Impact of marine hitchhiker load on host energy intake

Abstract Determining the energetic and fitness trade-offs associated with symbiotic relationships (mutualism, commensalism or parasitism) can reveal the implications of symbiosis for species and ecosystem health. To identify hitchhiker impact on sea turtles, this study reviewed global literature and examined the association between remoras ( Echeneis naucrates ) and green turtles ( Chelonia mydas ) at a high-density foraging site in the Red Sea using SCUBA and video ( n = 71 observations) in October 2023. Previous evidence of remora-sea turtle association is limited to qualitative observations from the Atlantic and Pacific Oceans. The results show that depth significantly impacted the number of remoras per turtle ( p < 0.05 ). Turtle grazing rate was affected by remora load ( p < 0.05) , decreasing by ~ 30% across the load range from a mean of 22.8 bites min −1 (0 remoras) to 15.6 bites min −1 (3 remoras). There was little evidence of benefit to turtles, with only one observation of a remora cleaning a turtle carapace. The observed reduction in grazing effort suggests potential impacts on green turtle body condition over time, which may affect growth, reproduction, and population health, warranting long-term investigation. These findings present the first quantitative evidence that the remora-sea turtle relationship shifts from commensalism to parasitism as remora load increases, demonstrating the potential costs of hitchhikers for sea turtles.