The ontogeny of Na+ uptake in larval rainbow trout reared in waters of different Na+ content.

Teleost fish have a remarkable capacity to maintain ion homeostasis against diffusion gradients in hypo-ionic freshwater. In adult teleosts the gills are the primary site for ion uptake; however, in larvae, the gills are underdeveloped, and as ion-regulation is primarily cutaneous, branchial mechanisms of plasticity are not yet available. In larval rainbow trout, the gills become the primary site for Na+ uptake at ~ 15 days post hatch (dph). To address how Na+ uptake develops in response to differences in water [Na+], the present study characterised the ontogeny of Na+ uptake in rainbow trout larvae, at a time when ion regulation transitions from being a primarily cutaneous to a primarily branchial process. Results indicate that initially (0-15 dph), when ion-regulation is cutaneous, low-[Na+] reared larvae had a higher Na+ affinity (lower Km) compared to the high-[Na+] treatment. In addition, larvae reared in low-[Na+] water had a lower internal Na+ content, despite similar Na+-uptake rates ([Formula: see text]) across treatments. But, once the gills became the dominant site for ion-regulation (> 15 dph), larvae in all treatments maintained the same Na+ content, despite large differences in [Formula: see text], indicating plasticity in those mechanisms that control Na+ efflux ([Formula: see text]). The mechanisms of Na+ uptake in larval rainbow trout showed plasticity during all stages of development. However, in young larvae that relied on cutaneous Na+ uptake, the internal Na+ content was significantly affected by the [Na+] in the water, perhaps revealing challenges to ion homeostasis and a period of heightened vulnerability to external stressors during early larval development.