Pillans Richard D, Franklin Craig E
School of Life Sciences, The University of Queensland, Queensland, QLD 4072, Australia.
Comp Biochem Physiol A Mol Integr Physiol. 2004 Jul;138(3):363-71. doi: 10.1016/j.cbpb.2004.05.006.
Bull sharks (Carcharhinus leucas) were captured across a salinity gradient from freshwater (FW) to seawater (SW). Across all salinities, C. leucas were hyperosmotic to the environment. Plasma osmolarity in FW-captured animals (642 +/- 7 mosM) was significantly reduced compared to SW-captured animals (1067 +/- 21 mosM). In FW animals, sodium, chloride and urea were 208 +/- 3, 203 +/- 3 and 192 +/- 2 mmol l(-1), respectively. Plasma sodium, chloride and urea in SW-captured C. leucas were 289 +/- 3, 296 +/- 6 and 370 +/- 10 mmol l(-1), respectively. The increase in plasma osmolarity between FW and SW was not linear. Between FW (3 mosM) and 24 per thousand SW (676 mosM), plasma osmolarity increased by 22% or 0.92% per 1 per thousand rise in salinity. Between 24 per thousand and 33 per thousand, plasma osmolarity increased by 33% or 4.7% per 1 per thousand rise in salinity, largely due to a sharp increase in plasma urea between 28 per thousand and 33 per thousand. C. leucas moving between FW and SW appear to be faced with three major osmoregulatory challenges, these occur between 0-10 per thousand, 11-20 per thousand and 21-33 per thousand. A comparison between C. leucas captured in FW and estuarine environments (20-28 per thousand ) in the Brisbane River revealed no difference in the mass of rectal glands between these animals. However, a comparison of rectal gland mass between FW animals captured in the Brisbane River and Rio San Juan/Lake Nicaragua showed that animals in the latter system had a significantly smaller rectal gland mass at a given length than animals in the Brisbane River. The physiological challenges and mechanisms required for C. leucas moving between FW and SW, as well as the ecological implications of these data are discussed.
牛鲨（白真鲨，Carcharhinus leucas）在从淡水（FW）到海水（SW）的盐度梯度中被捕获。在所有盐度条件下，白真鲨相对于周围环境都是高渗的。与捕获于海水中的动物（1067±21毫渗摩尔）相比，捕获于淡水中的动物的血浆渗透压（642±7毫渗摩尔）显著降低。在淡水中的动物体内，钠、氯和尿素的浓度分别为208±3、203±3和192±2毫摩尔/升。捕获于海水中的白真鲨的血浆钠、氯和尿素浓度分别为289±3、296±6和370±10毫摩尔/升。淡水和海水之间血浆渗透压的增加并非呈线性关系。在淡水（3毫渗摩尔）和盐度为24‰的海水（676毫渗摩尔）之间，盐度每升高1‰，血浆渗透压增加22%或0.92%。在盐度从24‰到33‰之间，盐度每升高1‰，血浆渗透压增加33%或4.7%，这主要是由于血浆尿素在盐度从28‰到33‰之间急剧增加所致。在淡水和海水之间移动的白真鲨似乎面临着三大渗透调节挑战，这些挑战分别出现在盐度为0 - 10‰、11 - 20‰和21 - 33‰之间。对捕获于布里斯班河淡水中的白真鲨与河口环境（盐度20 - 28‰）中的白真鲨进行比较，结果显示这些动物的直肠腺质量并无差异。然而，对捕获于布里斯班河的淡水白真鲨与圣胡安河/尼加拉瓜湖中的白真鲨的直肠腺质量进行比较发现，在相同体长情况下，后一系统中的动物直肠腺质量明显小于布里斯班河中的动物。本文讨论了白真鲨在淡水和海水之间移动时所面临的生理挑战及所需机制，以及这些数据的生态学意义。
