Figure 5. Relationship between tidal amplitude and time after sunrise on days with peak gamete release from a) Fucus vesiculosus, and b) Fucus ceranoides. Data are from Pemaquid, Maine, USA (Oct 8 and Nov 8, 1999; from Berndt et al. [9]), Viana do Castelo, Portugal (Jun 23, Jul 22, 2003; this study), and Port Erin, Isle of Man (Jul 17, Aug 1–3, 1989, and Jul 22, Aug 4, 1990; from Brawley [10]). doi:10.1371/journal.pone.0035978.g005 Intraspecific timing and synchrony of gamete release: hourly scale during the tidal cycle During the 4 neap tide periods studied, major egg release events (defined as .1000 eggs per bag in a 2 h sampling period) were observed on 8 days in F. vesiculosus (June 2–3, June 15–17, and July 17–19); 4 days in F. guiryi (June 3, 14, 17 and 18); and 6 days in F. spiralis (June 3, 5, 14, 17, 18 and July 3). While the three species showed co-occurring gamete release on several days, the largest events were shared mainly by the two hermaphrodites, F. spiralis and F. guiryi. (Figs. 7a and b), to the exclusion of F. vesiculosus (Fig. 7c). Moreover, the timing of gamete release within the tidal cycle was divergent between F. vesiculosus and the two hermaph- rodite species. While F. vesiculosus released gametes almost exclusively immediately prior to, and especially following, the high tide (mid-morning to early afternoon), F. spiralis and F. guiryi released gametes consistently at the earliest sampling time of 06:00. Although we were unable to measure gamete release during peak high tide in F. vesiculosus due to the difficulty of sampling immersed individuals in situ, our data are consistent with this being the case, as reported by Berndt et al. [9] who observed the onset of release occurring prior to immersion. The precise timing of release in F. spiralis and F. guiryi remains uncertain, with both the dark to dawn transition, or release in the dark after the last sampling bags were attached (i.e., after 21:30 on the previous day) being possible. Double logarithmic plots of cumulative frequency against ranked egg release magnitude for 2 hourly intervals (Pareto plots) revealed power-law distributions for all three entities (F. spiralis, F. guiryi and F. vesiculosus; Fig. 8). The relationship deviated from linearity only at values of egg release .ca. 105 eggs per sampling bag (2–3 receptacles), which probably approaches the upper limit of eggs contained in the sample receptacles. The regression fit to the F. vesiculosus data was improved (r2 = 0.987 versus 0.952) by Reproductive Synchrony as Hybridization Barrier Figure 6. Relationship between the timing of high tide and egg settlement (shown as a percentage of the maximum) in a) Fucus guiryi, and b) Fucus vesiculosus at site A and B. Data are daily samples taken between February 2002 and July 2003 (n = 10 and 15 disks at site A and B, respectively, 6 SE). doi:10.1371/journal.pone.0035978.g006 removing values $105, so this was chosen as a cut-off value for number of eggs sampled. The fit of the regressions were also high for hermaphrodite species (r2 = 0.9813 and 0.9701 for F. guiryi and F. spiralis, respectively), and were highly significant in all cases (P,0.0001). Pairwise comparisons of the regression slopes showed no significant difference between the two hermaphrodite species (P = 0.208), but that both were significantly different from F. vesiculosus (P,0.0001 in both cases). The same was found using Kolmogorov-Smirnoff tests; the distributions of egg release data did not differ between the hermaphroditic species (F. spiralis versus F. guiryi, p = 0.800) but differed between these and the dioecious F. vesiculosus (F. guiryi versus F. vesiculosus, p = 0.000; F. spiralis versus F. vesiculosus, p = 0.000). It follows that the exponent or scaling factor, a, for the power law relationships calculated from the regression slopes varied little between the two hermaphrodite species (0.4760 and 0.4883 for the F. spiralis and F. guiryi, respectively), but was lower for F. vesiculosus (0.3454). PLoS ONE | www.plosone.org 7 April 2012 | Volume 7 | Issue 4 | e35978