Moon jellyfish (Aurelia aurita), one of Earth’s oldest marine species, are revealing fascinating new secrets about their life cycle. Recent research by microbiologists at Kiel University (CAU) has uncovered that bacterial metabolites—especially beta-carotene—within the jellyfish’s microbiome are essential for successful reproduction.
Published in iScience, this groundbreaking study shows that without the natural microbiota, the moon jellyfish cannot complete strobilation, the vital transformation of polyps into young jellyfish (ephyrae). This disruption leads to severe developmental issues and reduced reproductive success.
Why the Jellyfish Microbiome Matters
The microbiome—the complex community of microorganisms living in and on the jellyfish—plays a pivotal role in supporting health, growth, and developmental transitions in many marine organisms. In Aurelia aurita, this microbial ecosystem directly influences the transition from sessile polyps to free-swimming medusae through asexual reproduction.
Researchers found that the presence of specific bacteria enables the production of beta-carotene, a compound that jumpstarts metamorphosis by activating the jellyfish’s retinoic acid signaling pathway.
Beta-Carotene: The Molecular Trigger for Jellyfish Metamorphosis
The study reveals that during strobilation, bacteria within the jellyfish’s microbiome produce beta-carotene, a provitamin A compound. The jellyfish absorbs and converts it into 9-cis-retinoic acid, a signaling molecule that triggers genetic pathways required for metamorphosis.
“Our findings highlight that microbial signals are not just supportive but essential to jellyfish development,” explains Dr. Nadin Jensen, lead author of the study.
In sterile lab conditions, polyps deprived of their natural microbiome showed severe developmental abnormalities, such as:
- Incomplete segmentation
- Shortened bodies
- Missing tentacles
- Low ephyra release
However, supplementing the water with beta-carotene or retinoic acid partially restored these defects. In contrast, blocking the retinoic acid pathway disrupted strobilation even in healthy jellyfish with an intact microbiome.
Microbial Genes Hold the Key
Genetic analysis confirmed that beta-carotene biosynthesis genes exist solely in the jellyfish’s microbiota—not in the jellyfish genome itself. This confirms the moon jellyfish’s complete dependence on microbial symbionts for successful life cycle progression.
“Without this microbial interaction, the polyp stays stuck in its early stage and cannot reproduce,” says Dr. Jensen.
Broader Implications for Marine Biology and Ecology
These findings open new avenues for understanding marine organism adaptability and jellyfish population dynamics. By linking jellyfish reproduction to microbiome health, scientists can better predict phenomena like jellyfish blooms and explore ways to mitigate their impact on marine ecosystems.
“Jellyfish are crucial to oceanic food webs. Understanding their microbiome-driven life cycle helps us grasp how they respond to environmental changes,” adds Professor Ruth Schmitz-Streit, head of the study at Kiel University.
Study Citation
Jensen, N. et al. (2025). Microbiota-derived β-carotene is required for strobilation of Aurelia aurita by impacting host retinoic acid signaling. iScience. DOI: 10.1016/j.isci.2024.111729