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 events in the life cycle for understanding population dynamics, we lack information on the phenology of most macroalgal species. The red macroalga Gracilaria vermiculophylla has become 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 the hard substrate. By contrast, in sites with soft substratum, thalli are free-living and either drift freely or are anchored by the tube-building polychaete Diopatra cuprea. The difference in substrate type has had profound implications on the evolutionary ecology of G. vermiculophylla populations, wherein the life cycle has been 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 polymerase chain reaction (PCR) assay for >4800 thalli over the course of 3 years at hard and soft bottom sites with fixed and free-living G. vermiculophylla thalli, respectively, along the Eastern Shore of Virginia and in Maryland. At hard-bottom sites, most thalli were reproductive in the summer, and sites were composed of gametophytes and tetrasporophytes, although there was a tetrasporophytic bias. By contrast, soft-bottom sites were overwhelmingly 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 also fluctuating through time. Phenology surveys are critical for understanding the spread of introduced macroalgae, including the ongoing spread of G. vermiculophylla.