A Baseline Model For Estimating the Risk of Gas Embolism in Sea Turtles During Routine Dives

Abstract

Sea turtles, like other air-breathing diving vertebrates, commonly experience significant gas embolism (GE) when incidentally caught at depth in fishing gear and brought to the surface. To better understand why sea turtles develop GE, we built a mathematical model to estimate partial pressures of N2 (PN2), O2 (PO2), and CO2 (PCO2) in the major body-compartments of diving loggerheads (Caretta caretta), leatherbacks (Dermochelys coriacea), and green turtles (Chelonia mydas). This model was adapted from a published model for estimating gas dynamics in marine mammals and penguins. To parameterize the sea turtle model, we used values gleaned from previously published literature and 22 necropsies. Next, we applied this model to data collected from free-roaming individuals of the three study species. Finally, we varied body-condition and cardiac output within the model to see how these factors affected the risk of GE. Our model suggests that cardiac output likely plays a significant role in the modulation of GE, especially in the deeper diving leatherback turtles. This baseline model also indicates that even during routine diving behavior, sea turtles are at high risk of GE. This likely means that turtles have additional behavioral, anatomical, and/or physiologic adaptions that serve to reduce the probability of GE but were not incorporated in this model. Identifying these adaptations and incorporating them into future iterations of this model will further reveal the factors driving GE in sea turtles.

Absence of atrial smooth muscle in the heart of the loggerhead sea turtle (Caretta caretta): a re-evaluation of its role in diving physiology

“…We suggest that constriction of sinus venosus smooth muscle impedes venous return and reduces filling of the right atrium, as opposed to being achieved via altering of atrial dimensions through endocardial smooth muscle contraction (Joyce et al, 2019). In the absence of atrial smooth muscle, smooth muscle in the sinus venosus may instead contribute to regulation of cardiac output and pulmonary blood flow in order to minimize nitrogen uptake during diving while still selectively exchanging oxygen and carbon dioxide (García-Parraga et al, 2018, Robinson et al, 2021).…”

“…This protective dive response has also been shown to be interrupted in struggling green sea turtles, Chelonia mydas , during involuntary forced dives (Berkson, 1967). Understanding the mechanisms of cardiac regulation, including the potential role for endocardial smooth muscle in sea turtles, is particularly pertinent given that effective regulation of cardiac output during diving is likely to affect the susceptibility to gas embolism (Robinson et al, 2021). The only previous insight into the possible presence of atrial smooth muscle in sea turtles was provided by Bottazzi (1906), who briefly remarked on tonus waves produced by a right atrial C. caretta preparation; however, it is not possible to discern if this represented a reproducible observation.…”