Regulation of body weight and serum composition by hagfish in various media

1. 1. When put in dilute sea water hagfish gained weight rapidly then slowly returned to their original weight. 2. 2. In concentrated sea water the animals shrank and did not return to their initial weight. 3. 3. Substitution of isosmotic sucrose for one-fifth of sea water volume caused no change in body weight and the blood ion concentrations unless sucrose penetrated the body. When sucrose permeation occurred serum ions tended towards equilibrium with external ion concentrations. 4. 4. Similar experiments with urea caused a weight gain, which was maintained without regulation. The hagfish were permeable to urea and appeared to suffer a “uremic” poisoning. 5. 5. During dilution stress, blood sodium and chloride declined in proportion to thier decrease in the medium, but magnesium and calcium remained at initial concentrations. 6. 6. The results indicate that hagfish are very permeable to water but not to ions, particularly calcium and magnesium. 7. 7. Hagfish drink sea water, but analysis of the gut fluid failed to reveal any substantial differences in composition between it and sea water. 8. 8. It is suggested that the permeability of hagfish to water and lack of regulatory mechanisms for sodium chloride are the primary causes of their lack of osmotic homeostasis an therefore, their limited weight regulation.