The shoot demography and rhizome growth of Syringodium filiforme Kutz. and Halodule wrightii Aschers. were studied, based on plant dating techniques, to account for their role as pioneer in the succession sequence of Canbbean seagrasses. Results demonstrated that these spe- cies are able to develop dense meadows, supporting bio- masses in excess of 500 g DW m" They produced more than 2000 g DW m-2yr-' due to their high leaf (5.0to 8 5 yr.') and rhizome (20 to 3.3 yr-') turnover. Rhizome growth and branch~ngrates were very high, allowing these seagrasses to rapidly occupy the space they colonise. The rapid rhizome turnover involved, however, a high shoot mortality rate and low 11fe expectancy (average shoot life expectancy 100 to 180d).This implies that, while these pioneer species are able to rapidly occupy the space they colonise, their established shoots cannot occupy that space for as long as the more long- lived species Thalassia testudinum. We suggest, therefore. that the role of seagrass species as pioneer or climax species is independent of their capacity to support dense, productive populations, and is closely related to shoot longevity and rhi- zome turnover.