Coral reefs have attracted much attention recently as climate change intensifies and environmental extremes such as marine heat waves disrupt the integral symbiosis between corals and their endosymbiotic algae. Furthermore, ocean acidification, or the decline in ocean pH due to absorption of CO2 from the atmosphere, is making it more challenging for corals and other marine calcifiers to build their skeletons and shells. As coral reefs are under threat by these global stressors and other local human impacts, new corals are needed to take the place of the corals that are lost.
New coral offspring, called larvae, are either produced through coral spawning and develop in the seawater, or through coral brooding, where they develop into larvae inside the parental polyp. In coral brooding, there is a tight connection between the environment of the parents and their larvae. In this way, coral offspring resilience may be attributed to stress exposure in the lives of their parents or during early development, which has been shown in mammals such as humans and rats.
We hypothesized that the environment the parent experiences, and/or the environment during the early development of the coral offspring, conditions the larvae to respond positively to stress in the future. After demonstrating that parental stress exposure benefits their offspring immediately after they are born, I teamed up with Dr. Raphael Ritson Williams, super tech Jennifer Davidson and stellar summer undergraduate student Jolly Ann Cruz to track coral offspring through the juvenile stage, so we could expand our understanding of the contribution of stress conditioning to coral ecology and coral reef resilience.
Pocillopora coral larvae being released by the parents and searching for a place to settle and grow. Photos: Hollie Putnam
Coral reproduction is amazing to watch and essential for coral reef maintenance. Studying coral reproduction, however, can be extremely intense, as all the action is packed into a few short days each month in the summer on Hawaiian Reefs. We used large mesocosm tanks at the Hawaiʻi Institute of Marine Biology to stress our parent corals with low pH while they were reproducing and brooding their young.
As the coral larvae are very small (~1mm in length x 0.5mm in width), we had to design special chambers to collect the larvae and to hold the larvae in their treatments. What most people don't see behind the scenes are the hours spent crafting each coral baby crib with care to get ready for the big event. While fingers were definitely burnt in the lid making, we couldn't complain about the view from our office!
After the corals released their offspring, the larvae were placed in the chambers with tiles for them to settle on. Settlement and survivorship was measured after several days and then the tiles were tagged, and the new corals were mapped and tracked for 6 months.
As hypothesized, parental conditioning to the low pH benefited the coral offspring. Not only did more of them settle and survive up through the first month, but they also grew to larger sizes if their parents were stressed. This parental or developmental carry over provides a buffer against stressful conditions at the earliest stages, which can help more corals to recruit into the population, grow faster to minimize mortality from competition or predation, and thereby enhance reef resilience.
Pocillopora coral larvae expanding their tentacles in the light. Videos: Hollie Putnam
Through this project, I am able to see the metaphorical parallels between conditioning in corals and conditioning for life. Our experiences truly do influence us. Those that come before, can contribute to our success by anticipating our future challenges and through their training and encouragement prepare us to face those challenges. I was so privileged to have worked with Dr. Ruth Gates for 10 years. This paper represents our last active project before her passing. I know I speak for myself and many others when I say our mentors and our sponsors matter! Ruth prepared us, she encouraged us, and she supported us. Not only has this project generated data on another piece of the complex puzzle that is coral reef resilience, but it also has reminded me to channel my mentor in the ongoing and challenging work to better prepare and support each new generation of scientists, and in this way enhance and improve the quality of the academic system.
We dedicate this work to our dear friend, colleague, and mentor, the irreplaceable Dr. Ruth D. Gates.