Osmoregulation in the estuarine diamond-backed terrapin across a broad range of naturally occurring salinities

Diamond-backed terrapins are exposed to a broad range of salinities within their estuarine habitats, ranging from brackish water to full-strength seawater. The diamond-backed terrapin's ability to live exclusively in highly dynamic estuarine habitats has motivated experiments to explore the species' osmotic strategy; however, most of these experiments have taken place under laboratory conditions. The purpose of this project was to investigate the osmotic status of free-ranging diamond-backed terrapins during the active season in coastal southeastern North Carolina, USA. We collected blood samples from diamond-backed terrapins captured in salinities ranging from 9-39 psu and used linear models to assess the responses of blood osmolality, organic osmolytes, inorganic ions, and blood proteins to environmental and morphological variables. Results indicate that organic osmolytes play an important role in maintenance of body fluid homeostasis during exposure to high salinity. We found that salinity and body size have significant effects on blood variables which may reflect plasticity in osmotic strategy depending on demographic characteristics. We consider our results in the context of the energetic costs of maintaining osmotic homeostasis and the implications for diamond-backed terrapin resilience when exposed to altered salinity profiles due to changes in coastal land use and rising sea levels.