Aims This study describes the effect of phage therapy on hatching of longfin yellowtail (Seriola rivoliana ) eggs challenged with Photobacterium damselae subsp. damselae . Methods and results A lytic phage (vB_Pd_PDCC‐1) against P. damselae subsp. damselae was isolated and characterized. The use of phage vB_Pd_PDCC‐1 increased the hatching rate of eggs, and reduced presumptive Vibrio species to non‐detectable numbers, even in non‐disinfected eggs. High‐throughput 16S rRNA gene sequencing analysis revealed that phage vB_Pd_PDCC‐1 caused significant changes in the composition and structure of the associated microbiota, allowing that members (e.g. those belonging to the family Vibrionaceae) of the class Gammaproteobacteria to be displaced by members of the class Alphaproteobacteria. Conclusions To the best of our knowledge, this represents the first study evaluating phage therapy to control potential negative effects of P. damselae subsp. damselae during hatching of longfin yellowtail eggs. Significance and Impact of the Study The Seriola genus includes several important commercial fish species due to its rapid growth and easy adaptability to confinement conditions. However, bacterial infections (especially those caused by Vibrio and Photobacterium species) are among the main limiting factors for the intensification of marine fish aquaculture, particularly during early development stages. Therefore, the use of phages, which are natural killers of bacteria, represents a promising strategy to reduce the mortality of farmed organisms caused by pathogenic bacteria.

Crassostrea gigas aquaculture depends on the capacity of hatcheries to produce microalgae to feed oysters. Alternative feeds for bivalves, such as the use of seaweeds single cell detritus (SCD), have been reported. However, this has not been tested as yet as a possible strategy for C. gigas hatcheries. This study was conducted to select an SCD as an alternative feed for C. gigas based on its effects on digestive activity, histomorphology, and microbiota. The seaweeds tested were Ulva lactuca, Ulva clathrata, and Porphyra sp. and compared with common feed consisting of microalgae Chaetoceros calcitrans and Isochrysis galbana. After 17 days of feeding, the mortality was 100% for oysters fed SCD of Porphyra sp., but oysters fed SCD of Ulva showed low mortality. Ulva SCDs were appealing for oysters, according to the filtration rate. Histomorphology of the epithelium height of oysters fed U. lactuca SCD was similar to that fed C. calcitrans. The ingested SCD modified the oyster’s microbiota composition, decreasing the alpha and beta diversity in oysters fed SCD of U. lactuca; however, species richness was similar to that of oysters fed microalgae. In oysters fed SCD, bacterial orders Vibrionales, Campylobacterales, and Clostridiales increased, whereas Pseudomonodales, Chromatiales, Rhizobiales, Sphingomonodales, and Burkholderiales decreased. The most abundant bacterial phyla Proteobacteria, Bacteroidetes, and Firmicutes were identified in all oysters independently of feed. Oysters fed with SCD decreased the lipase enzymatic activity, which is related to the low lipid content in SCD; however, the activity of enzymes involved in carbohydrate and proteases digestion increased in oysters fed U. lactuca SCD. These results suggest U. lactuca-derived SCD as a candidate for supplementation of C. gigas in future partial substitution tests.