PS 20-5 - Bacterial community trends associated with sea scallop, Placopecten magellanicus, larvae in a hatchery system.

Abstract: Atlantic sea scallops, Placopecten magellanicus, are the most economically important marine bivalves along the northeastern coast of North America, and wild-caught adults and juvenile spat are increasingly being cultured in aquaculture facilities and coastal farms. While adults can be induced to spawn successfully in hatcheries, the last two weeks of the larval maturation phase are plagued by large mortality events, making production unfeasible. Research into other scallop- and aquacultured-species point to animal loss from bacterial infections or from altered functionality of host-associated microbiota. There are no previous studies of the bacterial communities from biofilms growing in scallop hatchery tanks, nor even host-microbial studies with this species of sea scallops. We identified bacterial communities in veliger-stage wild larvae, hatchery larvae, and tank biofilms, using the V3-V4 region of the 16S rDNA gene, via Illumina MiSeq sequencing.

Hatchery larvae had lower bacterial richness (number of bacteria taxa present) than the wild larvae and tank biofilms, and hatchery larvae had a similar bacterial community (which taxa were present) to both wild larvae and tank biofilms. Bacterial richness was not significantly different between tanks which had been occupied by larvae for 48 hours, and those which had just been drained, scrubbed clean, and refilled with filtered seawater. Static-water-flow compared to continuous-water-flow (flow-through) did not generate different levels of bacterial richness overall, and only an equivocal difference when accounting for time as a smoothing feature in the model (GAM, p = 0.04).

Bacterial richness and community similarity between tank samples fluctuated over the trial in repeated patterns of rise and fall, which showed some correlation to lunar cycle where richness is high when the moon is about 50% and richness is low during new and full moon phases. This may be a proxy for the effects of spring tides and trends in seawater bacteria and phages which are propagated into hatchery tanks. The number of days since the full moon was significantly correlated with bacterial community richness in tanks (GAM, p < 0.01): low during the full moon, peaking ~ 21 days after the full moon, and decreasing again at the next full moon.

These results along with future work, will inform hatcheries on methods that will increase larval survival in these facilities, for example, implementing additional filtering or avoiding seawater collection during spring tides, to reduce certain bacterial taxa of concern or promoting a more diverse microbial community which would compete against pathogens.