Morphological Structure of the Aortic Wall in Deep Diving Cetacean Species: Evidence for Diving Adaptation

SIMPLE SUMMARY: Diving implies cardiovascular adaptations in marine mammals. The work aimed to analyze the aortic wall in nine cetacean species with deep diving habits belonging to four families. We hypothesize that variations in the aortic wall will reflect the diving capability of each species. The analyses showed that the elastic component was higher in the initial part of the artery, the muscular elements increased in the distal portion in all cases, and the apparent organization of the components in the aortic wall did not show essential modifications between the four families. All species presented a decrease in the arterial wall thickness along the aorta. The reduction was dramatic between the initial and thoracic aorta in the sperm whale specimens; meanwhile, the species of the other three families, beaked whale, pygmy sperm whale, and deep-diving dolphins, showed a more uniform decrease between the aortic segments. Because of the diving habits described for the different species, our findings support that a decrease in the arterial wall thickness between the aortic segments is the most relevant characteristic related to diving adaptation. ABSTRACT: This study analyses the aortic wall structure in nine cetacean species with deep diving habits belonging to four Odontoceti families: Ziphiidae, Kogiidae, Physteridae, and Delphinidae. Samples of ascending, thoracic and abdominal aorta were processed for histological and morphometric studies. The elastic component was higher in the proximal aortic segments, and the muscular elements increased distally in all cases. Morphometric analyses showed that all families presented a decrease in the thickness of the arterial wall and the tunica media along the aorta. The reduction was dramatic between ascending and thoracic aorta in the Physeteridae specimens; meanwhile, the other three families showed a more uniform decrease between the ascending, thoracic and abdominal aorta. The decline was not correlated with a reduced elastic or lamellar unit thickness but with a loss of lamellar units. The organization of the elements in the aortic wall did not show essential modifications between the four families, resembling the structure described previously in the shallow and intermediate diving dolphins. Our findings support that the difference in the morphometric characteristics of the different segments in the aortic wall is likely related to the diving habit more than the absolutes values of any other parameter.