Remoras, a group of fish known for attaching themselves to larger marine animals such as sea turtles, whales, and sharks, have been observed exhibiting a previously undocumented behavior involving manta rays. According to a recent study published in Ecology and Evolution, remoras sometimes enter the cloacal openings—located at the rear end of manta rays—an action researchers have termed "cloacal diving."

This behavior, documented in seven recorded instances between 2010 and 2025, spans all three known species of manta rays and was observed across three different ocean basins. The data was compiled by the Marine Megafauna Foundation and reported by a team including Catherine Macdonald, director of the shark research and conservation program at the University of Miami and senior author of the study.

Remoras, also called suckerfish, have been known to seek refuge in the cloacas of whale sharks before, but this is the first time such behavior has been confirmed in manta rays. The observations show remoras inserting themselves far enough into the ray’s cloaca that only the tip of their tails remains visible. In some cases, when the manta ray is too small to fully accommodate the remora, a portion of the fish’s body remains outside the host.

Researchers note that manta rays appear to be reluctant hosts in this interaction. Macdonald emphasized that the behavior does not seem to be comfortable or welcomed by the rays. The findings also add complexity to ongoing discussions regarding the ecological relationship between remoras and their hosts, which has long been debated. While remoras are often considered commensal, benefiting from transportation and food scraps without harming the host, the new evidence raises questions about the costs or stress this cloacal diving might impose on manta rays.

Although the functional purpose behind this unusual behavior remains unclear, the broad geographic and species range of the observations suggests it may be more common than previously recognized. The study underscores the need for further investigation into remora-host interactions, especially considering the conservation importance of manta rays and the diverse marine ecosystems they inhabit.