The minimal endosymbiosis between Spurilla braziliana (Aeolidiidae, Nudibranchia) and Symbiodiniaceae

Abstract 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.