To settle

Friends, as I've mentioned, there are some experiments going on in my lab at WHOI right now to examine how oyster larvae behave in different environmental conditions. Students and interns in the Mullineaux lab have been working on oyster behavior for several years now, in an effort to understand how turbulence, light, and water chemistry affect the swimming behavior of oyster larvae.

This mess of tubes is channeling compressed gas into our
flasks of seawater. 

This year, our question is how ocean acidification influences the behavior of oyster larvae when they're ready to settle and begin adult life on the seafloor. Settlement is a critical stage in the life-cycle of marine animals, and swimming larvae use a variety of physical and chemical cues to select the best place on the seafloor to attach, metamorphose, and spend the rest of their life. However, as the concentration of carbon dioxide in the atmosphere rises, more and more CO2 is becoming dissolved in the ocean, and the average pH of the surface waters is dropping. This acidification (decrease in pH) could affect how oyster larvae interpret chemical settlement cues.

We gathered larvae from a local hatchery and exposed them to filtered seawater and a chemical settlement cue (basically, seawater that adult oysters had been soaking in), at both ambient and low pH. To control the pH, we bubbled different blends of gases through the water: air for ambient pH, and air enriched with CO2  to simulate ocean acidification conditions. The bubbling process involved feeding compressed gases into the liquid solutions. We had a mess of plastic tubes stretching between gas tanks, flow controllers, and the flasks of seawater. It took about an hour for the pH in the flasks to equilibrate, and then we were off!

Example frame grab from a video recording
with larvae swimming down into
an experimental flask

With the pH-equilibrated solutions in hand, we headed over to WHOI's Shore Lab, where the experiments were to be run. We used 5 replicate flasks for each treatment. One by one, the flasks received a pipet full of larvae, and we filmed the larvae swimming for 10 minutes. We used a near-infrared camera to illuminate the flasks from behind so that the larvae were not exposed to light in the visible spectrum (light can affect their behavior). We also kept the flasks in a temperature-controlled chamber so that no convective currents would form in the flasks to disrupt swimming behavior. We were very careful to control the conditions so that the only difference between treatments was pH and the presence or absence of a settlement cue.

When entering the flasks, most of the larvae swam straight to the bottom, and then some swam back up. We'll use the videos to calculate parameters like the proportion of larvae swimming back up or staying on bottom, the proportion of larvae swimming in helices, and their average velocity. Together, these metrics will help us discern how ocean acidification affects the swimming behavior of larvae when they're ready to settle. I look forward to seeing the results!