Grant: Discovery Grants Program - Individual

The Eastern or Atlantic oyster (Crassostrea virginica) plays an important role in both the ecology and the economy of coastal North America. Oysters build large reefs that provide refuge to many other species, increasing species diversity in coastal waters. They also `armor' the shoreline against erosion, and filter large volumes of pollutants, protecting other species and fisheries and increasing human recreational use of coastal waters. Oysters are also the most valuable aquaculture shellfish species in Canada, valued at over $50 million annually. However, the free-swimming larval life stages of oysters are highly susceptible to infection by bacteria which also live and swim in the ocean. One common marine bacterium, Vibrio, kills oyster larvae in both the wild and in aquaculture facilities where oysters are reared (infiltrating flow-through water systems from the coastal ocean). We understand very little about the physics of how marine bacteria seek out and find swimming larvae, in order to infect them. In this work, we propose to investigate a strategy called `chemotaxis' used by marine bacteria to navigate in the ocean, which permits them to identify and swim towards nutrient or chemical sources which they need to survive. We will experimentally determine if Vibrio use chemotaxis to look for oyster larvae hosts, and also if this strategy can be disrupted in aquaculture facilities (by way of changing water temperature, salinity, or ambient water motion) to prevent infection and subsequent die-offs. First, we will use a `microfluidic' system in which experiments are set up at tiny scales (<1mm) on microscope slides, which permit us to carefully control the fluid and chemical environment experienced by swimming bacteria, to investigate whether bacteria can `sniff out' larvae using chemotaxis, and if stressed larvae (lack of food, wrong temperature) or age makes them more attractive to bacteria. Second, we will investigate how fluid flow impacts chemotaxis and subsequent interactions between bacteria and larvae; swimming larvae set up small currents around themselves as they move in the water, and the turbulent ocean around them also moves. Using high-speed cameras and microscope optics, in conjunction with an oscillating-grid tank which stirs the water, we will assess impacts of larval swimming and water motion on bacteria-larva interactions and infection success. This work will provide key information for natural resource managers, conservation professionals and ecologists who work with oysters. Since Vibrio attacks many species (e.g., other shellfish and coral reefs), this work will also be valuable to other conservationists. From an industry stakeholder perspective, shellfish aquaculture is a growing industry in Canada and bacterially-induced die-offs can cripple harvests. The proposed research represents an important step forward in mitigating these economically damaging occurrences.