Charting a Sustainable Course: Phaeobacter Inoculation as a Probiotic-Based Strategy for Common Octopus Aquaculture During Early Life Stages

The aquaculture of Octopus vulgaris faces high larval mortality, mainly due to nutritional limitations and susceptibility to pathogens, particularly Vibrio spp. As vaccination is not feasible in cephalopods, host-associated probiotics represent a promising and sustainable alternative to improve survival and reduce infections. This study evaluated bacteria from the Roseobacter clade as probiotic candidates during octopus embryonic and paralarval stages. Characterization of egg-associated microbiota revealed the absence of cultivable bacteria within eggs and a significantly lower bacterial load on egg surfaces under maternal care, highlighting the regulatory role of female cleaning behavior. No bacteria with antagonism against Vibrio lentus, a common pathogen to octopus, were isolated from egg surfaces. Therefore, selected Roseobacter clade strains were screened in vitro against relevant aquaculture pathogens. Phaeobacter strains showed strong inhibitory activity against Vibrio spp., including V. lentus, while Ruegeria strains exhibited higher specificity against Tenacibaculum maritimum. Based on these results, Phaeobacter sp. 4UAC3 was selected for in vivo assays. This strain successfully colonized eggs, water, and paralarvae; however, its application reduced hatching success in eggs by 33%, likely due to surface-associated accumulation of the bacteria linked to the administration method. In contrast, probiotic treatment significantly improved survival at the paralarval stage. Although high variability was observed, probably due to stressful rearing conditions, more than 50% was observed in treated vs. 0% in non-treated cases at day 6. Overall, Phaeobacter sp. 4UAC3 emerges as a promising probiotic candidate to improve O. vulgaris paralarvae survival, potentially contributing to solving this bottleneck in a sustainable way.