Treading thin lines: delineating species boundaries in corals

Abstract

Delimiting species is not only a central issue in evolutionary and systematic biology but also a prerequisite to physiological, ecological, and population genetic studies. However, species boundaries' delineation in highly diverse environments, such as shallow tropical coral reefs, remains challenging. Traditional hypotheses that separate lineages in corals have long been based on morphological traits that did not yield resolution at the species level. The plastic response of corals to environmental variations and reports of intermediate morphotypes in the field have also hindered this task, casting doubt on current species delimitations. Therefore, while anthropogenic disturbances threaten coral reef ecosystems, the taxonomy of vulnerable taxa that inhabit them remains obscure.

The difficulty to navigate this intricate taxonomic landscape is epitomized by the coral genus Acropora, which exhibits more than a hundred morphospecies and provides an excellent training ground to test and validate new practices. Species delimitation attempts in Acropora corals have so far been unsuccessful due to widespread genealogical incongruence among genetic markers and between morphological groupings, mating trials, and molecular clades obtained in phylogenetic analyses. Consequently, this thesis aimed to test the current species delimitation in scleractinian corals, outlining an integrative approach and establishing a methodology that paves the way for a taxonomic revision using the Acropora genus as a case study.

For this purpose, a historical overview highlighting the emergence of the main issues faced by coral taxonomy was compiled (Chapter I). Molecular approaches were then applied to delimitate sympatric species of ecologically important and closely related tabular Acropora species. The congruence of the resulting molecular species delineation with additional lines of evidence, such as mating trials and morphology, was then gauged to select the most robust taxonomic hypothesis (Chapter II). The potential of this integrative methodology was extended by assessing the discriminative power of novel 3D-based morphometrical approaches in Acropora species robustly delineated using other lines of evidence (Chapter III). Finally, the discussion, perspectives and conclusions of this thesis are presented (Chapter IV).

The results of this thesis support the feasibility of developing a robust coral taxonomy when combining approaches sensitive enough to detect species divergence amid the complexity of speciation scenarios. The agreement between Acropora species boundaries delineated using different lines of evidence challenged the widespread notion that coral morphospecies cannot be distinguished at the molecular level due to hybridization. Due to unwarranted expectations, allele sharing-based and coalescence-based multilocus approaches outperformed mainstream molecular approaches. Contrastingly, the discriminative power of 3D-based quantitative morphology was comparable to traditional morphometric analyses for differentiating closely related species. Ultimately, comparing multiple lines of evidence is crucial to address species delimitation and provide valuable support to conservation efforts of taxonomically confused and threatened keystone organisms like corals of the genus Acropora.