. Taken together, these findings raise uncomfortable (alebit fascinating) questions about one of the ocean’s most recognizable animal partnerships.

Remoras, members of the family Echeneidae, are famous for their modified dorsal fin that evolved into a suction disc. This disc acts almost like a biological vacuum cup, allowing them to attach to larger animals including sharks, whales, sea turtles and rays. The arrangement has traditionally been described as either commensalism (where the remora benefits while the host is unaffected) or mutualism (where both animals gain something from the relationship), but one thing is for certains: the perks for the remora are obvious. A large host means not only does it have free transportation across huge distances, they also get free acess to protection from predators and even food, as they snack on scraps left behind from feeding events, parasites on the host’s skin and even fecal material. Some studies suggest that by removing parasites, remoras may actually help keep their hosts healthy. But symbiosis in nature is rarely neat and tidy as it reads on paper. Recent research increasingly suggests that remoras may not always be harmless passengers, as their attachment can increase drag as a host swims, potentially making movement less efficient and requiring more energy. Large remoras or groups of them may also cause skin damage where they attach. And now thanks to Yeager, with evidence of remoras entering sensitive body openings like cloacas and gill slits… well, rightfully, scientists are reconsidering whether some of these relationships lean closer to parasitism than partnership.

If that’s the case, what does it mean for manta rays, who are already vulnerable animals? Giant manta rays and reef manta rays face threats from fishing pressure, boat strikes, habitat degradation and climate change. If remora behavior truly causes stress, injury or increased energy costs, it can help scientists piece together a more holistic view of what these animals are really up against in the wild. But the story is not as simple as “remoras bad, manta rays victims,” as these interactions likely exist along a continuum. In some situations, a remora may genuinely benefit its host by cleaning parasites or dead tissue. In others? The costs may outweigh those benefits. The relationship may even shift depending on the size of the remora, the species involved or the health of the host animal!

There is also a bit of an evolutionary mystery here. Why would remoras engage in this behavior at all? Are they seeking nutrients? Shelter? Opportunities for reproduction? Could manta rays themselves serve as entire mobile ecosystems supporting complex communities of hitchhiking fish? Scientists do not yet have definitive answers. Some evidence suggests manta rays may function as important habitat for remoras beyond simple transportation. In certain mobulid rays, remoras are almost constantly present. One study on sicklefin devil rays found nearly every individual observed had at least one remora attached. Researchers have even hypothesized that rays could serve as reproductive habitat for remoras, with mating pairs observed associating closely with hosts.

For decades, remoras have occupied a kind of background role in marine science, overshadowed by the sharks and rays they accompany. But studies like this one pull them back into focus and reveal that these “side characters” may have much larger ecological significance than anyone realized. Humans tend to categorize animal relationships into tidy boxes (predator versus prey, parasite versus host, mutualist versus partner), but the ocean often ignores those categories entirely, with a relationship that is beneficial one day possibly becoming harmful the next. Seems simple enough on the surface: we see a remora attached to a manta ray and assume we know the story is “One fish gets a ride. The other barely notices. Simple.”

Except it is not simple at all. So, how often are we mistaking familiarity for understanding in the marine world? Studies like this really make one think…