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

Coral recruitment depends on the ability of larvae to identify suitable benthic substrates, and non-geniculate coralline algae are among the most important natural facilitators of this process. Within this group, members of the Lithophyllum prototypum complex are repeatedly associated with high coral settlement responses, but the mechanisms underlying this effect remain incompletely understood. Most explanations have emphasized chemical and microbial cues, whereas the distinctive surface architecture of this morphotype has received little ecological attention. Here, we characterize the applanate surface architecture of the Lithophyllum prototypum complex using scanning electron microscopy and quantitative morphometry. Repeated thallus superimposition generates applanate branches, characterized by overlapping lamellae, elevated margins, narrow crevices and interstitial spaces distributed across the algal surface. The mean spacing between adjacent applanate branches was 533 ± 213 µm, while overhanging lamellar margins formed shallow cavities averaging 29.1 ± 5.6 µm in height. These dimensions suggest that the surface of this morphotype produces larval-scale microhabitats rather than deep cryptic shelters. We hypothesize that this architecture may enhance coral settlement by increasing contact opportunities, promoting larval retention and modulating the local expression of chemical and microbial cues. This physical–chemical–microbial microhabitat hypothesis provides a testable framework for re-evaluating how NGCA morphology contributes to coral recruitment.