Anemonia viridis (Forsskal, 1775)

Camillo, Cristina Gioia Di, Arossa, Silvia, Pica, Daniela, Bastari, Azzurra, Torsani, Fabrizio & Cerrano, Carlo, 2020, Phenology of Anemonia viridis and Exaiptasia diaphana (Cnidaria: Anthozoa) from marine temperate ecosystems Abstract, Mediterranean Marine Science 22 (1), pp. 40-50 : 43-46

Anemonia viridis

The species A. viridis was observed in sheltered areas, likely characterized by a low water exchange. Its upper limit of distribution was ~ 1.5 m depth. Individuals of this species were found on substrates with significant exposure to light, both on vertical and horizontal surfaces. They are usually weakly attached onto the substrate. Table 1 summarises main features of A. viridis .

Temporal variations in abundance

Anemonia viridis specimens were observed all year round in the studied area, however its cover varied monthly as shown in Figure 2A . The anemone was particularly abundant from July to September 2013 with a peak in August (percentage cover: 45.7% ± 26.8% SD), then it decreased steadily from October to December, reaching a minimum value in December (13.8% ± 12.1% SD). The abundance resulted higher in the quarter July-September (average in the quarter: 35.1% ± 22.3% SD) and minimal in the quarter October-December (16.9% ± 15.4% SD). Abundance of A. viridis was not correlated to temperature, waves height and irradiance values (Kruskal-Wallis, respectively r = 0.29, r = 0.38, r = 0.14, N = 11, p> 0.05). Moreover, there was no significant variation among all the considered quarters (Kruskal-Wallis, H = 5.38, p> 0.05).

Reproductive biology

Monthly variations in sex ratio are shown in Figure 3A . Female individuals occurred all year round except for July. The maximum number of females was reached in February 2014 (87.5%); then, females decreased in the period from March 2014 and the minimum value was observed in June 2014 (21.7%). Males were observed from January 2014 to June 2014 with a maximum value in May 2014 (42.1%). July 2013 was the only month in which all the specimens collected were infertile. The sex ratio (F/M) varied from 14: 1 in February 2014 to 1: 1 in June 2014. Males were more abundant than females only in May 2014 (42.1% male and 26.3% female). The average F/M ratio was 6:1. Hermaphrodites were never found.

In Figure 4A average monthly variations in the number of female gonads per individual is shown. Female gonads were visible starting from August 2013 (85.8 gonads individual-1 ± 40.4 SD); the number of gonads was higher from October to April, reaching a maximum during April 2014 (322.0 gonads individual-1 ± 85 SD). From May, their abundance decreased and dropped in June 2014 (99.8 gonads individual-1 ± 45.5 SD). Number of gonads individual-1 were inversely correlated to temperature (Pearson’s correlation, r = 0.70, df = 9, p <0.05). Considering the quarterly variations of gonads number individual-1, the highest values were observed from October to June, with a maximum in winter (265.3 ± 68.6 SD; when the mean temperature was equal to Tav=11.3 ± 0.02°C) and lower in July-September (154.9 ± 80.2 SD; Tav=21.7 ± 0.07°C). Statistical analysis showed a significant difference in the number of gonads during different quarters in females (One-way ANOVA, F = 3.92, p <0.05). Values observed in the period from July to September were significantly different from those of periods January-March and April-June (One-way ANOVA, F = 3.92, p <0.05, followed by Tukey HSD, p <0.05), but not from October-December (Tukey HSD, p <0.05). October-December values showed no significant difference with January-March and April-June and the same occurred between values occurred in January-March and April-June (Tukey HSD, p> 0.05).

Oocytes ( Fig. 4B ) were observed from August 2013 to June 2014; however, it was not possible to count cells from August to October due to their small size (and in November no samples were collected due to unfavorable weather conditions, as already mentioned). The number of female sex cells increased from December 2013 to May 2014 reaching a peak in April (44.0 oocytes per gonad ± 6.9 SD), followed by a sharp decrease in June (20 ± 7.9 SD). Variations in the number of oocytes per gonad in A. viridis were inversely correlated to temperature (Spearman’s correlation, r = 0.59, N = 11, p <0.05). The investigation of variations in the oocyte number per gonad in the considered quarters highlighted that the higher reproductive effort occurred from April to June (in average 31.6 oocytes per gonad ± 13.1 SD). Statistical analysis showed no significant difference between sample medians in all considered quarters (Kruskal-Wallis, H = 4.26, p> 0.05), but Mann-Whitney pairwise test showed a significant difference only between quarters April-June and October-December (p <0.05).

Concerning the size of oocytes ( Fig. 4B , Supplementary Material S3), it was observed that gametocytes are smaller from July to September (size range: 20-100 μm; size class more frequent (60%): 20-61 μm), while they reach the maximal sizes in April-June (size range: 61- 460 μm; size class more frequent (23%): 381-420 μm) with the highest value in May 2014 (421-460 μm).

The number of male gonads individual-1 is shown in Fig. 4C . No gonads were observed from July 2013 to December 2013. Male gonads were detectable from January 2014; then, their number started increasing, peaking in April 2014 (235.4 gonads per ind. ± 1.9 SD) and dropping in June 2014 (94.8 ± 40.3 SD). Monthly Variations in the number of gonads per male was not correlated to temperature (Pearson’s correlation, r = 0.55, df = 10, p> 0.05). Statistical analysis showed no significant difference between sample medians in quarters January-March and April-June (188.4 ± 181.9 SD and 181.9 ± 75.9 SD, respectively; One Way ANOVA, F = 0.027, p> 0.05). It was not possible to count the number of sperm cysts for each gonad due to their small size. However, we measured the diameter of the gonads containing sperms (spermatocysts). Biggest spermatocysts ( Fig. 4C , Supplementary Material S3) were observed in the quarter April-June (size range: 61-460 μm; size class more frequent (30%): 261-300 μm); on the contrary, in the period January-March gametocysts ranged from 20 to 180 μm (class more frequent (54%): 20-60 μm).

Figures

Fig. 2: Temporal variations in abundance of Anemonia viridis (A) and Exaiptasia diaphana (B) expressed as cover percentage in relation to monthly fluctuations of surface water temperature (°C), irradiance (W/m2) and wave heights (m).

Fig. 3: Sex ratio of Anemonia viridis (A) and Exaiptasia diaphana (B) throughout the period July 2013-July 2014 with relative percentages of male, female and infertile individuals.

Fig. 4: Reproductive biology of Anemonia viridis (A-C) and Exaiptasia diaphana (D-F). The histograms show variations in number of female (A, D) and male (C, F) gonads and number of oocytes per gonad (B, E) in each species throughout the period July 2013- July 2014.