Product release by mucous granules of land slugs: II. species diversity in triggering of mucous granule rupture

Abstract

Epithelial cells of the body wall of several species of terrestrial snails and slugs secrete mucus in membrane-bound granules. On emergence from mucous cells, the granules are triggered to rupture and release their contents, forming the protective mucous layer. In three species of slugs of the Superfamily Arionoidea (Ariolimax columbianus, Arion ater, Prophysaon foliolatum), ATP at micromolar concentrations triggered granule rupture through processes dependent on activation of calcium channels. Granules of a fourth species in this superfamily (Philomycus carolinianus) were insensitive to ATP. Granules secreted by slugs of the Superfamily Limnoidea (Limax maximus, Deroceras reticulatum) were also entirely unaffected by ATP, and showed no evidence of a role of calcium channels in mucous granule sensitivity. Instead, they ruptured when immersed in media of elevated pH, a process blocked by nigericin. The slugs of the two superfamilies represented taxa quite widely separated in evolutionary origin. Nevertheless, the diversity of granule function was not expected, considering the crucial and presumably ancient role of mucus in protection of cell surfaces, not only in slugs and other molluscs but throughout the range of animal taxa. © 1996 Wiley-Liss, Inc.