Effects of harmful microalgae on the behavior and morphology of ephyrae of the moon jellyfish Aurelia aurita

Although microalgae typically serve as prey for jellyfish ephyrae in marine food webs, this study investigated the potential of harmful microalgae to produce detrimental effects on the moon jellyfish Aurelia aurita. Understanding the biological interactions between Aurelia and microalgal species is crucial, particularly considering their common co-occurrence in coastal waters worldwide. We examined the effects of 11 protist strains, comprising seven species of harmful microalgae and two non-toxic microalgae, on A. aurita ephyrae. The rhythmic pulsation behavior of A. aurita was significantly suppressed when exposed to the raphidophytes Heterosigma akashiwo and Chattonella marina var. ovata and the dinoflagellates Amphidinium carterae, Coolia canariensis, and Pfiesteria piscicida. Notably, the media filtrates of all H. akashiwo strains and C. marina var. ovata killed ephyrae, implying a possible extracellular release of chemicals. This study discovered novel interactions between microalgae and jellyfish ephyrae, implying that harmful algal blooms may suppress mass occurrences of Aurelia medusae. •This study explored the effects of harmful microalgae on moon jellyfish (Aurelia aurita) ephyrae.•Harmful microalgae significantly suppressed the rhythmic pulsation of A. aurita ephyrae through diverse mechanisms.•Harmful algal blooms could suppress large populations of A. aurita medusae by negatively affecting their ephyrae.