The ontogeny of Na⁺ balance during rapid smoltification in pink salmon (Oncorhynchus gorbuscha)

Abstract

Pink salmon hatch in fresh water, but their highly anadromous life history requires them to migrate into the ocean immediately after gravel-emergence, at a very small size. During their down-river migration these larvae undergo rapid smoltification that completely remodels their osmoregulatory physiology. At this time, the larvae reportedly have high whole-body Na⁺ contents and we hypothesised that the active accumulation of internal Na⁺ occurs in preparation for ocean entry. Using a comparative approach, the present study characterised the ontogeny of Na⁺ regulation in larvae of the anadromous pink salmon and the fresh-water rainbow trout. Our results indicate that larvae from both species actively accumulated Na⁺; however, whole-body Na⁺ content was higher in rainbow trout larvae compared to pink salmon. The time-course of this response was similar in the two species, with highest Na⁺-uptake rates ($J^{{{\text{in}}}}_{{{\text{Na}}^{ + } }}$) shortly after yolk sac absorption, but the mechanism of Na⁺ accumulation differed between the species. Rainbow trout larvae greatly increased $J^{{{\text{in}}}}_{{{\text{Na}}^{ + } }}$ to overcompensate for a large simultaneous increase in Na⁺-efflux rate ($J^{{{\text{out}}}}_{{{\text{Na}}^{ + } }}$), whereas pink salmon mounted a smaller increase in $J^{{{\text{in}}}}_{{{\text{Na}}^{ + } }}$ while maintaining tight control over $J^{{{\text{out}}}}_{{{\text{Na}}^{ + } }}$, which is supported by a significantly lower paracellular permeability. Our results indicate that the transient accumulation of internal Na⁺ is not a unique feature of the highly anadromous life history in pink salmon and may be a common ontogenetic pattern during larval development in salmonids; and perhaps it is associated with the development of the cardiovascular system during the larvae’s transition to a more active lifestyle.