Competition for food reduces disease susceptibility in a marine invertebrate

Abstract

Competition between organisms interfere in host and pathogen dynamics in ways that are difficult to predict. By one side, competitors can reduce the food supply and cause nutritional stress. Such stress could further modulate the susceptibility to infection by altering immune response or metabolic rate of the host. Alternatively, competitors may trap pathogens before they reach the focal host, and therefore reduce, enhance, or have no effect on infection according to the competitor's susceptibility to the infection. To better understand how competition influences host and pathogen interactions, we experimentally assessed the relative importance of competition for pathogens and resources on the severity of a viral disease infecting the Pacific oyster Crassostrea gigas. We designed an open-flow system where food enriched seawater flowed to filter-feeding competitors (or empty controls) before being delivered to recipient oysters. We tested a range of competing species that exhibit both low (ascidians, European oysters, mussels) and high (Pacific oysters) susceptibility to the virus. We assessed the physiological condition of the recipient oysters during acclimation, we added virus-contaminated seawater upstream of the distribution system, and we monitored host and pathogen dynamics. We found that the presence of competitors, regardless of susceptibility to the virus, indirectly reduced the infection rate of hosts by decreasing their food ingestion and growth rates. Although competitors can reduce viral particles from the seawater, this had no effect on the host population. Our data suggest that the effect of competition for food overwhelmed that of competition for pathogens, thus emphasizing the importance of considering resource availability in host and pathogen dynamics. More particularly, resource availability can have positive effects at the individual level, fostering physiological condition and growth, but negative effects at the population level, increasing magnitude of epidemics.

Seaweeds influence oyster microbiota and disease susceptibility

“…Here we used an open-flow system developed in a previous study (Pernet et al, 2021; Figure 1a). Virus-contaminated seawater enriched with phytoplankton was evenly distributed to 12 experimental units, each consisting of one header tank containing the seaweeds (green, brown or red algae) or left empty as control connected to one holding tank containing the SPF oysters (Figure 1a).…”

“…They were kept without feeding in a 45 L tank in static oxygenized seawater for 24 h where they shed viral particles (Figure 1). At this point, the seawater surrounding the donors became contaminated with the virus and used as the source of infection connected to the seawater distribution network by flexible tubes fitted inside a peristaltic pump (Pernet et al, 2021). Infection therefore began on day 14 of the experiment (Figure 1b).…”

“…Mortality risk of oysters held in mussel farms is lower than in oyster farms or empty farms, suggesting that mussels reduce the infection pressure on the host (Pernet et al, 2014). It is also possible that species that live around the oysters harbour species‐specific bacterial microflora that influence the host microbiota, its immune response and its susceptibility to POMS (Pernet et al, 2021). The oyster pathobiome may be extended beyond the host species and its microbiota, leading to community models in which co‐occurring species influences the health of the host population.…”

“…The risk of OsHV-1 mortality differs depending on the species that live around the oysters (Pernet et al, 2014;Pernet et al, 2021).…”

“…The risk of OsHV‐1 mortality differs depending on the species that live around the oysters (Pernet et al, 2014; Pernet et al, 2021). Mortality risk of oysters held in mussel farms is lower than in oyster farms or empty farms, suggesting that mussels reduce the infection pressure on the host (Pernet et al, 2014).…”