Planula settlement and polyp morphogenesis in two bloom forming jellyfish species of the genus Cyanea Péron and Lesueur, 1810 and effects of abiotic factors on planulocysts

The negative effects of scyphozoan jellyfish blooms on ecosystems and economy are linked to planula survival, settlement success and subsequent polyp metamorphosis. A particular periderm covered stage formed by the newly settled planula, the planulocyst, has been reported for Cyanea lamarckii Péron and Lesueur, 1810 but not for Cyanea capillata (Linnaeus, 1758) from the Northeast Atlantic. Only a few planulocysts develop to polyps directly after settlement and the excystment process has not been understood in detail. By combining live observations with histological sections and scanning electron microscopy, present results clarified that the excysting C. lamarckii planula secretes a thin periderm stalk within the planulocyst and subsequently the polyp develops at the top of the stalk. No planulocysts but tiny periderm stalks appeared during the polyp development in C. capillata . Experiments with combined temperature (10, 15, 20 °C) and salinity (32, 25) treatments revealed significant effects of temperature on C. lamarckii planula settlement success (highest at 15 °C) and planulocyst excystment (highest at 20 °C) but no significant effects of salinity. Food supply did not affect excystment but enhanced the tentacle development of polyps. Our results demonstrate that studies on early life stages can reveal species-specific morphological differences in scyphozoan polyps which lack other distinct characters. The experimental results indicate that early C. lamarckii life stages are well adapted to environmental salinity changes and that increasing temperature due to global warming can be beneficial for their development which may support their northward distribution and increasing jellyfish populations in the Northeast Atlantic area.