Plasma osmolyte concentrations and rectal gland mass of bull sharks Carcharhinus leucas, captured along a salinity gradient

Abstract

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‰ SW (676 mosM), plasma osmolarity increased by 22% or 0.92% per 1‰ rise in salinity. Between 24‰ and 33‰, plasma osmolarity increased by 33% or 4.7% per 1‰ rise in salinity, largely due to a sharp increase in plasma urea between 28‰ and 33‰. C. leucas moving between FW and SW appear to be faced with three major osmoregulatory challenges, these occur between 0–10‰, 11–20‰ and 21–33‰. A comparison between C. leucas captured in FW and estuarine environments (20–28‰) 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.