The mycobiome of Cassiopea xamachana as determined by culture-based and metagenomic sequencing

Cassiopea xamachana, the upside-down jellyfish, exists in shallow environments resting on the seafloor with its oral arms facing towards the surface. This habitat creates a close association with the air–water interface, human interactions, and the microbial communities within the marine sediment. Nonetheless, the mycobiome of C. xamachana has never been studied. We sampled male and female C. xamachana medusae from three locations in Key Largo, Florida, USA, as well as lab-reared medusa kept in artificial seawater. Gene markers (ITS and 18S) of fungal isolates from different compartments (bell, oral arms, rhopalia, and gonads) were sequenced for taxonomic identification. Additionally, 18S metabarcoding of the host was carried out as an attempt to identify non-culturable fungal taxa. Our findings suggest that sampling location is the main determinant of fungal diversity in C. xamachana. Lab-reared jellyfish had fewer morphologically distinct isolates than any natural location, as well as a higher proportion of Cladosporium sp. The three natural locations had varying numbers of culturable isolates, with jellyfish collected on Coco Plum Road having the highest number of isolates. We found fungal taxa exclusive to certain jellyfish tissues and distinct from environmental samples, indicating some winnowing mechanism for the establishment of the endomycotal community. This study is the first characterization of fungal communities in a benthic jellyfish and sets the stage for further research into transmission modes and the relationships between Cassiopea and its mycobiome.