Reproductive Synchrony as Hybridization Barrier Figure 3. Total number of oogonia (closed symbols) and number of mature oogonia (open symbols) per conceptacle (n = 5 receptacles, ± SE) from Fucus spiralis (plot a), Fucus guiryi (plot b), and Fucus vesiculosus (plots c and d) between February 2002 and July 2003. doi:10.1371/journal.pone.0035978.g003 We compared the daily tidal phase on days of major gamete release in this study with those of Berndt et al. [9] for F. vesiculosus on western Atlantic shores in Maine, USA, because these authors observed release during spring tides, while we observed release exclusively during neap tides. However, the tidal phase (i.e., the timing of the low and high tides during the day) when gamete release occurred was very similar in both studies (Fig. 5a). Gamete release in F. vesiculosus, both in our study and in Berndt et al. [9], happens mainly on days when the first diurnal high tide occurs in the late morning. In Figure 5b we added tidal data from Brawley [10] for days with maximum gamete release in the estuarine dioecious fucoid, F. ceranoides. In this case peak gamete release appears shifted forwards by about 2 h relative to F. vesiculosus. PLoS ONE | www.plosone.org 5 April 2012 | Volume 7 | Issue 4 | e35978