Association between the parasitic larvae of the sea‑anemone Peachia sp. (Cnidaria: Haloclavidae) and hydromedusae (Cnidaria: Hydrozoa) in the temperate Southwestern Atlantic Ocean

This present study describes for the first time the annual seasonality of a peculiar association between two different groups of cnidarians in temperate waters of the Southwestern Atlantic Ocean: the parasitic larval phase of the sea anemone Peachia sp. (Anthozoa) with the hydromedusa Liriope tetraphylla and Eucheilota ventricularis (Hydrozoa) from Mar del Plata, Argentina. As part of the sea anemone description, we also detailed three life-phases of Peachia sp. (parasitic, postparasitic, and juvenile), as well as the cnidom (spirocysts, basitrichs, holotrichs I and II, and microbasic b-mastigophores). The interaction between sea anemone-hydromedusae was observed only in the spring–summer months, while the rest of the year was not observed. Out of the 97 larval sea anemones observed during the period of occurrence of the association, 65% (n = 63) of them were found in E. ventricularis and 35% (n = 34) in L. tetraphylla. Prevalence in E. ventricularis was almost 2% and in L. tetraphylla nearly 0.5%. Intensity of infection ranged 1–2 sea anemones per host. Parasitized organisms in both host species were significantly larger than non-parasitized. The parasite phase of Peachia sp. was observed mainly adhering to the ex-umbrellar surface in both host species. Also, we provide data on the detachment mechanisms of the larvae of Peachia sp. from its host species, the burial process of the postparasitic stage, and the feeding mechanisms of the juvenile polyp.