The Hard And Soft Of It: The Role Of Substrate In Patterns Of Phase Dominance And Phenology In Gracilaria Vermiculophylla

Despite the importance of the seasonal timing of life cycle events for understanding population dynamics, we lack information on the phenology of most macroalgal species. The red macroalga Gracilaria vermiculophylla is now common in both hard and soft bottom habitats following its invasion throughout the Northern Hemisphere. In sites with sufficient hard substratum, thalli are fixed by holdfasts to hard substrate. By contrast, in sites with soft substratum, thalli are free-living and either drift free-floating or anchored by the tube building polychaete Diopatra cuprea. The difference in substrate type has profound implications on the evolutionary ecology of G. vermiculophylla populations whereby the life cycle is disrupted in soft bottom habitats leading to tetrasporophytic dominance. To investigate these patterns across seasons, we determined the reproductive state via microscopy and phase via a sex-linked PCR assay for >4,800 thalli over the course of three years at hard and soft bottom sites with fixed and free-living G. vermiculophylla thalli, respectively, along the Eastern Shore of Virginia and Maryland. At hard bottom sites, most thalli were reproductive in the summer and sites were composed of gametophytes and tetrasporophytes, though there was a tetrasporophytic bias. By contrast, soft bottom sites were overwhelming tetrasporophyte dominated and fewer thalli were reproductive at any point in the year. Moreover, each soft bottom site displayed different patterns in the proportion of reproductive thalli, with G. vermiculophylla abundance fluctuating through time. Phenology surveys critical for understanding the spread of introduced macroalgae, including the on-going spread of G. vermiculophylla.