Osmotic and ionic regulation in the earthworm Lumbricus terrestris

Earthworms are capable of osmotic and ionic regulation while living in a dilute balanced salt solution. When transferred from soil to pond-water (PW) their body weight increases by 15 percent due to a net uptake of water. This results in an initial dilution of the coelomic fluid (CF). Subsequently, Na and Cl are accumulated in net amounts returning the CF ionic concentration to the same level as when the animals were in soil (75 mM Na/1, 47 mM Cl/1). Water is taken up across the skin. Although worms in PW do drink, the amount is minimal. In a steady state, the rate of water uptake and loss must be equal. Water is voided as a hypo-osmotic urine from the nephridia. The clearance of Inulin and Dextran from the CF is about 75 μl/10 g-hr. Since some water is probably reabsorbed from the nephridia, this gives a maximum estimate of urine excretion. Worms living in PW use their gastro-intestinal tract as an auxiliary excretory organ. Fluid excreted from the rectum is hypo-osmotic and hypo-ionic to the CF. It is hypothesized that fluid entering the esophagous, at the level of the calciferous gland, is an ultrafiltrate of the blood. Chloride is reabsorbed from the esophageal fluid in exchange for HCO₃ ions. The esophageal fluid enters the crop. Fluid in the crop is similar in composition to the CF except that the Cl concentration is lower (2 mM/l). As the fluid passes down the intestine Na is reabsorbed in exchange for NH₄, resulting in the formation of a dilute rectal fluid. Earthworms living in a dilute aquatic environment accumulate Na and Cl against an elecrochemical gradient by active transport across the skin. Sodium and Cl are transported independently. To maintain electrical neutrality, Na is probably exchanged for NH₄ or H ions and Cl exchanged for HCO₃. Kinetic analysis of the Na transport system for PW acclimated worms indicates a V[subscript m] of 1.1 μeq Na/10 g-hr and a K[subscript s] of 1.6 mM Na/1. These values suggest the epithelium is less permeable to Na than in most fresh-water animals. The ion transport systems are delicately regulated. A dilution of the CF increases the rate of active transport of both Na and Cl. Sodium efflux is unchanged but the Cl efflux is reduced. The mechanism of regulation is unknown; however, neuroendocrine mediation has been implicated. It is concluded that earthworms can adapt to an aquatic environment. However, they do possess some characteristics which distinguish them from truly fresh-water animals. The use of the gastro-intestinal tract for producing a dilute fluid excreta is unique. The relative low skin permeability to Na results in a Na transport rate much lower in magnitude than fresh-water organisms of comparable size.