Sucker fish are hiding in manta rays' rear ends, new study reveals

A new study published in the journal Ecology And Evolution documents a surprising behavior in which remoras, also called suckerfish, sometimes enter a manta ray’s cloaca—what researchers term “cloacal diving.” The work, based on seven instances recorded between 2010 and 2025, includes observations gathered by the Marine Megafauna Foundation in three ocean basins and across all three known manta species. Researchers say this is the first confirmed documentation of the behavior in manta rays, though remoras have previously been observed in whale-shark cloacae. In some cases, only the tip of the tail protrudes; in others, the remora’s width nearly matches the opening and may force much of its body to remain outside. Catherine Macdonald of the University of Miami led the study, while Emily Yeager, also at the university, authored it. The findings may reshape debate over whether remoras’ interaction is commensal or mutualistic.