After reviewing more than a decade of underwater footage, researchers have documented remoras, the suckerfish famous for hitching rides on larger animals, diving into the cloaca of manta rays. According to Science X, a study published in the journal Ecology and Evolution describes the behavior across all three known manta ray species. It is the first time remora “cloacal diving” has been documented in manta rays, though it had previously been seen in sharks.
An unusual twist on a familiar partnership
Remoras, members of the family Echeneidae, are ray-finned fish known for attaching themselves to whales, sharks and turtles. They use a specialized disk on top of the head that works like a suction cup, riding along for free transport and sometimes food, and in return they often clean parasites from their hosts. It is one of the more recognizable partnerships in the ocean.
What the new research adds is a far more intimate version of that relationship. Instead of clinging to the outside of a manta ray, some remoras were seen diving into the cloaca, the single opening the animal uses for waste and reproduction. In some cases the fish plunged so deep that only its tail remained visible; in others, half the body hung out of the cavity. The researchers wrote that these observations “add important documentation to the growing knowledge base on Echeneidae-host associations” and offer insight into what they called a “cryptic symbiotic relationship.”
How the behavior was found
The team did not stumble on this in a single dramatic encounter. They combed through more than a decade of underwater footage collected at several locations, including Florida and the Maldives, conducting thousands of surveys between 2010 and 2025. Across all that observation, they documented seven instances of cloacal diving and one instance of gill attachment. The behavior appeared in less than 1% of survey observations, making it genuinely rare and easy to miss.
Because the pattern showed up across all three manta species and at widely separated sites, the researchers treat it as a real, if uncommon, part of remora behavior rather than a one-off oddity. The documentation of gill attachment alongside cloacal diving suggests remoras exploit more than one hidden space on their hosts.
As for why the fish would seek out such a location, the researchers are candid that they are not sure, and they offer several theories rather than a firm answer. One is that the remora uses the host’s body as a shield against predators. Another is that it feeds on parasites or waste that the openings harbor. A third is energetic: by tucking inside, the fish escapes the rushing water generated as the ray moves through the ocean, so it spends less energy than it would clinging to the outside.
How the rays respond, and what is still unclear
Whatever the reason, the manta rays do not appear to welcome the behavior. In one video, a ray briefly shuddered after a remora entered its cloaca. In other observations, rays flicked their pectoral fins, which the scientists suggest could be an attempt to dislodge the fish. The authors concluded that “Echeneidae-host relationships are more physiologically and ecologically complex than previously understood,” while acknowledging that the exact mechanisms driving cloacal diving and gill attachment remain unclear.
Several specifics are unresolved. The researchers have not confirmed which of the competing explanations is correct, whether the behavior helps or harms the ray, or how the fish avoid injury inside their host. The rarity of the observations, fewer than one in a hundred surveys, also means the sample is small, and the study frames its findings as documentation and hypotheses rather than settled conclusions.
For readers, the study is a reminder of how much ocean behavior still escapes notice, and of the value of long-term footage. A partnership that most people picture as a fish stuck to a shark’s belly turns out to include a stranger, hidden version that only surfaced because scientists were willing to review fifteen years of video. The next step, the researchers indicate, is understanding what drives the behavior and what it costs the animals on both sides of this odd relationship.
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*This article was researched with the help of AI, with human editors creating the final content.
