Osmolyte processing in the gut and an important role of the rectum in the land slug, Ariolimax columbianus (pulmonata; arionidae)

Abstract

AbstractExperiments were carried out to investigate the role of the gut in the osmotic balance of the land slug Ariolimax columbianus. Previous investigators have shown that absorption of nutrients, calcium, and some heavy metals occurs in the slug's digestive gland and intestine. The only function suggested for the rectum, beyond transfer of the fecal mass, has been its possible involvement in lipid absorption. We collected hemolymph and fluids contained in different regions of the gastrointestinal tract and compared these with respect to osmotic pressure as well as to concentrations of ions (sodium, potassium, calcium, and chloride), glucose, and amino acids. The osmotic pressure of rectal contents was found to average one‐quarter the values of other fluids. In in vitro experiments sacs of intestine and rectum exposed to 1:4 mucosa:serosa osmotic gradients for 1 h showed transfer of fluid across the sac wall into the surrounding fluid. This transfer was significantly enhanced in the case of the rectum but not of the intestine, after dehydration or loss of hemolymph, and during immersion of the sacs in media containing the gastropod neurotransmitters FMRFamide, norepinephrine, and 5‐hydroxytryptamine, as well as arginine vasotocin. The data indicate that the rectum of the slug serves as a small but ready source of water available for use under conditions of acute water deficiency.