Structural characteristics of the epidermis in marine and freshwater finless porpoises adapted to distinct osmotic environments

The Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis, Pilleri & Gihr, 1972; YFP) is an exclusively freshwater cetacean species inhabiting the Yangtze River and its connecting large lakes. As the primary line of defense in maintaining physiological equilibrium, the epidermis of the porpoise is expected to have undergone structural adaptations due to the shift from the marine to the freshwater environment. This study compared the microstructural and ultrastructural features of the epidermis of YFP and its marine counterpart, the East Asian finless porpoise (Neophocaena asiaeorientalis sunameri, Pilleri & Gihr, 1975; EAFP). Microscopic structural observations and statistical analyses of the epidermal thickness demonstrated no significant differences in the cell structure or distribution between the two porpoise species. However, the epidermis of the YFP contained more abundant stratum basale cells. The outermost lipid stratum corneum exhibited a thinner cell layer with wider neutral lipid droplets to resist the passive entry of water molecules in the hypotonic environment. In contrast, for the EAFP, a more uniformly arranged stratum basale in the epidermis led to denser keratin fibers and robust desmosomes within each epidermal layer at the ultrastructural level. This tight arrangement of cells can reduce transepidermal water loss (TEWL) in an environment with higher osmotic pressure. In conclusion, the two finless porpoise species appear to employ different epidermal mechanisms to adapt to their distinct osmotic environments. The YFP appears to possess a "lipid waterproofing" epidermal structure, while the EAFP possesses a "thick and compact water-retaining" epidermal structure to cope with potential water loss.