While there is a considerable body of work describing osmoregulation by elasmobranchs in brackish and saltwater,
far fewer studies have investigated osmoregulation in hypersaline waters. We examined osmo- and ionoregulatory
function and plasticity in juvenile brown-banded bamboo sharks, Chiloscyllium punctatum, exposed to three
experimental salinities (25, 34 and 40‰) for two weeks. C. punctatum inhabits sheltered coastal areas and bays
which can naturally become hypersaline as a consequence of evaporation of water but can also become hyposaline
during flood events. We hypothesised that C. punctatum would demonstrate a phenotypically plastic
osmoregulatory physiology. Plasma osmolality, urea, Na+ and Cl− levels increased significantly with increasing
environmental salinity. Rectal gland and branchial sodium–potassium ATPase (NKA) activities were unaffected by
salinity. Using immunohistochemistry and Western Blotting we found evidence for the presence of the
key ion-regulatory proteins vacuolar H+-ATPase (VHA), pendrin (Cl−/HCO3
− co-transporter) and the
Na+–H+ exchanger isoform 3 (NHE3) in discrete cells within the branchial epithelia. These results indicate that
C. punctatum is a partially euryhaline elasmobranch able tomaintain osmo- and ionoregulatory function between
environmental salinities of 25‰ and 40‰. As suggested for other elasmobranchs, the gills of C. punctatum likely
play a limited role in maintaining Na+ homeostasis over the salinity range studied, but may play an important
role in acid–base balance.
While there is a considerable body of work describing osmoregulation by elasmobranchs in brackish and saltwater,far fewer studies have investigated osmoregulation in hypersaline waters. We examined osmo- and ionoregulatoryfunction and plasticity in juvenile brown-banded bamboo sharks, Chiloscyllium punctatum, exposed to threeexperimental salinities (25, 34 and 40‰) for two weeks. C. punctatum inhabits sheltered coastal areas and bayswhich can naturally become hypersaline as a consequence of evaporation of water but can also become hyposalineduring flood events. We hypothesised that C. punctatum would demonstrate a phenotypically plasticosmoregulatory physiology. Plasma osmolality, urea, Na+ and Cl− levels increased significantly with increasingenvironmental salinity. Rectal gland and branchial sodium–potassium ATPase (NKA) activities were unaffected bysalinity. Using immunohistochemistry and Western Blotting we found evidence for the presence of thekey ion-regulatory proteins vacuolar H+-ATPase (VHA), pendrin (Cl−/HCO3− co-transporter) and theNa+–H+ exchanger isoform 3 (NHE3) in discrete cells within the branchial epithelia. These results indicate thatC. punctatum is a partially euryhaline elasmobranch able tomaintain osmo- and ionoregulatory function betweenenvironmental salinities of 25‰ and 40‰. As suggested for other elasmobranchs, the gills of C. punctatum likelyplay a limited role in maintaining Na+ homeostasis over the salinity range studied, but may play an importantrole in acid–base balance.
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