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

Symbiotic relationships between dinoflagellates of the family Symbiodiniaceae and marine invertebrates underpin the functioning of 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 capacity of the nudibranch Spurilla braziliana . Histological analyses revealed only limited branching of the digestive gland that accommodates algal 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 minimal morphological and microbial adaptations to the symbiotic state. Consequently, S. braziliana may bridge the evolutionary gap between symbiotic and non-symbiotic nudibranchs, providing a valuable reference point for studies on the evolution of animal–algal symbioses.