Coralline Algae Surface Architecture May Explain Why Coral Larvae Settle There

Coral larvae face a formidable challenge: after spending days or weeks drifting in the plankton, they must find a suitable patch of seafloor on which to settle and begin life as a sessile polyp. Among the most reliable signposts on that journey are crustose coralline algae, the pinkish, stony marine algae that encrust reef surfaces. Now, a hypothesis-and-theory paper published in Frontiers in Marine Science proposes that a long-overlooked physical feature of one particular coralline alga may help explain why coral larvae so often choose it: the microscopic architecture of its surface.

The study, led by Rodrigo Tomazetto Carvalho of the Rio de Janeiro Botanical Garden together with colleagues from the Museu Nacional of the Federal University of Rio de Janeiro and the Fluminense Federal University, focuses on the Lithophyllum prototypum complex, a group of non-geniculate coralline algae. Members of this complex have repeatedly been associated with high coral settlement responses on reefs, yet the mechanisms behind that association have remained incompletely understood. Most previous explanations have emphasized chemistry and microbes.

Crustose coralline algae are known to release chemical signals that attract larvae, and the films of bacteria living on their surfaces are thought to contribute additional cues. According to the authors, however, the distinctive physical architecture of the L. prototypum morphotype has received little ecological attention, even though it may shape how larvae experience both the surface and its chemical environment. To characterize that architecture, the team used scanning electron microscopy combined with quantitative morphometry.

Their analyses revealed that the alga's applanate surface results from the repeated superimposition of thalli, the flattened bodies of the alga, one atop another over time. This growth pattern produces overlapping lamellae, plate-like layers with elevated margins, and between these layers run narrow crevices and interstitial spaces distributed across the algal surface. The measurements give a concrete sense of the scale of these features.

The mean spacing between adjacent applanate branches was 533 ± 213 micrometers, while the overhanging lamellar margins formed shallow cavities averaging 29.1 ± 5.6 micrometers in height. From these observations the authors build what they call a physical–chemical–microbial microhabitat hypothesis. The idea is that the surface architecture may enhance coral settlement in several complementary ways.

First, the ridges, crevices and interstitial spaces increase the number of contact opportunities available to a searching larva, effectively enlarging the functional surface area of the alga. Second, the shallow cavities and overhanging margins may promote larval retention, giving larvae places where water motion is reduced and where they can remain in prolonged contact with the substrate long enough to test its chemical suitability. Third, and perhaps most intriguingly, the architecture may modulate the local expression of chemical and microbial cues.

The authors are careful to frame their proposal as a hypothesis rather than a demonstrated mechanism. That is precisely the point of the framework: by describing the geometry in quantitative terms, the study generates testable predictions.

Subject of Research: Microscopic surface architecture of the coralline alga Lithophyllum prototypum as a proposed driver of coral larval settlement

Article Title: Why might corals settle on Lithophyllum prototypum? A hypothesis linking non-geniculate coralline algal architecture to coral recruitment

Article References: Carvalho, R. T., Bahia, R. D. G., Moraes, F. C., Santos, J. A., Salgado, L., & Pereira, R. C. (2026). Why might corals settle on Lithophyllum prototypum? A hypothesis linking non-geniculate coralline algal architecture to coral recruitment. Frontiers in Marine Science, 13, 1996042. https://doi.org/10.3389/fmars.2026.1996042

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.1996042

Keywords: coral recruitment, larval settlement, crustose coralline algae, Lithophyllum prototypum, microhabitat, microtopography, settlement cues, coral reefs, scanning electron microscopy, non-geniculate coralline algae, hypothesis, might