The tropical Matutinao River was investigated to study factors determining the distribution of the extant gastropods. Since ecological zonation and life-cycles are somewhat correlated, we followed both research avenues and in particular focused on early ontogenetic strategies, discriminated by detailed studies of corresponding shell features. Twenty-six gastropod species, almost all of which are distributed over whole Cebu Island, occurred in the small Matutinao River. The specific associations of river sections, and ecological demands and life-cycles of gastropods are characterized. Of special interest are planktotrophic neritoidean larvae, which are usually carried to the sea and develop in the marine environment, but in rare cases remain and successfullymetamorphose in freshwater. The biogeographic background and evolutionary consequences of the results are discussed.
