A review of laboratory and field data, together with recent growth experiments, show that Chelydra serpentina, the common snapping turtle, is unable to hypoosmoregulate in salinities more concentrated than their internal osmotic concentration, about one third that of seawater (100% seawater is defined as 35 parts per thousand = 1000 milliosmoles). Circumstantial evidence suggests an understanding of the nascent stages of adaptation of freshwater vertebrates to high salinity habitats should include incidental immune system effects. Recent advances in the study of autoimmunity and ecoimmunology indicate the immune system of vertebrates plays an integrative role in maintaining homeostasis in the face of changing internal and external stimuli and may clarify why a small percentage of snapping turtle hatchlings can grow at relatively high salinities, at least up to 40% seawater.