Salt and water balance in the sipunculan phascolopsis gouldi: is any animal a “simple osmometer”?

1. 1. Phascolopsis gouldi, the commonly studied sipunculan of the Woods Hole area, Massachusetts, can tolerate a salinity range from about 45% seawater to at least 100% SW, with literature records extending to about 160% SW. There was no survival at 40% SW. 2. 2. Over this salinity range, P. gouldi is an osmotic and ionic conformer. 3. 3. The osmotic and sodium concentrations of the coelomic fluid are the same as that of the medium; the chloride concentration is about 9% less than that of the medium; the potassium concentration is about 30% higher. 4. 4. Analysis of water content regulation by two different approaches shows that P. gouldi does have a limited ability for volume regulation, restricted to salinities higher than 58% SW. 5. 5. P. gouldi is not a “simple osmometer”.