Remoras Slip Into Manta Ray Cloacae, Prompting Parasitic Concerns

A study published May 11 in *Ecology and Evolution* documents “cloacal diving,” where remoras enter the cloacal opening of manta rays. Video footage captured the fish slipping into the vent of both juvenile and adult individuals, revealing a previously undocumented internal hitchhiking behavior.

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Remoras, also called suckerfish, attach to hosts via a suction cup on their heads. In many cases they remove parasites, creating a mutualistic link. Other observations describe a neutral commensalism, where the host gains no benefit. However, physical injury from suction and increased drag have been reported, and the new cloacal entries suggest a potentially parasitic dimension.

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Researchers examined survey data (2010-2025) from the Marine Megafauna Foundation and the Manta Trust at aggregation sites in Florida, the Maldives and Mozambique. Seven cloacal-diving events were recorded across all three manta-ray species, with large remoras fully filling the opening. The team also documented a remora beneath a gill slit and several gill-injury cases likely linked to the fish. Lead ecologist Emily Yeager (University of Miami) coordinated the analysis; David Shiffman and Brooke Flammang offered external commentary.

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The cloaca is a sensitive reproductive and excretory opening; intrusion could damage tissue, hinder waste elimination, and impair breeding. Observed gill injuries suggest additional health risks. Researchers suspect the behavior may be a fear response, yet the physical impacts raise parasitic concerns. If such injuries are common, they could reduce individual fitness and, over time, impair reproductive success and waste excretion, potentially affecting manta ray health.

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Emily Yeager said remoras seem to hide from predators, noting they “jump straight into the manta ray’s cloaca opening,” causing the host to shudder. David Shiffman called the behavior fascinating yet distressing, adding it is “no fun for the manta.” Brooke Flammang warned that remora suction cups can damage host skin.

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• “My first reaction was a combination of amazement and horror—it’s so cool that remoras can do that, but I imagine it’s no fun for the manta,” — David Shiffman, independent marine conservation biologist

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• “The remoras are pretty much as wide as the cloaca is,” — Emily Yeager, marine ecologist, University of Miami

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• “That remora jumps straight into the manta ray’s cloaca opening,” — Emily Yeager, marine ecologist, University of Miami

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Only seven cloacal entries were documented, limiting statistical confidence. The study could not directly assess the manta’s physiological response or long-term reproductive impact, leaving a gap in understanding the true cost of the behavior.

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