Innate immune function in the diamondback terrapin (Malaclemys terrapin)

The innate immune system serves as a critical first line of defense in reptiles, particularly under the energetic constraints of ectothermy. This study characterizes complement-mediated innate immune responses in the diamondback terrapin ( Malaclemys terrapin ), an estuarine turtle species facing increasing environmental and disease pressures. We evaluated the antibacterial capacity of terrapin plasma against Aeromonas hydrophila , Vibrio vulnificus , Staphylococcus aureus , and Streptococcus pyogenes and assessed complement activity through sheep red blood cell (SRBC) hemolysis assays. Plasma showed strong, concentration-dependent antibacterial activity, with complete inhibition of S. aureus at 100% plasma and substantial inhibition of the other species at ≥25%. Hemolytic activity was rapid, occurring primarily within 10 min, and peaked at 25°C–30°C. Complement activity was abolished by heat, protease, and EDTA treatments and restored with divalent cations, indicating a heat-labile, protease-sensitive mechanism consistent with the alternative complement pathway. Mannan affinity chromatography and SDS-PAGE revealed two major lectins (~25.8 and ~33.4 kDa), consistent with known complement-associated lectins in other emydid turtles. These findings demonstrate that diamondback terrapins possess robust, complement-mediated innate immune responses, potentially shaped by evolutionary exposure to estuarine pathogens like Vibrio spp., and support their use in comparative eco-immunological research and conservation health assessments.