Exaiptasia diaphana

Individuals of E. diaphana were mostly retrieved in sheltered substrates, with high water exchange and current exposure. The maximum depth that limits their distribution at Passetto was 3 m. Contrary to A. viridis, E. diaphana was usually observed on shady and vertical/ subvertical surfaces, where it tends to strongly attach and to colonize the micro asperities found on the substrate.

Temporal variations in abundance

Its variations Exaiptasia diaphana were always present during the study period, but its abundance fluctuated as shown in Figure 2B, reaching a maximum value in September 2013 (31.6% ± 18.1% SD) and a minimum value in December 2013 (5.0% ± 7.6% SD). Table 1 summarises main features of E. diaphana. Their abundance was positively correlated with temperature patterns (r = 0.59, N = 11, p <0.05), but not with wave heights and irradiance (Kruskal- Wallis, respectively with r = 0.35, N = 11 and r = 0.57, N = 11, p> 0.05). A. diphana’s abundance peaked during the quarter July-September (on average: 24.4% ± 17.8% SD), while the minimal value occurred between October and December (15.3% ± 20.4% SD). Statistical analysis showed significant variations in E. diaphana coverage among quarters (Kruskal- Wallis, H = 22.6, p <0.001). Table 1 summarises main features of E. diaphana.

Reproductive biology

Figure 3B summarizes data about sex ratio in the species E. diaphana. The first fertile individuals (females) were recorded in December 2013 (10%). The following months showed an increasing in the number of females (Fig. 4d-f), reaching a peak in April and May 2014 (35%), while males were observed only in June 2014 (54.5 %), with a sex ratio (F/M) 1:2.

Monthly variations in the number of female gonads individual-1 were shown in Figure 4D. The minimal value was recorded in December 2013 (29 gonads individual-1 ± 4.2 SD) reaching a maximum in May 2014 (94.0 ± 12.4 SD), then, their abundance dropped in June (71 ± 21.2 SD). Considering the quarterly variations, the number of gonads peaked in April-June (76.8 ± 24.3 SD) and decreased in October-December (29.0 ± 2.1 SD). Variations in the number of gonads individual-1 in E. diaphana were negatively correlated to temperature (Pearson’s correlation, r = 0.71, df = 28, p <0.001). Differences among quarters were statistically significant (One Way ANOVA, p <0.01, followed by Tuckey HSD, p <0.05).

The number of oocytes gonad-1 was counted only from April to June 2014, as the increase of size of the oocytes allowed us to count them. In April the number was 44.3 ± 36.5 SD, in May it was 37.7 ± 6.0 SD and in June it was equal to 13.9 ± 8.5 SD. It showed a decreasing pattern in time.

Sizes of female gametes are shown in Figure 4E and Supplementary Material S3. Their size ranged from 60 to 180 μm in January-March, with the 55% of the measured cells within the class 101-140 μm; while sizes varied between 60 and 220 μm in April-June (size class more frequent (46%): 101-140 μm). Finally, Male individuals were not found in the sampling site; however, we observed males in June 2013 in some specimens collected nearby for other purposes.