Remoras are peculiar fish, and a recent study reveals they have few limits when it comes to taking advantage of their manta ray hosts.
These tropical suckerfish are renowned for latching onto sharks, whales, and even turtles.
Remoras attach themselves using a sucker located on the back of their head, a modified dorsal fin.
This adaptation allows them to travel far distances, benefiting from a steady stream of oxygenated water across their gills and feeding on leftovers from their hosts’ meals.
Ecologists view these fish as freeloaders.
Although dolphins and green turtles can become burdened by too many remoras, these fish generally do not cause much trouble under normal circumstances.
However, issues arise when remoras become overly familiar.
While examining images and videos of manta rays (Mobula yarae, Mobula birostris, and Mobula alfredi) gathered from various global locations, a group of marine biologists occasionally observed strange behavior from the remoras.
In just seven cases, out of thousands of surveys conducted from 2010 to 2025, remoras were seen hiding inside the cloaca of their manta ray hosts.
A cloaca serves as a multi-purpose exit used by many animals for waste elimination and reproduction.
It is not typically intended as a refuge for fish, but some crafty remoras seem to have chosen it as an ideal hiding spot.
A remarkable video, recorded by a freediver in Florida, USA, captured a remora diving directly in.
The footage shows an adult Atlantic manta ray (M. yarae) with a medium-sized remora (Remora remora) lingering near its pelvic fins.
“Once the diver passed into the ventral plane of the manta ray, the remora appeared to startle and quickly inserted itself into the manta ray’s cloacal opening,” marine biologist Emily Yeager and her colleagues report.
“In response to this intrusion, the manta ray briefly shuddered before continuing to swim away with the remora still inside of its cloacal opening.”
This odd behavior is termed ‘cloacal diving’. See it for yourself:
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>“While this video clip was brief and does not show the remora reemerging from the cloacal opening, it is possible the unexpected presence of the freediver prompted the cloacal diving behavior, indicating the possibility that this behavior could be a response to perceived predation risk or other threat by the remora,” the team writes.
More disturbingly, the scientists believe the manta rays may not actually welcome the behavior.
Remoras have been spotted entering the cloacas of whale sharks too, but whale sharks and their cloacas are considerably larger than the remoras.
Since manta rays are closer in size to the hitchhiking remoras, the medium-sized remora in that video might pose a problem for its host.
“While this behavior was documented through short interactions and the amount of time a remora may spend inside a cloacal opening is unknown, the presence of a moderately-sized remora in a manta ray’s cloacal opening could impede mating behavior, live birth, or defecation if the cloacal diving behavior occurs for extensive periods of time,” the team warns.
Related: Expired Cans of Salmon From Decades Ago Contained a Big Surprise
Determining whether remoras linger in the cloacas of fish for extended periods is challenging through photographs. Some observations show the remora’s tail protruding from the manta rays, but they can also become completely hidden.
This behavior raises questions about the previously assumed mutually beneficial relationship between rays and remoras.
“Evidence of cryptic harmful behaviors in new species indicates that these symbioses may be more parasitic, at least in some contexts, than previously understood,” the authors conclude.
The research was published in Ecology and Evolution.
