Variations of natremia in sea kraits (Laticauda spp.) kept in seawater and fresh water

Marine tetrapods evolved specific excretory structures (e.g. salt glands) that maintain salt concentrations within a narrow range of variation. However, recent investigations showed that in some lineages (sea snakes), individuals dehydrate in seawater and cannot equilibrate their hydromineral balance without access to fresh water. How these marine species cope with salt gain is therefore puzzling. We sampled two species of amphibious sea kraits (Laticauda saintgironsi and L. laticaudata) in the field. We also experimentally investigated patterns of salt regulation, specifically variations in natremia (plasma sodium) and body mass (net water flow), in individuals transferred first to fresh water and then to seawater. Our results show that free-ranging sea kraits display hypernatremia (up to 205mmol·l(-1)). Experimental data showed that natremia markedly decreased in snakes exposed to fresh water and increased when they were transferred to saltwater, thereby demonstrating a marked flexibility in their relation to environmental conditions. A literature survey indicated that all free-ranging marine snake species usually display hypernatremia despite having functional salt glands. Overall, sea snakes exhibit a marked tolerance to salt load compared to other marine tetrapods and apparently trigger substantial salt excretion only once natremia exceeds a high threshold. We hypothesise that this high tolerance significantly decreases energetic costs linked to salt gland functioning.