Are Remoras Parasitic Hitchhikers? New Research Challenges Symbiotic Myth

Remoras, known for their distinctive suction cup-like heads, have long been considered beneficial hitchhikers in the ocean. These fish attach themselves to larger marine animals—such as sharks, rays, whales, dugongs, turtles, and even parrotfish—using their adhesive foreheads to secure free rides across vast ocean distances.

Traditionally, remoras were thought to engage in a mutually beneficial relationship with their hosts. The fish were believed to eat parasites and dead skin off their hosts, thereby reducing disease risk, while benefiting from shelter and transportation. They also scavenged leftover food scraps and feces from their hosts. While a few remoras might slightly reduce a manta ray’s aerodynamics, their small size and the hosts’ massive proportions made this a seemingly negligible trade-off.

However, new research is challenging this long-held assumption. Emily Yeager, a PhD student at the University of Miami, argues that the narrative around remoras is shifting. In a 2025 study, researchers found that sea turtles carrying one to three remoras grazed less frequently. Even more striking, they observed only a single instance of a remora cleaning a turtle’s shell.

Yeager’s findings, published in the journal Ecology and Evolution, suggest that remoras may be more parasitic than previously believed. The study documents remoras entering the bodies of their hosts, including their mouths, gill slits, and cloacae—the all-purpose openings used for excretion, reproduction, and, in some species, birthing.

One 2023 paper detailed observations of remoras wriggling inside whale sharks, including their cloacae. Yeager’s research expands on this, describing remoras swimming inside the cloacae of manta rays, sometimes inserting half their bodies into the helpless ray’s cloaca. The team has coined this behavior “cloacal diving.”

The study presents some of the strongest evidence yet that remoras may be parasitic rather than symbiotic. As Yeager notes, the act of “cloacal diving” is far from the harmless behavior once assumed. The paper underscores the need for a reevaluation of remora-host relationships, highlighting how even small organisms can have significant impacts on their larger counterparts.