The regulation of body fluid volume in the estuarine annelidNereis succinea

Summary The process of volume readjustment following swelling after transfer to dilute sea water was examined in the estuarine polychaeteNereis succinea by measuring body weight changes. Volume readjustment is an active process that requires O₂ and uninhibited cellular metabolism, and the process is retarded at low temperature. O₂ uptake increases during the swelling phase and remains high throughout the readjustment phase. The intracellular osmolytes lost during volume readjustment include free amino acids, some of which appear to be deaminated and some extruded intact into the medium. Although it is known that intracellular inorganic ions, including Na⁺ and Cl^–, are lost into the extracellular fluid against a concentration gradient, volume regulation in an intact animal is insensitive to a number of drugs that influence ion movements at the cellular level.Two possible mechanisms of extracellular fluid readjustment implicated by previous findings have been eliminated. The increased urine flow known to accompany volume readjustment is not brought about by increased hydrostatic pressure in the coelom. In fact, coelomic fluid pressure decreases, which is consistent with the hypothesis that extracellular volume readjustment is accomplished by enhanced ciliary activity in the nephridia. Removal of the supraesophageal ganglion, an important neuroendocrine organ in annelids, has no effect on organismal volume readjustment.Abbreviations FAA free amino acids - NPS ninhydrin positive substances