Reproductive Synchrony as Hybridization Barrier Figure 2. Monthly egg settlement from Fucus spiralis and Fucus guiryi (a) and Fucus vesiculosus (b) in site A (dark bars) and site B (grey bars) between February 2002 and July 2003. Periods when sampling was not carried out are represented by n.d. (no data). Insets show the data plotted on a Logarithmic scale. doi:10.1371/journal.pone.0035978.g002 observed (in August) and when settlement was near zero (October– November). Abundance of mature receptacles declined following the main reproductive period, coinciding with declining repro- ductive output (pers. obs.). However, the continued presence of mature oogonia following the end of the main reproductive period in October suggests that gamete release may not be directly related to the availability of gametangia. Consistently lower settlement occurred in F. spiralis (site A), which had low density of reproductive individuals in the immediate vicinity of settlement disks. We included these data in Fig. 2a to illustrate that settled eggs originated from reproductive individuals immediately adjacent to sampling disks. Egg settlement quantification at the study site at various distances from the nearest females also consistently demonstrated low dispersal (Monteiro, Pearson, Serra˜o, unpubl. data). In addition to supporting the absence of interspecific contamination between sites monitored for each species, the ca. 2-fold lower settlement observed in F. vesiculosus compared with F. guiryi (site B, Fig. 2) may also be related to the low egg dispersal, since all individuals of hermaphroditic species produce eggs, compared with only half of dioecious F. vesiculosus (assuming equal sex-ratios); randomly- placed settlement substrates closer to males than females may therefore account for the reduced overall egg count for F. vesiculosus. Periodicity of gamete release on a daily scale within the semilunar cycle On a finer daily scale during the semilunar cycle, gamete release was highly discrete, with peaks of release coinciding with neap (minimum amplitude) tides in both 2002 and 2003 (Fig. 4). In 2002, 9 major settlement events, and in 2003 a further 6 were recorded over a total of 311 days. In most cases, release occurred over 1–2 days, more rarely with an additional 1–2 days of low release before or after the main peak. In 2003, release events were somewhat broader in time (5–6 days, compared with 1–2 days in 2002) and peak settlement was higher than in 2002. Greater gamete release in 2003 may have contributed to the temporal broadening of release events as a result of sampling efficiency (i.e., increased detection efficiency of larger release events may have contributed to apparent peak broadening). Overall, the data show that the large majority of days on which release occurred were shared between species, and reveal no temporal barriers to interspecific gamete encounters in the water column at a semilunar time scale. PLoS ONE | www.plosone.org 4 April 2012 | Volume 7 | Issue 4 | e35978