Freshwater to seawater acclimation of juvenile bull sharks (Carcharhinus leucas): plasma osmolytes and Na+/K+-ATPase activity in gill, rectal gland, kidney and intestine

Abstract

This study examined the osmoregulatory status of the euryhaline elasmobranch Carcharhinus leucas acclimated to freshwater (FW) and seawater (SW). Juvenile C. leucas captured in FW (3 mOsm l(-1) kg(-1)) were acclimated to SW (980-1,000 mOsm l(-1) kg(-1)) over 16 days. A FW group was maintained in captivity over a similar time period. In FW, bull sharks were hyper-osmotic regulators, having a plasma osmolarity of 595 mOsm l(-1) kg(-1). In SW, bull sharks had significantly higher plasma osmolarities (940 mOsm l(-1) kg(-1)) than FW-acclimated animals and were slightly hypo-osmotic to the environment. Plasma Na(+), Cl(-), K(+), Mg(2+), Ca(2+), urea and trimethylamine oxide (TMAO) concentrations were all significantly higher in bull sharks acclimated to SW, with urea and TMAO showing the greatest increase. Gill, rectal gland, kidney and intestinal tissue were taken from animals acclimated to FW and SW and analysed for maximal Na(+)/K(+)-ATPase activity. Na(+)/K(+)-ATPase activity in the gills and intestine was less than 1 mmol Pi mg(-1) protein h(-1) and there was no difference in activity between FW- and SW-acclimated animals. In contrast Na(+)/K(+)-ATPase activity in the rectal gland and kidney were significantly higher than gill and intestine and showed significant differences between the FW- and SW-acclimated groups. In FW and SW, rectal gland Na(+)/K(+)-ATPase activity was 5.6+/-0.8 and 9.2+/-0.6 mmol Pi mg(-1) protein h(-1), respectively. Na(+)/K(+)-ATPase activity in the kidney of FW and SW acclimated animals was 8.4+/-1.1 and 3.3+/-1.1 Pi mg(-1) protein h(-1), respectively. Thus juvenile bull sharks have the osmoregulatory plasticity to acclimate to SW; their preference for the upper reaches of rivers where salinity is low is therefore likely to be for predator avoidance and/or increased food abundance rather than because of a physiological constraint.

Observations on the distribution, biology, short-term movements and habitat requirements of river sharks Glyphis spp. in northern Australia

“…which rivers this species occurs in) given their wide salinity tolerance (0.8 to 28 and 2 to 36 for G. glyphis and G. garricki, respectively). Tracked G. glyphis showed rapid movements across salinity gradients, providing additional evidence that this species is capable of osmoregulating in a range of salinities and probably osmoregulates in a similar manner to bull sharks (Pillans & Franklin 2004, Pillans et al 2005). Turbidity appears to play an important role in the distribution of Glyphis spp., in particular for G. glyphis.…”

DNA capture reveals transoceanic gene flow in endangered river sharks

“…Although it was assumed that Glyphis represent true freshwater species, our results contribute to a growing body of evidence that suggests that the ecology of the genus Glyphis might be similar to that of the bull shark. Adult bull sharks live in marine environments but use freshwater habitats, where juveniles are frequently encountered, for reproduction (31). It has been speculated that high predation pressure on juveniles in marine environments is promoting the use of freshwater nursery areas in bull sharks (31).…”

“…Adult bull sharks live in marine environments but use freshwater habitats, where juveniles are frequently encountered, for reproduction (31). It has been speculated that high predation pressure on juveniles in marine environments is promoting the use of freshwater nursery areas in bull sharks (31). It is possible that the same selective pressure is behind the adaptation to euryhaline conditions in the ancestor of the genus Glyphis.…”

Hormonal control of salt and water balance in vertebrates

“…These salinity-induced changes apparently require an intact hypophysio-adrenocortical axis, though the role of corticosteroids appears to be permissive. The size and Na + , K + -ATPase activity of the NaCl secreting rectal gland of euryhaline elasmobranchs also varies in response to environmental salinity (Piermarini and Evans, 2000;Pillans et al, 2005). It would be of interest to determine if GH and/ or IGF-I have a role in rectal and salt gland development and diVerentiation that accompanies salinity acclimation of elasmobranches and birds.…”