Physiological and molecular ontogeny of branchial and extra-branchial urea excretion in post- hatch rainbow trout (Oncorhynchus mykiss) Authors:

1 All teleost fish produce ammonia as a metabolic waste product. In embryos, ammonia 2 excretion is limited by the chorion and fish must detoxify ammonia by synthesizing urea via the 3 ornithine urea cycle (OUC). Although urea is produced by embryos and larvae, urea excretion 4 (Jurea) is typically low until yolk sac absorption, increasing thereafter. The aim of this study was 5 to determine the physiological and molecular characteristics of Jurea by post-hatch rainbow trout 6 (Oncorhynchus mykiss). Following hatch, whole-body urea concentration decreased over time 7 while Jurea increased following yolk sac absorption. From 12-40 days post-hatch (dph), extra8 branchial routes of excretion accounted for the majority of Jurea, while the gills became the 9 dominant site for Jurea only after 55 dph. This represents the most delayed branchial ontogeny of 10 any process studied to date. Urea transporter (UT) gene expression in the gills and skin increased 11 over development, consistent with increases in branchial and extra-branchial Jurea. Following 12 exposure to 25 mmol/l urea, the accumulation and subsequent elimination of exogenous urea was 13 much greater at 55 dph than 12 dph, consistent with increased UT expression. Notably, UT gene 14 expression in the gills of 55 dph larvae increased in response to high urea. In summary, there is a 15 clear increase in urea transport capacity over post-hatch development, despite a decrease in OUC 16 activity. 17