Reproductive Synchrony as Hybridization Barrier Figure 7. Egg release during 2 h sampling intervals (n = 5 ± SE) by a) Fucus spiralis, b), Fucus guiryi, and c) Fucus vesiculosus at two replicate sites/species (open and closed symbols) during four neap tide periods in June and July 2009. Grey lines are tidal amplitude and grey bars the dark periods during the daily cycle. doi:10.1371/journal.pone.0035978.g007 Discussion This study provides one of the most comprehensive descriptions to date of reproductive timing (egg release) at annual to hourly scales in the marine environment. We had two main goals; firstly to investigate the potential for interspecific variation in spawning time to act as a prezygotic ecological barrier to hybridization between sympatric, externally-fertilizing congeners. Our data show that, while coincident at seasonal and semilunar scales, differences in the timing of gamete release during single tides are consistent with a partial ecological barrier to hybridization. Secondly, we addressed the hypothesis that selfing species are under reduced selective constraint for spawning synchrony relative to obligately outcrossing species. We discovered that while reproductive synchrony remains a feature in selfing hermaphro- ditic species, it is reduced relative to sister species with obligate outcrossing. Reduced synchrony was identified on daily time scales as increased variance of major spawning periods during the semilunar reproductive cycle, and from a relative excess of small spawning events during single favourable tides. Finally, we provide evidence from these data and previously published studies on both sides of the Atlantic [9,10] that gamete release in natural fucoid populations is controlled by environmental cues arising from the interaction of tidal and diurnal cycles, rather than semilunar cycles. Spawning patterns at seasonal to daily temporal scales were highly coincident for F. vesiculosus, F. spiralis and F. guiryi. Reproductive output, estimated as daily egg settlement, a close PLoS ONE | www.plosone.org 8 April 2012 | Volume 7 | Issue 4 | e35978