Data from: What is the primary function of the early teleost gill? Evidence for Na+/NH4+ exchange in developing rainbow trout (Oncorhynchus mykiss)

Post-hatch fishes lack a functional gill and use cutaneous surfaces for
exchange with the surrounding environment. The ionoregulatory hypothesis
posits that ionoregulation is the first physiological process to be
limited by cutaneous exchange, necessitating its shift to the gills. We
hypothesized that the ontogeny of branchial ammonia excretion (Jamm) is
coupled to Na+ uptake (Graphic) in accordance with the current model for
Graphic exchange in freshwater. Using divided chambers, branchial and
cutaneous Jamm, Graphic and oxygen consumption (MO2) by larval rainbow
trout were assessed. Following hatch, the skin accounted for 97% and 86%
of total Jamm and Graphic, respectively. Jamm and Graphic shifted to the
gills simultaneously at 15 days post-hatch (dph) and were highly
correlated (R2 = 0.951) at the gills, but not the skin, over development.
Contrastingly, MO2 shifted significantly later at 27 dph, in agreement
with the ionoregulatory hypothesis. Moreover, the mRNA expression and/or
enzymatic activity of Rhesus proteins, Na+/H+-exchanger, H+-ATPase,
Na+/K+-ATPase and carbonic anhydrase, all key components of the
Graphic-exchange system, increased in the gills over larval development.
We propose that the ontogeny of branchial Graphic occurs as Graphic
exchange and provide evidence for a novel element to the ionoregulatory
hypothesis, the excretion of potentially lethal metabolic ammonia.