Volume regulation in an intertidal nemertine, Procephalothrix spiralis coe. I. Short‐term effects and independence of decerebration

Abstract

AbstractIn the Nemertina both neurosecretory cells in the cerebral ganglia and neuroglandular structures (cephalic glands and cerebral organs) have been found to be responsive, on a cytological level, to changes in the osmolality of the worm's environment. However, the volume or osmoregulatory capacities of these worms are scarcely known. The present study examines water, ion, and volume regulation in an intertidal nemertine and the effects of removal of the cerebral ganglia and the cephalic glands on these parameters.Control, sham‐operated, and decerebrated (ablated) Procephalothrix spiralis were acutely transferred to diluted seawaters. Osmolality and Na, K, and Cl (mEq/liter) concentrations were determined in total tissue water after various exposure periods. Extracellular volume was determined, in vitro, as the fraction of the total tissue water in which 14C‐polyethylene glycol was distributed.Control, sham‐operated, and ablated P. spiralis all responded to diluted seawaters in the same manner with no significant difference among groups. In diluted media, the worms demonstrated hyperosmotic conformity. Tissue osmolality decreased rapidly in all groups at similar rates indicating no effect of ablation on integumental water permeability. Significant volume regulatory responses were observed in all worms starting 15 minutes after exposure. Based on the very small extracellular volume (∼5% of total body water) of P. spiralis, measured volume regulatory events in this species primarily reflect intracellular phenomena. Initially, osmolality decreased while water content (gm H20/gm solute free dry weight) increased. During this period, volume regulation was accomplished by the elimination of Na, Cl, and unmeasured solutes (μmoles or μosmoles/gm s.f.d.w.) which limited water gain. Subsequently, osmolality was constant and all worms demonstrated a slow regulatory volume decrease. The solutes that accompanied volume loss were also Na, Cl, and substantial amounts of unmeasured solutes.