Comparing Functional Immune Responses in Three Aquatic Turtle Species

Reptiles are increasingly faced with conservation challenges and ecoimmunological techniques would be a beneficial tool in monitoring and evaluating populations that are at-risk. However, the reptilian immune system is poorly understood, and few studies have made intraspecies comparisons, making generalizations difficult. To help address this gap, innate immune function across three conspecific freshwater turtle species was evaluated. Red-eared sliders (Trachemys scripta elegans), Mississippi mud (Kinosternon subrubrum hippocrepis), and musk turtles (Sternotherus odoratus), are found throughout the Southeastern USA and represent different ecological microhabitats and life histories. In spring 2024, male aquatic turtles were caught using hoop nets, and blood samples were taken to assess immune characteristics. Microbial killing assays were conducted using multiple blood serum and buffy layer (hereafter referred as "serum + BL") manipulations (fresh, frozen, and frozen + heat manipulated serum + BL) as well as three microbes that activate specific immunological responses: Gram-positive bacterium (Staphylococcus aureus), Gram-negative bacterium (Escherichia coli), and a fungus (Candida albicans). By using this suite of microbial assays, differences in immune prioritization can be observed across species. This study revealed that there are differences in immunocompetence in each species of freshwater turtle that varied by microbe and serum + BL manipulation. We determined that because of the contribution of complement proteins when challenged against Gram-negative bacteria, frozen manipulated serum + BL appears to be a reliable way to assess immunocompetence in individuals across turtle species. Conducting intraspecies comparisons in immune function using integrative approaches can provide valuable insight into the underlying patterns of physiological variability within wild organisms, especially those that are of conservation concern.