A clear distinction and presence of Acropora aff. divaricata within Acropora cf. solitaryensis species complex along their biogeographic distribution in East Asia

In the Anthropocene, scleractinian corals face unprecedented threats from synergistic stressors, including rising seawater temperatures that surpass critical thresholds that lead to global coral reef degradation. With over 1,698 coral species in the order Scleractinia, their conservation is increasingly complex due to their morphological plasticity and the challenge of accurate species identification. The genus Acropora, with approximately 400 nominal species, exemplifies these challenges, as morphological traits often vary within and among species, complicating taxonomic efforts. Traditional methods based on skeletal characteristics are insufficient for delineating Acropora species, prompting the use of integrative approaches combining morphology, reproduction, and molecular data. In this study, we employ multi-locus phylogenetic analyses and morphometric measurements to distinguish different growth forms of Acropora cf. solitaryensis as distinct species and delineate the species range boundaries of A. aff. divaricata and A. cf. solitaryensis in East Asian coral ecosystems. We identify arborescent and intermediate morphotypes belonging to A. aff. divaricata, which is distributed from tropical reefs in southeastern Taiwan to temperate non-reefal communities in Shikoku, Japan. Conversely, the solid-plate morphotype aligns with A. cf. solitaryensis from the holotype locality at Solitary Island, Australia, found primarily in subtropical non-reefal regions in northern Taiwan and Japan. The distinct distribution patterns of A. aff. divaricata and A. cf. solitaryensis underscore the necessity for biogeographic sampling in Acropora taxonomy, considering the Kuroshio Current's impact on coral distributions, and a re-evaluation of poleward coral migration or expansion due to climate change. Our findings challenge the traditional taxonomy of A. aff. divaricata and A. cf. solitaryensis, revealing that they may instead encompass multiple species. This has significant implications for coral conservation strategies, as accurate species identification is crucial for understanding coral responses to environmental changes and informing conservation efforts in the face of climate change.