Mechanisms and energetic implications of osmoregulation in embryos and larvae of chum and coho salmon

Mechanisms and the energetic implications of osmoregulation were investigated in the early life stages of chum (Oncorhynchus keta) and coho salmon (O. kisutch) at three different developmental stages: eyed embryo, prehatch embryo, and yolk sac larva. Embryos and larvae were acclimated (>7 d) to selected ranges of saltwater (SW) concentrations (0 to 30‰S) in order to examine whole-animal (Chapter 1), cellular (Chapter 2), and biochemical (Chapter 3) aspects of early life osmoregulation.;Chum embryos and larvae survived exposure to SW at higher salinities than coho embryos and larvae. Saltwater-challenged chum fry were able to successfully hypo-osmoregulate in 35‰S for 24 h (plasma osmolality ∼350 mOsm) whereas coho fry were only able to tolerate 24‰S and were less capable at effective hypo-osmoregulation (plasma osmolality ∼440 mOsm). Chum salmon were markedly more tolerant to SW than coho salmon and SW acclimation had markedly different effects at each of the three developmental stages tested. Saltwater acclimation had the most marked effect on chum salmon larvae with increased oxygen consumption rates of up to 53% in 30‰S. This increased energy expenditure did not appear to be the result of increased swimming activity or changes in energy allocation to growth processes.;Cutaneous and branchial epithelia showed an extensive distribution of cutaneous chloride cells (CCs) with relatively high densities on all major embryonic and larval cutaneous surfaces. CCs in prehatch chum and coho embryos showed cellular fine structure typical of CCs in adult fish acclimated to similar conditions. However, SW-acclimated coho embryos appeared to have fewer CCs with typical SW-acclimated fine structure than chum embryos. These data support the theory that cutaneous epithelia are the site of early life osmoregulatory processes until the gills are fully developed.;This study is the first to examine Na+,K+-ATPase and H+-ATPase activity in embryos and larvae and shows that these enzymes function at activity levels that are similar to adult fish. Ontogenic changes in NA+,K+-ATPase and H +-ATPase activity in chum salmon incubated in FW increased by 16 times progressively throughout development, from the eyed embryo stage to the fry stage. Acclimation to SW appeared to increase Na+,K +-ATPase activity in the branchial epithelia of chum larvae but not coho. No obvious trends related to SW acclimation were observed in the cutaneous tissues at any of the three developmental stages. H+-ATPase activity was highest in the branchial epithelium of yolk sac larvae in FW specifically coho salmon. Collectively the evidence supports the theory that cutaneous osmoregulation is significant during the early life stages of salmonid development but that the energy consumed by ion transport processes is not high in relation to whole-animal metabolism. (Abstract shortened by UMI.).