THE PARTITIONING OF BODY WATER IN OSTEICHTHYES: PHYLOGENETIC AND ECOLOGICAL IMPLICATIONS IN AQUATIC VERTEBRATES

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Abstract
The major body fluid compartments were measured in 7 marine species of teleosts and 7 fresh-water species of fish including chondrosteans, holosteans and teleosts. Data were compared with previous measure-ments of an agnathan species and four species of Chondrichthyes. Plasma volume was measured by T-1824 dilution, extracellular fluid volume by sucrose dilution, and total body water by complete desiccation at 105[degree]C. Whole blood volume was calculated from plasma volume and hematocrit; interstitial (tissue) fluid was estimated as the difference between plasma and extracellular fluid; and intracellular water was approximated by subtracting extracellular fluid from total body water. The extracellular fluid compartment and both of its sub-compartments (plasma and inter-stitial fluid), as well as whole blood volume, all showed a progressive reduction proceeding from primitive to more advanced groups. This was evident in the series, Agnatha, Chondrichthyes, Osteichthyes, as well as within the latter, in the progression, Chondrostei, Holostei, Teleostei. Conversely, the intracellular fluid volume exhibited a relative increase in the same phylogenetic progressions. The over-all pattern suggests that, in general, the more advanced forms function with a smaller proportion of mediating fluid and relatively more protoplasmic water than the primitive forms. Comparison of the water partitioning of marine species and fresh-water species discloses a remarkable similarity. Where small differences do occur, they are in the direction of primitiveness in the marine forms.

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