Brischoux François, Kornilev Yurii V
Centre d'Etudes Biologiques de Chizé, CEBC UMR 7372 CNRS-ULR, Villiers en Bois, France.
Bulgarian Society for the Protection of Birds, NCC "Poda", Burgas, Bulgaria.
PLoS One. 2014 Mar 21;9(3):e92617. doi: 10.1371/journal.pone.0092617. eCollection 2014.
The widespread relationship between salt excreting structures (e.g., salt glands) and marine life strongly suggests that the ability to regulate salt balance has been crucial during the transition to marine life in tetrapods. Elevated natremia (plasma sodium) recorded in several marine snakes species suggests that the development of a tolerance toward hypernatremia, in addition to salt gland development, has been a critical feature in the evolution of marine snakes. However, data from intermediate stage (species lacking salt glands but occasionally using salty environments) are lacking to draw a comprehensive picture of the evolution of an euryhaline physiology in these organisms. In this study, we assessed natremia of free-ranging Dice snakes (Natrix tessellata, a predominantly fresh water natricine lacking salt glands) from a coastal population in Bulgaria. Our results show that coastal N. tessellata can display hypernatremia (up to 195.5 mmol x l(-1)) without any apparent effect on several physiological and behavioural traits (e.g., hematocrit, body condition, foraging). More generally, a review of natremia in species situated along a continuum of habitat use between fresh- and seawater shows that snake species display a concomitant tolerance toward hypernatremia, even in species lacking salt glands. Collectively, these data suggest that a physiological tolerance toward hypernatremia has been critical during the evolution of an euryhaline physiology, and may well have preceded the evolution of salt glands.
排盐结构（如盐腺）与海洋生物之间广泛存在的关系有力地表明，在四足动物向海洋生活过渡的过程中，调节盐分平衡的能力至关重要。在几种海蛇物种中记录到的高血钠症（血浆钠升高）表明，除了盐腺的发育外，对高血钠症耐受性的发展也是海蛇进化过程中的一个关键特征。然而，缺乏来自中间阶段（缺乏盐腺但偶尔利用咸水环境的物种）的数据，无法全面了解这些生物广盐性生理特征的进化情况。在本研究中，我们评估了来自保加利亚沿海种群的自由放养的骰子蛇（棋盘蛇，一种主要生活在淡水的水游蛇科蛇类，缺乏盐腺）的血钠水平。我们的结果表明，沿海的棋盘蛇能够出现高血钠症（高达195.5 mmol x l(-1)），而对几种生理和行为特征（如血细胞比容、身体状况、觅食）没有任何明显影响。更普遍地说，对处于淡水和海水之间连续栖息地利用范围内的物种的血钠水平进行的综述表明，蛇类物种即使在缺乏盐腺的物种中也表现出对高血钠症的相应耐受性。总体而言，这些数据表明，对高血钠症的生理耐受性在广盐性生理特征的进化过程中至关重要，并且很可能早于盐腺的进化。
