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

中文翻译：

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不同Na+含量水域养殖虹鳟鱼幼体Na+吸收的个体发育

硬骨鱼具有在低离子淡水中针对扩散梯度维持离子稳态的非凡能力。在成年硬骨鱼中，鳃是吸收离子的主要部位；然而，在幼虫中，鳃不发达，由于离子调节主要是皮肤，鳃可塑性机制尚不可用。在虹鳟鱼幼体中，在孵化后约 15 天 (dph)，鳃成为 Na + 吸收的主要部位。为了解决 Na + 吸收如何随着水 [Na + ] 的差异而发展，本研究表征了虹鳟鱼幼虫中 Na + 吸收的个体发育，此时离子调节从主要的皮肤过程转变为主要的鳃过程. 结果表明，最初（0-15 dph），当离子调节是皮肤时，与高 [Na + ] 处理相比，低 [Na + ] 饲养的幼虫具有更高的 Na + 亲和力（较低的 K m ）。此外，在低 [Na + ] 水中饲养的幼虫具有较低的内部 Na + 含量，尽管 Na + 吸收率相似（ $${J}_{{\mathrm{Na}}^{+}}^{ \mathrm{in}}$$ J Na + in ) 跨治疗。但是，一旦鳃成为离子调节的主要部位（> 15 dph），所有处理中的幼虫都保持相同的 Na + 含量，尽管 $${J}_{{\mathrm{Na}}^{ +}}^{\mathrm{in}}$$ J Na + in ，表明控制 Na + 流出的那些机制的可塑性 ( $${J}_{{\mathrm{Na}}^{+}}^{ \mathrm{out}}$$ J Na + out )。虹鳟鱼幼体吸收钠离子的机制在各个发育阶段均表现出可塑性。然而，在依赖皮肤 Na + 吸收的幼虫中，