Rhizostoma pulmo (Scyphozoa; Rhizostomeae) planulae and polyps: production, settlement preferences and development

Rhizostoma pulmo (Macri 1778) is a common and large scyphomedusa in the Mediterranean Sea with a metagenetic life cycle. The polyps of this species have not been observed in natural habitats and there is limited knowledge regarding its benthic stages. Therefore, maintaining polyp stocks in the laboratory is essential for conducting experiments aimed at acquiring ecological insights. In this study, we collected R. pulmo polyps from wild medusae to investigate the environmental factors that promote their development in natural habitats. Sexually mature R. pulmo medusae (34.8 ± 7.4) were collected in the harbour of Sète (43°24’06.3"N 3°43’10.5"E) in August 2023. To investigate the early life stages of their benthic phase, the medusae were placed in tanks with controlled conditions to stimulate reproduction. We examined substrate preferences and vertical recruitment of planulae, and further tracked the developmental stages of polyps. We did not detect planula preference for a specific depth in the laboratory experiments, instead we observed that planulae exhibited a preference for settling on glass slides and polystyrene Petri dishes over mussel shells, with a greater abundance observed on the undersides of the settlement substrates.