Marine scientists have documented an unusual and invasive behavior in the ocean: remora fish hiding inside the cloacas (butts) and gill cavities of manta rays. Based on seven observations collected over 15 years in Florida, Mozambique, and the Maldives, the study provides some of the first photographic evidence that the hitchhiking fish may sometimes have a more parasitic relationship with their hosts than previously understood.
Researchers at the Shark Research and Conservation Program (SRC) at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science and collaborators from the Marine Megafauna Foundation and The Manta Trust, observed remora tails protruding from manta rays and, in one case, captured video of a remora rapidly diving into a manta ray's cloaca, the opening on its underbelly for reproductive and excretion needs, after appearing startled by a nearby diver.
"This is one of those discoveries that reminds us how little is known about the interactions and behavior of marine wildlife," said Emily Yeager, lead author of the study and a Ph.D. candidate in the Department of Environmental Science and Policy at the Rosenstiel School. "We've long assumed remoras and manta rays had mostly beneficial relationships, but these observations suggest the interaction may sometimes be more intrusive and potentially harmful than previously thought."
Remoras, also known as suckerfish, are known for hitchhiking on sharks, whales, and rays using a specialized suction disc on top of their heads. Scientists have generally considered the symbiotic relationship either harmless, or beneficial because remoras consume parasites, along with leftover food, and fecal matter from their hosts. The new observations add to growing evidence that the relationship may exist on a spectrum ranging from mutualistic to parasitic.
The study documented cloacal diving behavior in manta ray species across juvenile and adult animals between 2010 and 2025. The team also recorded one observation of a remora lodged inside a manta ray's gill slit, along with several manta rays displaying injuries consistent with remora intrusion.
"These weird interactions only underscore how much we don't yet know about how different species interact with each other in the ocean," said Catherine Macdonald, research associate professor of environmental science and policy, senior author of the study and the director of the SRC. "Opportunistic observations and routine monitoring programs are essential to understanding and uncovering interactions between highly mobile marine organisms."
The behavior could serve several purposes for remoras, including predator avoidance, feeding opportunities, or reducing drag while swimming, researchers said.
While the direct impacts on manta rays remain unclear, scientists note that intrusive behaviors involving the cloaca or gills could potentially interfere with respiration, reproduction, or other important biological functions if they occur frequently or for long periods of time.
The study, titled "Hiding in plain sight: Evidence of Echeneidae cloacal and gill diving behavior in manta ray hosts" was published May 11, 2026, in the journal Ecology and Evolution.
The authors are Emily A. Yeager1,2, Jessica Pate3, Guy M.W. Stevens4,5, Bryant Turffs3 and Catherine Macdonald1,2
1Department of Environmental Science and Policy, Rosenstiel School of Marine, Atmospheric, and Earth Science, 2Shark Research and Conservation Program, Rosenstiel School of Marine, Atmospheric, and Earth Science, 3Marine Megafauna Foundation, 4The Manta Trust, 5Maldives Manta Conservation Programme.
