A new study describes “cloacal diving,” where remoras enter manta rays’ rear opening—possibly to hide, feed, or hitchhike.

A new study is forcing marine biologists to rethink the strange relationship between remoras and manta rays. Researchers report that remoras sometimes dive into a manta ray’s cloacal opening—the rear passage used for excretion and reproduction—behavior they call “cloacal diving.”

The observations come from Florida and involve a suction-feeding fish known for attaching itself to a wide range of marine animals. including sharks. dolphins. other rays. boats. and even divers.. In this case, the remoras did not simply latch on to the outside.. Instead. they were documented plunging into manta rays’ cloacal orifice. an act the researchers say appears unwelcome from the manta rays’ perspective.

Across seven recorded cases, the study team noted remoras entering the cloacal opening of Mobula yarae manta rays.. The researchers emphasize that. while the manta ray is used here as the “vehicle. ” the behavior is not a harmless detour in any obvious way.. “It does not look like the manta ray likes it. ” the senior study author. Catherine Macdonald. a marine biologist at the University of Miami. said.

For remoras. the same behavior could serve multiple purposes. though the authors frame these ideas as possibilities rather than confirmed functions.. They speculate that cloacal diving may provide a convenient hiding place from predators.. It may also act as a feeding strategy. consistent with what remoras typically eat: material associated with their hosts. such as feces. scraps. and parasites.

Another benefit the researchers propose is hitchhiking. By moving into the manta ray’s rear, the remora could gain “reduced drag,” making it easier to move with less resistance in the water—an idea aligned with the broader way remoras interact with fast-moving animals.

The paper also links cloacal diving to a larger. less understood pattern: remoras have been found in other semi-internal spaces of marine species.. The authors note occurrences in the gills and cloacal openings of whale sharks. as well as in the mouths of lemon sharks.. Still. they say that because such behaviors are rarely observed. researchers have had limited opportunities to describe what drives these cryptic events and how they work in detail.

One key unknown is how long a remora might remain inside a manta ray’s cloaca.. Even without that timeline, the study authors argue that duration could matter.. If a remora stays in place for extensive periods. even a “moderately-sized” fish could potentially disrupt essential processes such as mating behavior. live birth. or defecation.

The researchers also highlight why the behavior is scientifically challenging to pin down: cloacal diving is inherently hard to monitor. and remoras are small and adept at concealing themselves.. In that sense, the seven documented cases offer rare direct evidence, even as the underlying mechanisms remain open to investigation.

The study was published in the journal Ecology and Evolution. drawing attention to the broader ecological implications of host–symbiont interactions that go beyond external attachment.. If cloacal diving does prove to be a consistent strategy—whether for refuge. feeding. or transport—it would add another surprising chapter to how remoras exploit the bodies and movements of larger marine animals.

For now. the observations raise a simple but consequential question: what looks like a nuisance from the manta ray’s standpoint may be part of a sophisticated remora routine.. Future work will likely need more systematic observations to determine frequency. timing. and outcomes for both sides of the interaction. including how often the behavior ends quickly versus lasting long enough to affect reproductive or digestive activity.

In the broader field of marine biology. the report underscores how much still remains unseen in the ocean’s hidden relationships.. Behaviors that involve semi-internal entry can easily escape notice. meaning entire strategies may go unrecognized until someone happens to document them—frame by frame. case by case. in the field.

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