Factors affecting infection of corals and larval oysters by vibrio coralliilyticus

Authors: USHIJIMA, BLAKE - Oregon State University; Richards, Gary; Watson, Michael; SCHUBIGER, CARLA - Oregon State University; HASE, CLAUDIA - Oregon State University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/12/2018
Publication Date: 6/19/2018
Citation: Ushijima, B., Richards, G.P., Watson, M.A., Schubiger, C.B., Hase, C.C. 2018. Factors affecting infection of corals and larval oysters by vibrio coralliilyticus. PLoS One. https://doi.org/10.1371/journal.pone.0199475.
DOI: https://doi.org/10.1371/journal.pone.0199475

Interpretive Summary: Vibrio coralliilyticus is a naturally occurring marine bacterium which infects and kills corals and larval shellfish. It is best known as the cause of coral bleaching, which has contributed to the loss or damage to coral reefs worldwide. More recently, it was determined that V. coralliilyticus strains also infect and kill larval oysters and other shellfish, particularly in hatcheries, causing shortages of seed oysters needed for commercial shellfish operations. This study compared the effects of four strains of V. coralliilyticus on Pacific oyster (Crassostrea gigas) larvae in regard to dose of Vibrio required to cause infection, the effects of temperature on mortality rates, the symptoms of infection, and genetic factors involved in both coral and larval oyster mortalities. Overall, this was the first study to show that all four strains were infectious toward larval oysters with some more infectious than others. Higher mortality was observed for all the strains at elevated temperature (27 degrees C) compared to 23 degrees C. Symptoms of illness were essentially the same at both temperatures. Some genes responsible for disease in corals were also found to be required for infection of larval oysters, while another gene was essential for coral disease but not for larval oyster disease. Together, this study provides a better understanding of important factors that impact coral and shellfish disease.

Technical Abstract: The bacterium Vibrio coralliilyticus can threaten vital reef ecosystems by causing disease in a variety of coral genera, and, for some strains, increases in virulence at elevated water temperatures. In addition, strains of V. coralliilyticus (formally identified as V. tubiashii) have been implicated in mass mortalities of shellfish larvae causing significant economic losses to the shellfish industry. Recently, strain BAA-450, a coral pathogen, was demonstrated to be virulent towards larval Pacific oysters (Crassostrea gigas). However, it is unclear whether other coral-associated V. coralliilyticus strains can cause shellfish mortalities and if infections are influenced by temperature. This study compared dose dependence, temperature impact, and gross pathology of four V. coralliilyticus strains (BAA-450, OCN008, OCN014 and RE98) on larval C. gigas raised at 23 and 27 degrees C, and evaluated whether select virulence factors are required for shellfish infections as they are for corals. All strains were infectious to larval oysters in a dose-dependent manner with OCN014 being the most pathogenic and BAA-450 being the least. At 27 degrees C, higher larval mortalities (p is less than 0.05) were observed for all V. coralliilyticus strains, ranging from 38.8-93.7 percent. Gross pathological changes to the velum and cilia occurred in diseased larvae, but there were no distinguishable differences between oysters exposed to different V. coralliilyticus strains or temperatures. Additionally, in OCN008, the predicted transcriptional regulator ToxR and the outer membrane protein OmpU were important for coral and oyster disease, while mannose sensitive hemagglutinin type IV pili were required only for coral infection. This study demonstrated that multiple coral pathogens can infect oyster larvae in a temperature-dependent manner and identified virulence factors required for infection of both hosts.