Abstract
ABSTRACTIn chelonids, oxygen is primarily stored in the lungs during a dive. Therefore, management of blood oxygen transportation to peripheral tissues by cardiovascular adjustments during submergence is crucial to maximize their dive duration, and consequently, the time spent for ecological activities such as foraging. However, the cardiac response to exercise has rarely been examined in sea turtles. In this study, heart rate and its relationship with exercise during voluntary dives were determined in six captive green turtles (19.4±1.5 kg) by simultaneously recording depth, acceleration and electrocardiogram. Our results demonstrated that the heart rate of green turtles was generally low (11.1±0.4 bpm) during resting dives, but they often exhibited instantaneously extreme tachycardia (up to 78.4 bpm). Green turtles elevated their heart rate up to 39.8±1.5 bpm during ventilation after resting dives, while up to 33.1±1.4 bpm after active dives. The heart rate immediately elevated with onset of exercise, and increased linearly with exercise. This result may indicate that turtles immediately need to transport oxygen from the lungs to peripheral tissues by pulmonary and systemic circulations to meet the metabolic demands of exercise because they mainly store oxygen in their lungs.
Highlights
Many air-breathing vertebrates are known to exhibit bradycardia when submerged, which allows them to regulate their blood oxygen depletion rate, thereby conserving onboard oxygen stores (Kooyman, 1989; Butler and Jones, 1997)
Our results demonstrated that the heart rate of green turtles was generally low (11.1 ± 0.4 bpm) during resting dives, but they often exhibited instantaneously extreme tachycardia
Extreme tachycardia during resting dives Our study found that extreme tachycardia, i.e. an instantaneous and temporary elevation in heart rate, up to 78.4 bpm, often occurred during resting dives in green turtles
Summary
Many air-breathing vertebrates are known to exhibit bradycardia when submerged, which allows them to regulate their blood oxygen depletion rate, thereby conserving onboard oxygen stores (Kooyman, 1989; Butler and Jones, 1997). The potential for conflict between diving bradycardia and exercise responses has been investigated for marine mammals (Fedak et al, 1988; Williams et al, 1999, Goldbogen et al 2019), aquatic birds (Millard et al, 1973), and sea turtles (Butler et al, 1984; Southwood et al, 2003; Williams et al, 2019) These studies have found that air-breathing vertebrates elevated their heart rate during exercise even while being submerged. Such an instantaneous cardiac response during submergence has not yet been examined in many air-breathing divers except for a few species of marine mammals (Weddell seals, Leptonychotes weddellii; bottlenose dolphins, Tursiops truncatus [Davis and Williams, 2012], narwhals, Monodon monoceros [Williams et al, 2017])
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