In vivo ion fluxes across the eggs of Armadillidium vulgare (Oniscidea: Isopoda): the role of the dorsal organ.

The thin-walled, lecithotrophic eggs of land isopods (suborder Oniscidea) are brooded in a fluid-filled maternal marsupium until a few days following the second embryonic molt. Eggs of Armadillidium vulgare possess a well-developed dorsal organ underlying a broad silver-staining saddle on the vitelline membrane. Based on its chloride permeability and known transport functions in planktotrophic crustaceans, we hypothesized that the dorsal organ functions in passive or active ion movements. To study this, we employed the automated scanning electrode technique with self-referencing ion-selective microelectrodes to measure ion fluxes across the dorsal organ and adjacent egg poles. Stage 1 (chorionated) eggs revealed only small ion fluxes, indicating low permeability. Early stage 2 eggs--between the first embryonic molt and blastokinesis--showed evidence for active uptake of Ca(2+) and Cl(-) and possibly Na(+) against low bathing concentrations, and uptake fluxes were predominantly localized over the dorsal organ. Late stage 2 eggs revealed no capacity for ion uptake, consistent with the atrophy of the dorsal organ at blastokinesis, but high ion permeability. In all stages, the silver-staining saddle showed a sustained outward proton flux indicating that it is the primary site for metabolic acid/CO(2) excretion. The emerging picture is that the embryo dorsal organ in A. vulgare serves important functions in ion regulation, calcium provisioning, and acid excretion.