Cardiac responses to underwater sound playback in juvenile green and loggerhead sea turtles: an exploratory biologging study

Anthropogenic underwater noise is an increasing concern in marine environments, yet physiological responses of sea turtles remain poorly understood. We examined cardiac responses to underwater sound playback in three juvenile green turtles (Chelonia mydas) and four juvenile loggerhead turtles (Caretta caretta) maintained after incidental capture. Electrocardiogram, acceleration, depth, and turtle-borne acoustic data were recorded during 44 trials. Green turtles showed prolongation of the RR interval (the time between successive heartbeats) on average after playback, particularly under vessel noise, whereas loggerhead turtles showed RR interval shortening. Generalized estimating equation models showed contrasting vessel-control response patterns between the two datasets. These patterns remained after accounting for mean changes in activity or depth, suggesting that responses were not explained solely by broad-scale movement or vertical displacement. Interpretation is limited by small sample size, differing experimental settings, variable playback durations, and estimated rather than directly measured sound pressure levels. Cardiac biologging can reveal physiological responses to underwater sound.