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ORIGINAL PAPER

The ontogeny of ­Na+ uptake in larval rainbow trout reared in waters of different ­Na+ content Emily J. Gallagher1 · Till S. Harter1,2   · Jonathan M. Wilson3,4 · Colin J. Brauner1 Received: 18 November 2019 / Revised: 26 July 2020 / Accepted: 9 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020

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 ionregulation is primarily cutaneous, branchial mechanisms of plasticity are not yet available. In larval rainbow trout, the gills become the primary site for N ­ a+ uptake at ~ 15 days post hatch (dph). To address how N ­ a+ uptake develops in response to + + differences in water ­[Na ], the present study characterised the ontogeny of N ­ a 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 in ­Na+-uptake rates ( JNa + ) across treatments. But, once the gills became the dominant site for ion-regulation (> 15 dph), larvae in in all treatments maintained the same ­Na+ content, despite large differences in JNa + , indicating plasticity in those mechanisms + + out that control ­Na efflux ( JNa   ) . The mechanisms of N ­ a 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. Keywords  Ion regulation · Cutaneous · Branchial · Ion hypothesis · Teleost · Plasticity

Introduction

Communicated by Bernd Pelster. Electronic supplementary material  The online version of this article (https​://doi.org/10.1007/s0036​0-020-01311​-3) contains supplementary material, which is available to authorized users. * Till S. Harter [email protected] 1



Department of Zoology, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada

2



Marine Biology Research Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA

3

Department of Biology, Wilfrid Laurier University, Waterloo, ON N2L 3C5, Canada

4

Centro Interdisciplinar de Investigação Marinha e Ambiental (CIIMAR), 4450‑208 Matosinhos, Portugal



Teleost fish are hyperosmotic compared to freshwater environments and thus, face continuous diffusive ion efflux and water influx across their semi-permeable external surfaces, primarily the skin and the gills. To maintain h