G. atlanticus and P. porpita 16S and COI sequencing from the Mediterranean sea
Glaucus atlanticus (“blue dragon”), is an aeolid nudibranch that acquires chemical defense through sequestration of nematocysts from its primary prey, the highly toxic Physalia physalis (“Portuguese man-of-war”). In August 2025 hundreds of G. atlanticus individuals appeared near Alicante (Spain), prompting temporary beach closures due to concerns about potential toxicity. This unusual occurrence, in a region where P. physalis is typically absent, raises questions about the trophic ecology and defensive strategies of this species under novel environmental conditions. Here, we used COI metabarcoding of gut and cerata to characterize the diet of G. atlanticus and combined it with 16S rRNA gene sequencing of cerata, digestive tract, mucus, and central dorsal epidermis to assess potential microbial transfer from alternative prey that could contribute to the synthesis of defensive compounds, given that the role of microbes in chemical defense of aeolid nudibranchs was not assessed previously. We detected a trophic shift toward alternative cnidarian prey, including Pelagia noctiluca and Porpita porpita, which are not considered highly hazardous. No P. physalis nematocysts were observed in the cerata of the collected individuals by microscopy. Microbiome comparisons with P. porpita revealed no evidence of prey-derived transfer of mutualistic bacteria involved in chemical defense. Instead, G. atlanticus harbored distinct microbial communities dominated by environmentally widespread taxa, including potential opportunistic pathogens, with no indication of symbionts associated with defensive functions. Altogether, these findings indicate that, despite trophic flexibility, G. atlanticus may have limited capacity to maintain its defensive mechanisms in the absence of its primary prey.
Glaucus atlanticus (“blue dragon”), is an aeolid nudibranch that acquires chemical defense through sequestration of nematocysts from its primary prey, the highly toxic Physalia physalis (“Portuguese man-of-war”). In August 2025 hundreds of G. atlanticus individuals appeared near Alicante (Spain), prompting temporary beach closures due to concerns about potential toxicity. This unusual occurrence, in a region where P. physalis is typically absent, raises questions about the trophic ecology and defensive strategies of this species under novel environmental conditions. Here, we used COI metabarcoding of gut and cerata to characterize the diet of G. atlanticus and combined it with 16S rRNA gene sequencing of cerata, digestive tract, mucus, and central dorsal epidermis to assess potential microbial transfer from alternative prey that could contribute to the synthesis of defensive compounds, given that the role of microbes in chemical defense of aeolid nudibranchs was not assessed previously. We detected a trophic shift toward alternative cnidarian prey, including Pelagia noctiluca and Porpita porpita, which are not considered highly hazardous. No P. physalis nematocysts were observed in the cerata of the collected individuals by microscopy. Microbiome comparisons with P. porpita revealed no evidence of prey-derived transfer of mutualistic bacteria involved in chemical defense. Instead, G. atlanticus harbored distinct microbial communities dominated by environmentally widespread taxa, including potential opportunistic pathogens, with no indication of symbionts associated with defensive functions. Altogether, these findings indicate that, despite trophic flexibility, G. atlanticus may have limited capacity to maintain its defensive mechanisms in the absence of its primary prey.
Institute for Integrative Systems Biology