Water and ion transport across the eversible vesicles in the collophore of a springtail Orchesella cincta

Abstract

Springtails (Collembola) are ancient close relatives of the insects. The eversible vesicles are their unique paired transporting organs, which consist of an epithelium located inside a tube-like structure called the collophore on the first abdominal segment. The vesicles can be protruded out of the collophore and several lines of evidence indicate that they have a vital function in water uptake and ion balance. However, the amount of water absorbed by the vesicles and which other ions apart from sodium are transported remain unknown. Using Orchesella cincta as a model, we developed protocols for two assays that enabled us to study water and ion movement across the eversible vesicles in whole living springtails. Using an inverse Ramsay assay we demonstrate that the eversible vesicles absorb water from a droplet applied onto their surface. Using the scanning ion-selective electrode technique (SIET) we show that the vesicles absorb Na+ and Cl− from the bathing medium, secrete NH4+, and both absorb and secrete K+. H+ is secreted at a low level in the anterior part and absorbed at the posterior part. We did not detect transport of Ca2+ at significant levels. The highest flux was the absorption of Cl−, and the magnitude of ion fluxes were significantly lower in fully hydrated springtails. Our data demonstrate that the eversible vesicles are a transporting epithelium functioning in osmo- and ionoregulation, nitrogenous waste excretion and likely acid-base balance.