A study published in the journal Ecology and Evolution on May 11, 2025, has documented remoras (family Echeneidae) entering the cloacae and attaching beneath the gill slits of manta rays.
Led by PhD student Emily A. Yeager from the University of Miami's Shark Research and Conservation Program, the research analyzed 15 years of footage from Florida, the Maldives, and Mozambique. The findings present evidence that the relationship between remoras and manta rays may sometimes be parasitic, challenging traditional classifications of the interaction as purely commensal or mutualistic.
Observed Behaviors
Researchers documented seven instances of remoras entering the cloacae of manta rays, a behavior termed "cloacal diving." This behavior was observed across all three manta ray species (reef manta ray Mobula alfredi, giant oceanic manta ray Mobula birostris, and Atlantic manta ray Mobula yarae), in both juveniles and adults, and across multiple ocean basins.
In one instance captured on video in 2023, a remora, after being startled by a free-diver, inserted itself into a manta ray's cloaca. The ray shuddered and then swam away with the remora inside.
- The study also recorded remoras attaching beneath manta ray gill slits. Injuries consistent with remora intrusion into the gills were observed in both juveniles and adults.
- Co-author Jessica Pate of the Marine Megafauna Foundation noted that observations of remora tails protruding from manta ray cloacae occurred approximately once every two to three years.
Background on Remora-Host Relationships
Remoras possess a modified dorsal fin that has evolved into a suction disc, allowing them to attach to larger marine animals such as sharks, whales, sea turtles, and rays. This relationship has historically been described as commensalism (remora benefits, host unaffected) or mutualism (both benefit). Remoras gain transportation, protection, and food, including scraps, parasites, and fecal material. Some studies have suggested remoras help hosts by removing parasites.
Recent research, however, has indicated that remora attachment can increase drag on hosts, potentially reducing swimming efficiency and increasing energy costs, and that large or numerous remoras can cause skin damage. A 2023 study documented remoras entering the mouths, gill slits, and cloacae of whale sharks. A 2025 paper found that sea turtles with one to three attached remoras grazed less.
Potential Explanations and Open Questions
Scientists have not established a definitive explanation for why remoras enter cloacae and gill slits. Possible reasons include seeking nutrients, shelter, or reproductive opportunities.
- Lead author Emily Yeager suggested that remoras may feed on host feces from inside the cloaca.
- Researchers have hypothesized that manta rays could serve as a reproductive habitat for remoras.
- Yeager observed that the fit between the remora and the cloaca is often tight, raising questions about how the remora breathes or exits.
- The behavior may also be a startle response, as one remora was observed to enter the cloaca after being startled by a diver.
Implications for Manta Rays
Manta rays are subject to existing threats including fishing pressure, boat strikes, habitat degradation, and climate change. If remora behavior causes stress, injury, or increased energy costs, it may add to the pressures these animals face.
- The study notes that manta rays have been observed trying to dislodge remoras by leaping or scraping against the sand, which may indicate the attachment is detrimental.
- Potential detriments to the manta ray include physical damage, interference with defecation, mating, and birthing.
- Since publication, three other researchers have reported similar observations to the study's authors.
Shifting Scientific Understanding
The study suggests that the remora-ray relationship spans a spectrum from mutualistic to parasitic, rather than fitting a single category. Yeager stated that "the narrative is shifting" regarding remoras as exclusively benign or helpful.
The findings rely on opportunistic observations due to the difficulty of directly studying remora behavior. Further research is needed to understand the ecological significance of these behaviors and the factors that cause the relationship to shift along the continuum from beneficial to harmful.