Written by: Jill Ashey
Edited by: Tim Bateman
Paper that inspired the post: Levenstein et al. (2022). Composite substrates reveal inorganic material cues for coral larval settlement. ACS Sustainable Chemistry & Engineering. doi: https://doi.org/10.1021/acssuschemeng.1c08313
Settlement is an essential step in the life cycle of a coral, as it is time for the larvae to find a good place to settle down, metamorphosis, and begin to grow. This step is also a switch from the mobile planktonic phase to the sessile benthic phase, making the choice of where to settle even more important, as the corals will be unable to move once they have settled. The perfect settlement spot depends on a number of factors including light availability, substrate materials, and currents, among others. If no suitable settlement locations are found, the larvae will die and reefs will be unable to grow.
Artificially-induced coral settlement has grown in interest over the last decade as a reef restoration option. Researchers induce coral spawning in laboratory environments, where they provide the larval corals with optimal settlement options to maximize the number that will settle. The settled corals are then reared in a safe environment and, when they are large enough, outplanted onto wild reefs for restoration purposes. To induce more larvae to settle, different natural cues, such as crustose coralline algae (CCA) and its associated bacteria, are used. CCA is hard calcareous red algae naturally found on reef environments. CCA is popular to use in coral settlement trials, as it emits chemical cues that induce larval settlement, and has a high settlement success rate (>50% in some coral species; Whitman et al. 2020).
Ceramic plugs are currently the go-to inorganic material for most coral settlement projects. While this is the established norm for coral settlement, investigation into other inorganic options is lacking and could provide improved settlement success for restoration efforts. Levenstein et al. (2022) sought to address this issue by evaluating the settlement preferences of coral larvae when given a range of inorganic materials.
First, the authors chose what kinds of inorganic materials to evaluate (Figure 1). One of the materials chosen was sand, which typically adds strength to cement and other substrates. Both quartz-rich sand and high aragonite sand were used; these choices were intended to compare non-calcium based sand (quartz) and sand typically found on reefs (aragonite). Synthetic glasses, specifically bioactive glass power and borosilicate glass fibers, were also chosen as another inorganic material. Glass fibers are typically added to materials to increase the strength of composite resins, and bioactive glass aids in bone growth and coral skeletal mineralization. Lastly, strontianite (SrCO3) and dolomite (CaMG(CO3)2), which are similar to materials that corals build their skeletons with, were evaluated in the larval settlement experiment. Both strontianite and dolomite are alkaline earth carbonates and are often used in aquariums to maintain coral mineralization. These inorganic materials were mixed with lime mortar to make settlement plugs. Additionally, the authors used earthenware clay ceramic plugs, which are typically used in settlement trials, as a reference material.
For settlement trials, the authors used two Caribbean reef-building corals, Acropora palmata and Diploria labyrinthiformis, to compare species-specific settlement preferences. A. palmata, a branching coral, and D. labyrinthiformis, a massive coral, are both hermaphroditic and reproduce by broadcast spawning. The different settlement options were placed in bins containing filtered seawater, and 200 larvae were added to each bin. After one week, the authors examined larval settlement choices.
They found that both species preferred to settle on specific materials, which were independent of bulk surface properties. Both species preferred quartz sand and materials with low glass content; additionally, D. labyrinthiformis preferentially settled on strontianite. Interestingly, the reference earthenware plugs had the lowest settlement, raising questions regarding their common application in reef restoration.
Differences in settlement preferences were not related to the bulk surface properties measured here.The authors hypothesized that larvae were attracted to specific materials because of the soluble ions that they released into the water. They developed a chemical equilibrium model to measure how soluble different materials were in seawater. The model found that the quartz sand, strontianite, and bioactive glass fibers were all releasing ions into the water, which may have been sensed by the coral larvae and induced settlement. Despite similar ion release rates, the coral larvae preferred to settle on strontianite-only material instead of strontianite/dolomite material. Using microscopes and 3D confocal surface maps, the authors observed microscopic differences between the topography of the strontianite only material and the strontianite/dolomite material, which may have contributed to differences in settlement rates.
These findings suggest that both soluble ions and micro-topography are important components of larval settlement (Figure 2). They also underscore the complex interactions and processes that contribute to coral settlement. A combination of organic and inorganic materials may be optimal when settling larvae in the lab for restoration purposes, but the nuances of these interactions must be explored in future research.
Gómez-Lemos et al. (2018). Coralline algal metabolites induce settlement and mediate the inductive effect of epiphytic microbes on coral larvae. Scientific Reports. 8, 17557. doi: https://doi.org/10.1038/s41598-018-35206-9
Whitman et al. (2020). Settlement of larvae from four families of corals in response to a crustose coralline alga and its biochemical morphogens. Scientific Reports. 10, 16397. doi: https://doi.org/10.1038/s41598-020-73103-2
Featured image: Gómez-Lemos et al. 2018.