Shellfish hatcheries are critical to bivalve aquaculture, yet frequent larval crashes significantly hinder seed production. While the exact causes of these crashes remain unclear, evidence suggests that the microbiome plays a key role. Understanding the interactions between pathogens and beneficial bacteria is essential for mitigating these crashes and improving hatchery production. To investigate this, we collected larval samples from bivalve hatcheries to isolate and screen for both pathogenic and probiotic bacteria. From March 2023 to July 2024, 381 isolates were identified from oyster, clam, and scallop larvae. The most abundant genera among the 35 identified were Vibrio (35%), Pseudoalteromonas (26%), and Alteromonas (12%). Isolates were screened for antimicrobial activity, hemolytic activity, quorum quenching, and biofilm formation. Several isolates were identified as potential pathogens, including Vibrio neptunius DEN11, V. tubiashii DEN41 and DEN12, V. toranzoniae CH7, V. rotiferianus CH3, V. chagasii CH4, V. alginolyticus CH1, Tenacibaculum ascidiaceicola Clam15, and Cellulophaga lytica CH30, which exhibited pathogenicity on either hemocytes or larvae, highlighting their threat to hatchery production.
Four isolates (Algoriphagus winogradskyi DEN5, Marinomonas gallaica Clam9, Glutamicibacter soli Clam16, and Pseudooceanicola nitratireducens NEH7) were selected for their probiotic potential and tested for immunomodulatory effects on larvae. Although no significant differences were observed between the probionts, larvae treated with these isolates and challenged with Vibrio coralliilyticus RE22 and Aliiroseovarius crassostreae (JOD) showed a 16-42% and 56-81% increase in relative percent survival (RPS), respectively. Larvae treated with MgClam9 (46%) and AwDEN5 (52%) had higher RPS when challenged with VtDEN41. Probiotic treatment did not improve survival (1-9% RPS) when larvae were challenged with VnDEN11, as high mortality rates indicated that VnDEN11 is a highly pathogenic strain (Table 1). A combination of all selected probiotic strains, along with Phaeobacter inhibens S4, resulted in better larval survival than PiS4 alone (Table 2). This research highlights the importance of identifying pathogens and developing targeted probiotics to enhance larval resilience and support sustainable bivalve aquaculture.