Stress

"It's not stress that kills us, it is our reaction to it." - Hans Selye

This trip, we are absolutely determined to achieve our main goal: an investigation of the factors causing selective mortality in the post-settlement stage for Porites lobata corals. We modified our transplant experiment to ensure at least some corals survived to the end this time. As an additional back-up, we kept some corals in the lab and simulated the conditions they might encounter on the reef. 

There are several things that can stress corals out - water that's too hot or too cold, too much light, too little light, too much sedimentation, predators, competitors like turf algae - honestly, it's a wonder any of them survive at all. The post-settlement stage is an absolute gauntlet for marine invertebrates. That's why I've gotten so interested in studying it and even invented a special camera to do so. 

Some of our coral babies on glass slides in the experiment.
Each round beige dot is a coral. Photo by Matthew-James
Bennett.

For this experiment, we settled the corals on glass microscope slides and kept them in tanks in the lab. In one tank, the temperature is pretty normal, a cool 30 C, and they have a normal level of light for 12 hours a day. In another tank, they have normal light but water that's a toasty 33 C. The third tank has 4x the light, simulating the brightest, sunniest day on the reef, but normal temperature. And the fourth tank is the worst: high temperature, high light. Good luck surviving there. There's an additional group of corals in the high-temperature, normal-light tank that's shaded beneath their exposed brethren, to figure out if shade helps them tolerate heat. We're taking photos of all the slides every 2 days to keep track of who survives. 

Planning stress experiments is always a challenge because you want the conditions to be stressful enough that you get differences between your treatments, but not so stressful that everything dies right away. You also have to run the experiment long enough to actually see things die. We were very careful to plan this experiment with conditions the corals might actually encounter in nature (it's meant to complement our field transplant experiment, after all), so we weren't sure if we could run it long enough to get differences between the treatments. 

Thankfully, we have. Matthew's been doing the photos and told me just the other day we have quite a few corals dying in the most stressful treatment (high-temperature, high-light) and a few beginning to die in the high-temperature, normal-light treatment too. Unsurprisingly, the death is occurring in the stressful treatments first. 

Hopefully this experiment will provide a good parallel to our field transplant. I love getting good data!