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

Abstract

1. The major body fluid compartments were measured in two species of fresh-water Chondrostei, two species of fresh-water Holostei, three species of fresh-water Teleostei and seven species of marine teleosts. These were compared with previous measurements of an agnathan species and four species of Chondrichthyes.2. A general correlation was shown between the relative rates of respiratory movements and pulse rates, but neither of these appeared to be related to the taxonomic series. A faster pulse was more characteristic of marine than of fresh-water species.3. Plasma volume was measured by the dye dilution method, using T-1824. Whole blood volume was calculated from plasma volume and hematocrit. A progressive reduction in plasma and whole blood volume was noted, proceeding from the primitive to the more advanced groups. This is true both among the three classes of aquatic vertebrates and also among the three groups within the Osteichthyes. These volumes were remarkably similar in fresh-water and marine teleost species, although slightly higher in the latter.4. Extracellular fluid volume was approximated by sucrose dilution. A comparison of inulin, raffinose and sucrose spaces in one species showed most thorough penetration by sucrose and least by inulin, the volumes being in inverse order to the molecular weights of the substances. Sucrose did not penetrate the minor fluid compartments (coelomic, cerebrospinal and ocular fluids) so these are not included in the extracellular data. The minor fluids probably do not exceed one fourth of one per cent of the body weight in any species studied. The sucrose space (extracellular fluid volume), like plasma volume, was found to be greatest in primitive forms and least in the most advanced. The fresh-water and marine forms have similar extracellular volumes, although slightly greater in the latter.5. Interstitial (tissue) fluid was estimated by subtracting plasma from extracellular fluid. Exactly the same relationship with the taxonomic series and habitat obtained here as for plasma and extracellular fluid.6. Total body water was measured by complete desiccation at 105° C. It was found to be fairly uniform among osteichthyan species, with two major exceptions, probably related to high oil content of the body and a heavy investiture of scales. The correlation of water content with the taxonomic series, is not as clear in this case, although there is a greater water content in the Chondrostei than in the other two groups of Osteichthyes, and in the Agnatha and Chondrichthyes than in Osteichthyes. Marine species of teleosts have a slightly smaller total water content than fresh-water species.7, Intracellular water was calculated by subtracting extracellular fluid from total body water. The relationship of this compartment to the taxonomic series is almost exactly reversed from that of plasma and extracellular fluid: a relatively small intracellular volume is characteristic of primitive groups and a larger volume of more advanced groups. Marine teleosts have a somewhat smaller intracellular volume than fresh-water teleosts, likewise a reversal of the relation in plasma and extracellular fluid.8. The phylogenetic and ecological patterns become somewhat more distinct when the volumes are expressed as per cents of total water rather than of body weight.9. Some of the differences shown are small and statistically questionable, and comparisons involving Agnatha are based on only one species. However, the over-all pattern suggests that, in general, the more advanced forms, as opposed to the primitive, and the fresh-water teleosts, as opposed to the marine teleosts, function with a smaller proportion of mediating fluid and relatively more protoplasmic water. The circulating fluid thus appears to be utilized more effectively in the advanced aquatic vertebrates and, to a lesser degree, in fresh-water than in marine forms.