Phylogenomics, Origin, and Diversification of Anthozoans (Phylum Cnidaria)

Area of Science:

• Evolutionary Biology
• Phylogenetics
• Marine Biology

Background:

• Anthozoans (corals, sea anemones) are crucial foundation species in reef ecosystems.
• Previous molecular phylogenies using mitochondrial or rDNA genes failed to resolve key evolutionary relationships within Anthozoa due to homoplasy.
• Morphological characters traditionally used for classification are often homoplastic.

Purpose of the Study:

• To construct a fully resolved, time-calibrated phylogeny for Anthozoa using a large dataset of ultraconserved elements and exon loci.
• To explore the evolutionary origins of major anthozoan clades and their key morphological features.
• To investigate the timing of key evolutionary events such as coloniality, skeleton formation, and photosymbiosis.

Main Methods:

• Phylogenetic analysis of 234 anthozoan species using hundreds of ultraconserved elements and exon loci.
• Construction of a time-calibrated phylogeny.
• Ancestral state reconstructions to infer the evolution of morphological traits.

Main Results:

• A resolved phylogeny supporting monophyletic Hexacorallia and Octocorallia, with Ceriantharia as the earliest diverging hexacoral.
• Divergence dating estimates place the origin of Anthozoa in the Cryogenian to Tonian periods (648-894 Ma).
• The ancestral anthozoan was a solitary, bilaterally symmetric polyp without a skeleton; coloniality and calcium carbonate precipitation evolved independently multiple times.
• Photosymbiosis originated by the Devonian (383 Ma) and has been repeatedly gained and lost.

Conclusions:

• The study provides a robust phylogeny that resolves long-standing questions about anthozoan evolutionary history.
• Key traits like reef-building capabilities and photosymbiosis evolved multiple times independently, highlighting evolutionary flexibility.
• The findings have significant implications for understanding early metazoan evolution and Precambrian environments.