Reproductive Synchrony as Hybridization Barrier Figure 4. Daily egg settlement from Fucus spiralis (panels c and d), Fucus guiryi (panels e and f) and Fucus vesiculosus (panels g–j), between 1 April and 31 September 2002 (left hand panels) and 1 April and 31 July, 2003 (right hand panels). Values are means 6 SE from n = 10 (Site A) or n = 15 (site B) sampling disks. Tidal amplitude (black line) and lunar phase for the sampling period are shown in a) and b); grey shading indicates the timing and duration of major settlement events from both species/sites. doi:10.1371/journal.pone.0035978.g004 Interspecific comparison of synchrony on a daily scale within the semilunar cycle To compare interspecific reproductive synchrony within semilunar cycles, we used the daily egg settlement data from 2002–2003 to compare the variance in the timing of settlement events above certain threshold values. The data were normalized as a percentage of the maximum value observed, and plotted against the time of the first daylight high tide (i.e., phase in the semilunar cycle) on the day of release (Fig. 6). Gamete release from hermaphroditic F. guiryi had a higher variance (i.e., was less synchronous) with respect to semilunar phase than that from F. vesiculosus. Mean settlement values $10% of the maximum, occurred on days when the maximum level of the day-time high tide (defined as the first high tide after sunrise) fell within the time interval 07:41–14:27 h in F. guiryi (a range .6 h), but were restricted to days with peak high tides within the time interval 08:27–13:13 h in F. vesiculosus (ranging ca. 4:46 h) (two-tailed F- test; F(32, 41) = 2.083, P = 0.027). For settlement events of $20% of the maximum the values were 07:41–13:49 h in F. guiryi (06:08 h) and 09:01–12:31 h in F. vesiculosus (03:30 h) (two-tailed F-test; F(26, 30) = 2.772, P = 0.008). PLoS ONE | www.plosone.org 6 April 2012 | Volume 7 | Issue 4 | e35978