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

Abstract

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 K_m) 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 (${J}_{{\mathrm{Na}}^{+}}^{\mathrm{in}}$) 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 ${J}_{{\mathrm{Na}}^{+}}^{\mathrm{in}}$, indicating plasticity in those mechanisms that control Na⁺ efflux (${J}_{{\mathrm{Na}}^{+}}^{\mathrm{out}}$). 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.