Symbiotic relationships between dinoflagellates of the family Symbiodiniaceae and
marine invertebrates underpin the functioning of certain shallow-water marine ecosystems.
Although the Symbiodiniaceae-Cladobranchia nudibranch association has been proposed
as a promising model for symbiosis research, interspecific variation in the extent
of this association remains poorly resolved. Here, we assessed the algal symbiotic
characteristics of the nudibranch Spurilla braziliana. Histological analyses revealed
limited branching of the digestive gland and the presence of intact Symbiodiniaceae
cells within lysosome-rich epithelial digestive cells. Metagenomic profiling further
showed a complete absence of Endozoicomonas-bacteria typically linked to Symbiodiniaceae
symbioses-in this species. These findings indicate that S. braziliana can harbor Symbiodiniaceae
but exhibits only primitive morphological and microbial adaptations to the symbiotic
state. Collectively, these results provide new insights into the evolutionary and
structural diversity of nudibranch-algal symbioses.
