A modern scleractinian coral with a two-component calcite–aragonite skeleton

Abstract

One of the most conserved traits in the evolution of biomineralizing organisms is the taxon-specific selection of skeletal minerals. All modern scleractinian corals are thought to produce skeletons exclusively of the calcium-carbonate polymorph aragonite. Despite strong fluctuations in ocean chemistry (notably the Mg/Ca ratio), this feature is believed to be conserved throughout the coral fossil record, spanning more than 240 million years. Only one example, the Cretaceous scleractinian coral Coelosmilia (ca. 70 to 65 Ma), is thought to have produced a calcitic skeleton. Here, we report that the modern asymbiotic scleractinian coral Paraconotrochus antarcticus living in the Southern Ocean forms a two-component carbonate skeleton, with an inner structure made of high-Mg calcite and an outer structure composed of aragonite. P. antarcticus and Cretaceous Coelosmilia skeletons share a unique microstructure indicating a close phylogenetic relationship, consistent with the early divergence of P. antarcticus within the Vacatina (i.e., Robusta) clade, estimated to have occurred in the Mesozoic (ca. 116 Mya). Scleractinian corals thus join the group of marine organisms capable of forming bimineralic structures, which requires a highly controlled biomineralization mechanism; this capability dates back at least 100 My. Due to its relatively prolonged isolation, the Southern Ocean stands out as a repository for extant marine organisms with ancient traits.

The calcifying interface in a stony coral’s primary polyp: An interplay between seawater and an extracellular calcifying space

“…This should be considered in risk assessment and management of coral-reef ecosystems around the globe. By exploiting new experimental approaches, our observations add up to several recent studies ( 15, 37, 68 ), revealing a larger and more versatile biological toolkit exploited by corals for their biomineralization process than previously recognized.…”

“…A major missing link for predicting the degree of stony coral resilience to environmental changes is the understanding of the basic mechanisms, at the tissue, cellular and molecular levels, by which corals calcify. Stony coral skeletons are made of calcium carbonate almost entirely of the polymorph aragonite ( 14, 15 ). The elemental and isotopic compositions of the aragonitic coral skeletons record the external seawater chemistry in which they were formed, but with an offset due to the biological control of the coral animal over this biomineralization process.…”

Consensus Guidelines for Advancing Coral Holobiont Genome and Specimen Voucher Deposition

“…But therein lies the conundrum: while molecular analyses commonly achieve superior taxonomic resolution, they rely on initial expert review and annotation to prevent error-propagation through incorrect phylogenetic annotations of sequence database entries (Tripp et al, 2011). It is important to acknowledge that taxonomic identification 7 https://www.sanger.ac.uk/collaboration/aquatic-symbiosis-genomics-project/ is challenging because morphological characteristics that differentiate species in one genus may not be applicable to other genera, and the same is true for molecular markers (Veron, 2000;Shearer et al, 2002;Stolarski et al, 2021). In the case of many coral lineages, species-level molecular markers are simply not (yet) available (Quattrini et al, 2018;Cowman et al, 2020;Erickson et al, 2021), partially due to ongoing taxonomic revisions, but also due to corals exhibiting low levels of congeneric divergence for commonly employed (mitochondrial) gene markers, effectively hampering specieslevel resolutions (Shearer et al, 2002; Supplementary Table 2).…”

Molecular and mineral responses of corals grown under artificial Calcite Sea conditions

“…In corals, skeletons are composed of different calcium carbonate (CaCO 3 ) polymorphs. Members of the order Scleractinia produce aragonite exoskeletons, with the sole exception of Paraconotrochus antarcticus , a recently discovered species that exhibits a two-component (i.e., aragonite and calcite) skeleton (Stolarski et al, 2021). In the subclass Octocorallia, an opposite pattern occurs: the vast majority of species produce calcitic endoskeletal structures, while aragonite exoskeletons are deposited by members of the order Helioporacea (i.e., the blue corals).…”