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The relative timing of skin-to-gill transitions for sodium and oxygen uptake and the plasticity of gill ionoregulatory development in larval rainbow trout, Oncorhynchus mykiss Fu, Clarice Hei Lok

Abstract

This study is the first to provide direct physiological evidence that larval fish gills play a primary role in ionoregulation considerably earlier than they do for gas exchange. Resting unidirectional sodium (Na+) and oxygen (O₂) uptake across the skin and gills were measured simultaneously in rainbow trout Oncorhynchus mykiss between 0 to 18 days post-hatch (dph). The age at which the gills accounted for more than 50% of total uptake occurred at 15 and 16dph for Na+ uptake while that for O2 uptake occurred at 23 and 28 dph in soft and hard water, respectively. The finding that Na+ uptake transitioned to the gills in about half the time required to do so for O₂uptake suggests that the gills are required for ionoregulation prior to gas exchange in rainbow trout. The ratio of Na+ to O₂uptake rates (ion/gas ratio; IGR) increased with age indicating that the ionoregulatory components of the gills developed more rapidly than those for gas exchange. The timing of transition for Na+ uptake, gill Na+, K+-ATPase (NKA) protein concentration and gill NKA enzyme activity levels were not significantly different between larvae reared in soft and hard water. These results suggest that gill ionoregulatory development is non-plastic. Approximately 90% (hard water) and 75% (soft water) of gill Na+ uptake is estimated to have balanced diffusive Na+ loss; the remaining was incorporated into the body. The apparent lesser Na+ efflux in soft water suggests the presence of specific mechanisms that may reduce ion loss, such as tightening of paracellular junctions. This investigation is the first to provide direct physiological evidence in support of the ionoregulatory hypothesis, challenging the long-held assumption that teleost gills develop initially for gas exchange. In rainbow trout, the gills clearly take on a primary role in ionoregulation in advance of gas exchange, which suggests that ionoregulation may be the initial driving force for gill development in this species. Further investigation would be required to determine how wide spread this relationship may be in teleosts and more basal fishes.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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