Salt transport by the gill Na+-K+-2Cl- symporter in palaemonid shrimps: exploring physiological, molecular and evolutionary landscapes

Palaemonid shrimps include species from distinct osmotic niches that hyper-regulate hemolymph osmolality and ionic concentrations in dilute media but hypo-regulate in saline media. Their gill epithelia express ion transporters like the Na+-K+-2Cl- symporter (NKCC) thought to play a role in salt secretion. Using a palaemonid series from niches including marine tide pools through estuaries (Palaemon) to coastal and continental fresh waters (Macrobrachium), we established their critical upper salinity limits (UL50) and examined their short- (24 h) and long-term (120 h) hypo-regulatory abilities at salinities corresponding to 80% of the UL509s (80%UL50). We tested for phylogenetic correlations between gill NKCC gene and protein expression and hemolymph Cl- hypo-regulatory capability, and evaluated whether niche salinity might have driven gill NKCC expression. The Palaemon species from saline habitats showed the highest UL509s and greatest hypo-regulatory capabilities compared to the Macrobrachium species among which UL509s were higher in the diadromous than in the hololimnetic species. While basal gill NKCC mRNA transcription rates differed among species, expressions were unaffected by exposure time or salinity, suggesting post-transcriptional regulation of protein synthesis. Unexpectedly, hemolymph Cl- hyper-regulatory capability correlated with gill NKCC gene expression, while gill NKCC protein synthesis was associated with hyper-regulation of hemolymph osmolality at the 80%UL509s of almost all Macrobrachium species, suggesting a role for the gill NKCC symporter in salt uptake. The evolutionary history of osmoregulation in these palaemonid shrimps suggests that, while some molecular and systemic mechanisms have accompanied cladogenetic events during radiation into different osmotic niches, others may be driven by salinity.