Physiological adjustments to dilution of the external medium in the lip‐shark Hemiscyllium plagiosum (bennett). I. Size of body compartments and osmolyte composition

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Abstract
The lip‐shark Hemiscyllium plagiosum can survive gradual dilution of the external medium to a salinity of about 12% sea water.Throughout the range in salinity from undiluted sea water to 25% sea water a plasma osmolarity greater than that of the medium is maintained. In undiluted sea water the concentrations of the plasma electrolytes Na+, K+, Ca++, Mg++ and Cl− are much lower than those of the medium. The high plasma osmolarity is due largely to urea.In dilute media, lip‐sharks gain considerable amounts of water. There is a marked expansion of the blood volume and intramuscular fluid space. Although the massive influx of water causes a significant decline in electrolyte concentrations in plasma and intramuscular space, the total electrolyte content is maintained. Urea content, on the other hand, declines significantly following adaptation to dilute media.Near the lowest salinity in which survival can occur there is a rapid decline in concentration of Ca++ in the plasma and intramuscular fluid. The concentration gradient of K+ and Cl− between the extracellular and intracellular compartments of the parietal muscle also declines so that the calculated diffusion potential across the myolemma for both ions decreases significantly. These changes may be related to the inactivation of muscle and respiratory failure prior to death.