A serious disease was recorded in juvenile Pacific abalone in Fujian Province, China in 2018. Although this disease caused no obvious external lesions, affected abalone exhibited bleached pedal epithelial cells and a lack of attachment ability. Bacterial strains were collected and cultured from the mucus of moribund and healthy abalone. A novel method was developed for screening abalone pathogens, based on the important role of mucus in the innate immunity of marine organisms. Using bacterial isolation, sequence analysis, and experimental challenges in vitro and in vivo, we identified the bacterial strains pathogenic to abalone. We verified that abalone mortality rates were high when exposed to Pseudoalteromonas shioyasakiensis strain SDCH87 at high temperatures. This opportunistic pathogen had an outstanding growth ability in mucus, and disrupted first line mucosal immunity in the foot within three days. The unprecedented sea surface temperatures associated with the record-breaking 2018 heatwave in south China may have induced opportunistic pathogenic behavior in P. shioyasakiensis. To our knowledge, this is the first report to show that P. shioyasakiensis is a serious opportunistic pathogen of abalone, or possibly mollusks in general, in the context of a heatwave.

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