Transplant day

We collected Porites lobata corals and brought them into the lab. We monitored them for spawning for 10 straight nights. We collected their gametes and mixed them together to fertilize. We removed any unfertilized eggs, diluted, and cleaned the cultures. We changed their water. We concentrated the larvae and distributed them among settlement bins. We provided them with limestone tiles and a chemical cue. We checked to make sure they had settled. We counted and measured them. 

Coral settlers on a limestone tile.

And then we could start our experiment. 

Larval biology is a multi-step process. Any experiment you do has so many components: spawning, rearing, settling. Since our experiment focuses on the post-settlement stage - right after the larvae settle on the reef - we had to go through almost all steps in the life-cycle before we could start our experiment. But we made it. The larvae settled on our tiles, and we were good to go.

Our main experiment - the one that got us funded - is a juvenile transplant experiment. Basically, we're trying to figure out why Porites populations are so genetically different between inner lagoon and outer reef sites. There are three different genetic groups (we call them "lineages") of Porites at our sites. One of them is really sensitive to heat; one of them is pretty flexible; and the third one loves hot water. The two lineages that can handle heat better are more common inside the Rock Island lagoons, where the water heats up on a regular basis. That all pretty much makes sense - the heat-tolerant corals live in the hot water. But how did they get there? They had to arrive as larvae. Do larvae from all lineages disperse to inner and outer sites equally? Do they settle on the reefs close to their parents or far away? If you're a heat-sensitive coral and you settle in a lagoon, are you doomed? What if you're a heat-tolerant coral and you settle on one of the cooler outer sites - can you thrive there, or is there some trade-off to your thermal tolerance? 

Understanding those processes is not just interesting from a scientific perspective; it has strong implications for the future of coral reefs. Bleaching events are becoming more frequent, so the heat-sensitive corals on the outer reefs are potentially at risk. If we find that heat-tolerant larvae can settle on the outer reefs and survive there, then maybe larvae from the lagoons could repopulate those outer reefs after a bleaching event. If they can disperse and settle there themselves - great. But if there's some barrier or trade-off, maybe humans would need to help the larvae along in the future. It's best for us to study the processes at play now so we can act when it's time. 

Some of our transplant tiles at the lagoon site, Taoch.
Photo by Carsten Grupstra.

Some of you might remember that we tried our juvenile transplant experiment last year. We spawned Porites in the lab in April, got the larvae to develop and settle on tiles, outplanted them at two sites, and came back to check on them in November. Everything was dead. Every single coral we had outplanted as part of our experiment was dead. So we're trying again.

This year, we're not leaving the corals all the way until November. In fact, we're only giving them two weeks on the reef. We oriented the tiles horizontally rather than vertically so that the corals will be sheltered from sedimentation and light stress (our settlers are on the bottom), and we put the corals at a deeper spot on the reef so there wouldn't be as many predators. 

Cross your fingers, guys. We need at least some individuals to survive this time!