A Host's Sea Floor Germs and Unique Gut Bacteria

Greg Howard
13th August, 2025

A Host's Sea Floor Germs and Unique Gut Bacteria

This sampling design separated the gastrovascular cavity and bell of the Upside-Down Jellyfish (Cassiopea xamachana) to demonstrate that the internal microbiome is dominated by Vibrio, Endozoicomonas, and Mycoplasma, whereas the external bell community mirrors the surrounding benthos.

Image adapted from: Muffett et al. / CC BY (Source)

Key Findings

  • Research on Upside-Down Jellyfish in the Florida Keys revealed their internal bacterial communities are highly specific and low in diversity, dominated by just three types
  • In contrast, their outer surface bacteria were diverse and largely resembled the surrounding environment, highlighting distinct host control versus environmental influence
Marine ecosystems are teeming with life, and beneath the surface, a complex web of interactions shapes the health and survival of organisms. A crucial part of this involves the microscopic communities of bacteria, fungi, and viruses that live within and on larger animals. These microbial partners, collectively known as the microbiome, are so fundamental that the host animal and its associated microbes are often considered a single functional unit, or a "holobiont." This is particularly true for cnidarians, a diverse group of aquatic animals that includes corals, sea anemones, and jellyfish. Many shallow-water cnidarians, like corals and some anemones (known as anthozoans), host symbiotic algae called zooxanthellae, which perform photosynthesis and provide energy to their hosts. While the microbial communities of these anthozoans have been increasingly studied, there has been limited information on the microbiomes of photosymbiotic scyphozoans, or true jellyfish. To address this gap, recent research conducted by scientists from the University of California Merced, Texas A&M University at Galveston, and Université de Perpignan focused on the Upside-Down Jellyfish, Cassiopea xamachana[1]. This jellyfish, common in shallow tropical waters, was sampled from eight sites across the Florida Keys. The study aimed to characterize the bacterial communities living inside and on the surface of these jellyfish. Using a technique called 16S rRNA barcoding, which identifies different types of bacteria by analyzing a specific genetic marker, the researchers examined bacterial DNA from the jellyfish's internal tissues and external mucus. The study revealed that Cassiopea xamachana possesses distinct microbial communities depending on their location within the jellyfish. The internal microbiomes were found to be remarkably low in diversity, meaning only a few types of bacteria dominated. These internal communities were primarily composed of just three specific bacterial groups: Endozoicomonas cf. atrinae, an uncultured novel Mycoplasma, and Vibrio cf. coralliilyticus. This finding suggests a strong selection process by the jellyfish host for a very specific internal microbial consortium. In contrast, the bacterial communities on the jellyfish's external bell mucus were much more diverse and largely resembled the microbial composition of the surrounding sediment, with the addition of Endozoicomonas cf. atrinae. This distinction between internal and external microbiomes in Cassiopea provides interesting insights when compared to previous research on other cnidarians. For instance, a review of octocorals, another type of coral, highlighted that these organisms often maintain remarkably stable bacterial communities across different geographical locations and even under environmental stress[2]. This stability, often dominated by a few microbial species, is thought to be a result of the octocorals' ability to actively regulate their microbiome, potentially through the production of antimicrobial compounds. The low diversity and dominance by a few specific taxa in the Cassiopea internal microbiome aligns with this observation in octocorals[2], suggesting that some cnidarians employ similar strategies to cultivate a stable, core internal microbiome. However, these findings also contrast with studies on other cnidarians, such as the sea anemone Exaiptasia pallida[3]. Research on Exaiptasia across various worldwide populations indicated that while bacterial diversity at a broad level was conserved, the specific species makeup varied drastically across locations. No single bacterial type was found ubiquitously in all Exaiptasia samples, and environmental settings, rather than host specificity, appeared to dictate the bacterial community structure[3]. This suggests that Exaiptasia might associate with a wide range of bacterial species as long as they provide necessary physiological benefits. The Cassiopea study, by showing a highly specific and low-diversity internal microbiome, indicates a greater degree of host selection or specificity compared to what was observed for Exaiptasia[3]. Yet, the external Cassiopea microbiome's similarity to the environment still supports the influence of environmental factors on microbial communities, creating a more nuanced understanding of host-microbe interactions across different cnidarian groups. The identification of Vibrio cf. coralliilyticus as a dominant member of the healthy Cassiopea internal microbiome is particularly noteworthy. Vibrio species are well-known in marine environments, and some, like those identified in a study on Stony Coral Tissue Loss Disease (SCTLD) in Florida corals, are associated with disease lesions[4]. The presence of Vibrio in a healthy jellyfish's core microbiome highlights the complex and often context-dependent nature of microbial roles; a bacterium that can be a pathogen in one host or under certain conditions might be a beneficial symbiont in another. Ultimately, the specific and stable internal microbiome observed in Cassiopea supports the broader concept that these microbial communities are not just passengers but active contributors to host biology. This aligns with findings from studies on other cnidarians, such as the sea anemone Nematostella vectensis, where the microbiome was shown to contribute to the animal's ability to adapt rapidly to varying temperatures[5]. In Nematostella, thermal tolerance could even be transferred to non-acclimated animals through microbiota transplantation, and specific beneficial microbes were passed down to offspring[5]. While the Cassiopea study focused on community composition, its findings suggest that the distinct internal microbiome likely plays a crucial role in the jellyfish's physiology and health, potentially contributing to its ability to thrive in its environment, much like the adaptive functions seen in Nematostella. By characterizing the bacterial communities of Cassiopea xamachana, this research expands our understanding of cnidarian holobionts beyond corals and anemones, establishing ecological links between these diverse groups and providing a foundation for future investigations into the functional roles of jellyfish microbiomes.

EcologyMarine Biology

References

Main Study

1) Florida Keys Cassiopea host benthos-like external microbiomes and a gut dominated by Vibrio, Endozoicomonas and Mycoplasma

Published 12th August, 2025

https://doi.org/10.1371/journal.pone.0330180

Related Studies

2) Host-microbe interactions in octocoral holobionts - recent advances and perspectives.

https://doi.org/10.1186/s40168-018-0431-6

3) Worldwide exploration of the microbiome harbored by the cnidarian model, Exaiptasia pallida (Agassiz in Verrill, 1864) indicates a lack of bacterial association specificity at a lower taxonomic rank.

https://doi.org/10.7717/peerj.3235

4) Microbial Community Shifts Associated With the Ongoing Stony Coral Tissue Loss Disease Outbreak on the Florida Reef Tract.

https://doi.org/10.3389/fmicb.2019.02244

5) Microbiota mediated plasticity promotes thermal adaptation in the sea anemone Nematostella vectensis.

https://doi.org/10.1038/s41467-022-31350-z


Related Articles

An unhandled error has occurred. Reload 🗙