While generally recognized as peculiar marine organisms, a recent investigation has revealed that remoras exhibit a remarkable lack of restraint when it comes to exploiting their manta ray associates.
These pelagic adhesive fish are predominantly known for their habit of attaching themselves to larger marine fauna such as sharks, cetaceans, and even sea turtles.
Their method of adherence involves a specialized suction disc situated on the posterior aspect of their cranium, which is, in fact, a modified dorsal fin.
This adaptation facilitates extended migratory journeys far beyond their intrinsic capabilities, while simultaneously ensuring a consistent influx of oxygenated water across their branchial structures and access to sustenance fragments derived from their hosts’ repasts.
Entomologists generally categorize these opportunistic voyagers as essentially availing themselves of a gratis passage.
While certain species like cetaceans and green sea turtles can experience a substantial burden from an excessive number of these piscine passengers, the majority of the time, remoras appear to cause minimal or no discernible disturbance to their hosts.
This passive coexistence, however, can transition when the remoras become excessively emboldened.
During a review of photographic and video documentation pertaining to various manta ray species (Mobula yarae, Mobula birostris, and Mobula alfredi) collated from manta aggregation sites globally, a cadre of marine biologists observed, on a limited number of occasions, an unusual phenomenon involving the remoras.
Across thousands of surveillance records compiled between 2010 and 2025, a mere seven instances documented remoras seeking refuge within the cloaca of their manta ray hosts.
For those unfamiliar with the term, a cloaca is a multi-functional posterior aperture utilized by numerous animal species for the expulsion of metabolic waste and for reproductive purposes.
It is ostensibly not designed as a sanctuary for piscine life, yet certain audacious remoras appear to have identified it as an ideal concealment spot.
Remarkably, a single video recording, obtained by a free diver in Florida, USA, captured a remora actively entering the cloaca.
The footage depicts an adult Atlantic manta ray (M. yarae) with a medium-sized remora (Remora remora) positioned proximate to its pelvic fins.
“As the diver approached the ventral aspect of the manta ray, the remora appeared to become agitated and rapidly entered the manta ray’s cloacal opening,” marine biologist Emily Yeager and her associates reported.
“In response to this infringement, the manta ray evinced a brief convulsion before resuming its course, with the remora remaining within its cloacal aperture.”
This notable piscine comportment has been designated as ‘cloacal diving’.
“Although this video segment was concise and does not illustrate the remora’s egress from the cloacal opening, it is plausible that the unexpected presence of the free diver precipitated the cloacal diving behavior, suggesting that this action might be a defensive response to potential predatory threats or other perceived dangers by the remora,” the research team stated.
More concerningly, the researchers posit that the manta rays may not find this behavior agreeable.
Instances of remoras entering the posterior orifices of whale sharks have been documented previously; however, whale sharks (and their cloacas) are of a considerably larger scale relative to their intrusive passengers.
Manta rays, in contrast, are more closely matched in size to their hitchhikers, and the medium-sized remora observed in the video could potentially inflict discomfort or harm upon its host.
“While this behavior was documented through brief encounters and the duration of a remora’s presence within a cloacal opening remains uncertain, the presence of a moderately sized remora within a manta ray’s cloacal opening could obstruct reproductive activities, parturition, or defecation if the cloacal diving behavior persists for extended durations,” the research group cautioned.
Should remoras inhabit the cloacas of fish for prolonged periods, such an occurrence would be challenging to ascertain from static imagery. Although a few observations noted the remora’s caudal appendage projecting from the manta ray, it appears they can achieve complete concealment.
This behavior casts a pall over the symbiotic relationship between manta rays and remoras, which was previously considered to be mutually advantageous.
“The emergence of clandestine detrimental behaviors in novel species suggests that these symbioses may be more akin to parasitism, at least under certain circumstances, than was previously comprehended,” the authors concluded.
