Effects of Ablation of the Cerebral Organs and Cerebral Ganglia on Volume Regulation in an Intertidal Nemertine Paranemertes peregrina

Abstract

In the Nemertina, cytological and physiological evidence points to an involvement of the cerebral organs and neurosecretory system (cerebral ganglia) in isoosmotic volume regulation. The solutes involved and the compartmentalization of the response between extra- and intracellular spaces have not been previously investigated. Intact and decerebrated (i.e., lacking cerebral organs and cerebral ganglia) Paranemertes peregrina were subjected to hypoosmotic (100% seawater [SW] → 70% SW for 72 h) as well as fluctuating salinity conditions (100% SW → 70% SW for 3 h → 100% SW for 12 h → 70% SW for 3 h). Replacement therapy experiments were not performed. In hypoosmotic media, intact individuals of P. peregrina regulate volume via putative neuroendocrine (1) stimulation of nephridial or gut elimination of extracellular water, Na, and Cl; (2) reduction in the permeability of the body wall or gut to inorganic ions; or (3) both mechanisms. The experiments performed do not allow for conclusive separation of these effects. There was also evidence for intracellular regulatory volume decrease (RVD) via organic solutes, but no direct effect of decerebration was observed. Under fluctuating salinity conditions, intact individuals of P. peregrina do not undergo regulatory volume increase (RVI) upon return to full-strength SW. However, RVD accomplished by Na and Cl excretion is potentiated by prior exposure to a diluted medium. Decerebration results in inability of the worms to restore their original water content upon return to full-strength SW as well as in the absence of RVD upon reexposure to hypoosmotic conditions. The excess water and inorganic solutes gained by decerebrated worms during initial exposure to diluted SW contribute substantially to the effects observed in these organisms under fluctuating salinity conditions. The relative contributions of the cerebral organs and the cerebral ganglia to volume regulation are not elucidated by the present study.