When we think about animal reproduction, we envision a complex process involving finding a partner, mating, and giving birth, carried out by organisms that are able to move. However, how can corals, which are attached to the bottom of the ocean, choose their partners? How do they mate and make sure that their offspring will find good places to live, allowing them to survive, grow and eventually reproduce themselves? Well, the truth is, they can’t. However, nature has thought it all and has developed a mechanism to allow corals to reproduce. 

Corals, like most benthic marine invertebrates, produce a planktonic larva which, after a dispersal period that ranges from a few minutes to weeks, tries to locate a suitable benthic surface to attach (termed settlement) and metamorphose. However, as coral larvae (known as planulae) don’t have annoying parents teaching them what is good and what isn’t, nature made them extremely picky at selecting their settlement substrate. And this happens for a good reason, as their choice of settlement substrate represents an integral step in their life, influencing their post settlement survival, growth and reproduction. As a result, planulae meticulously investigate reef surfaces prior to settlement, exhibiting a high level of specificity in the selection of attachment sites and employing an array of strategies and behaviors to enhance their likelihood of success. 

Accordingly, during the course of my PhD conducted in the lab of Dr. Julie Olson at the University of Alabama, I investigated the ecology, taxonomy and microbiology of crustose coralline algae and their role in mediating the settlement and metamorphosis of scleractinian coral larvae.  Crustose coralline algae (CCA) are a group of red seaweeds often overlooked by SCUBA divers because they are not as charismatic as many other marine creatures (Figure 1). Nonetheless, they are fundamental components of the reef benthos, helping to stabilize the reef, absorb wave energy, and reduce reef erosion while also providing habitat, refuge and grazing areas for fish and other invertebrates. Some species of CCA are well known for their ability to elicit settlement of a wide range of scleractinian coral species (Figure 2), but the exact conditions necessary for corals to settle remain highly debated. For instance, there is there is an ongoing debate between chemical ecologists trying to understand if the settlement abilities of CCA are mediated by chemical cues produced by the CCA itself (intrinsic factors), by cues produced by the extremely diverse microbial communities living on the algal surface (extrinsic factors), or by an interaction of both. 

To shed light into this topic, with the generous support of the American Academy of Underwater Sciences Doctoral Student Research Scholarship, in September 2021 and May-July of 2022 I travelled to the Florida Keys and the Smithsonian Marine Station (SMS) in Fort Pierce (Florida) where I conducted experiments for my dissertation.  I performed reef surveys while diving in the Florida Keys using point intercept transects and the resulting data were used to calculate the percent cover of major benthic organisms, including scleractinian corals, octocorals, macroalgae, turf, sponges and CCA, and determined the relative percent cover of each genus of CCA encountered along the transects (Figure 3). Also, I collected CCA from the reefs with the support of Mote Marine Lab’s International Center for Coral Reef Research and Restoration and transported the CCA to the SMS where coral larval settlement experiments were carried out.
I first wanted to tease apart the role of the CCA and their associated microorganisms in coral settlement through a series of settlement experiments (Figure 4) aimed at testing the ability of five species of CCA commonly found in the Florida Keys, with or without their associated bacterial communities, to induce settlement in coral larvae from Dendrogyra cylindrus, Orbicella faveolata and Colpophyllia natans. Results demonstrated that different species of corals responded differently to the presence of CCA for settlement and that removal of the CCA-associated bacteria elicited different responses in coral larvae based on the coralline species tested. Interestingly, the larvae of most coral species responded favorably to species of CCA that are not normally abundant on the reefs while only low levels of settlement were observed with species of CCA which are commonly encountered. Additionally, analysis of the CCA-associated prokaryotic communities revealed that the five species of CCA hosted distinct prokaryotic communities on their surface, which may help to explain the differences observed in larval settlement. 

Secondly, I wanted to further investigate the origin of the compound(s) responsible for coral settlement through a series of experiments to evaluate the response of Porites astreoides and Diploria labyrinthiformis larvae to two species of CCA following chemical and physical modifications. These experiments necessitated the identification of effective extraction methods for the natural products involved in coral settlement and their appropriate testing concentrations. Once again, results confirmed the important role of CCA in the settlement of scleractinian coral larvae and identified species-specific differences among corals and CCA tested. Additionally, this study provided valuable information about which extraction techniques best recovered the chemical cues produced by the CCA and highlighted the utility of testing the extracts over a broad range of concentrations to avoid erroneous results.

I am eternally grateful to the American Academy of Underwater Sciences for their generous support of my research. As coral reefs are facing unprecedented losses, my results contribute to understanding the necessary requirements for coral settlement to occur, offering essential information to help coral babies – which are the reefs of the future – have the best chance for survival. The recognition that coral species have species-specific requirements in order to survive and thrive and understanding which CCA species induce settlement across a wide variety of coral species will strengthen reef restoration efforts. The funds provided by the AAUS scholarship program enabled me to complete two chapters of my dissertation, furthering my research dramatically, while also giving me the opportunity to conduct research in the Florida Keys and at the SMS in Fort Pierce. As a result, I was able to meet and work with amazing scientists, including Drs. Valerie Paul, Jennifer Sneed, and Maggie Johnson. Based on these interactions, Maggie offered me a postdoctoral fellowship in her lab at the King Abdullah University of Science and Technology in Saudi Arabia, where I will continue with my studies of CCA in 2023.