Morphological Investigation of Uncharacterised Cardiovascular Structures in Shallow-Diving, Semi-Aquatic Freshwater Turtles (Chelidae:)

Abstract

Simple Summary Reptiles, particularly species capable of diving, have developed a range of physiological and anatomical adaptations that enable them to survive for extended periods underwater. This study aimed to characterise previously undescribed features of the cardiovascular system in the Murray River turtle. Thirteen healthy adult females were examined, revealing distinct valve-like structures within muscular arteries, as well as small intramural channel-like formations within arterial walls, suggestive of structural adaptations that may support blood flow under changing physiological conditions. Additionally, smooth muscle cells were confirmed lining cardiac atria, indicating a potential role in regulating cardiac blood movement. Measurements of these valve-like structures showed that they could occupy a large proportion of the blood vessel lumen, in some cases narrowing the vessel by more than 90%. Their consistent presence across multiple turtles and organs, together with the degree of vascular narrowing observed, supports the interpretation that these are true anatomical structures rather than tissue processing artefacts. Collectively, these findings suggest that turtles possess specialised cardiovascular features that may assist in managing fluctuations in blood pressure and circulation during diving. This study broadens current understanding of how vertebrates tolerate low-oxygen environments and provides a foundation for future research into cardiovascular adaptations. plain-language-summary Abstract Reptiles with diving capabilities have evolved physiological adaptations as well as conformational changes to temporarily sustain life underwater. Despite the importance of both respiratory and cardiovascular systems during diving, most studies have focused on respiratory adaptations. Thus, characterisation of previously undescribed cardiovascular anatomical variations in diving vertebrates is of broad interest. Thirteen clinically healthy, free-ranging adult female Murray River turtles (Chelidae:) were collected for research purposes, euthanised and autopsied. Prominent, valve-like structures, comprised exclusively of smooth muscle myocytes, were identified in medium- and large-calibre muscular arteries of all individuals. Additionally, multiple intramural vascular channels, mimicking post-thrombotic recanalization, were observed within medium-calibre muscular arteries. Further, we confirmed the presence of alpha-smooth-muscle actin-positive cells lining the cardiac atria in. Quantitative morphometric analyses demonstrated that the valve-like structures frequently occupied a substantial proportion of the vascular lumen, in some cases exceeding 90% luminal narrowing. Their consistent presence across multiple individuals and organ systems supports the interpretation that these are physiological vascular structures rather than artefacts. This study examines the potential physiological and evolutionary roles of these vascular structures, providing a basis for further research into cardiovascular adaptations in vertebrates subjected to postural changes and diving-related haemodynamic challenges.