Fine-Scale Skeletal Banding Can Distinguish Symbiotic from Asymbiotic Species among Modern and Fossil Scleractinian Corals

PLoS One. 2016 Jan 11;11(1):e0147066. doi: 10.1371/journal.pone.0147066. eCollection 2016.


Understanding the evolution of scleractinian corals on geological timescales is key to predict how modern reef ecosystems will react to changing environmental conditions in the future. Important to such efforts has been the development of several skeleton-based criteria to distinguish between the two major ecological groups of scleractinians: zooxanthellates, which live in symbiosis with dinoflagellate algae, and azooxanthellates, which lack endosymbiotic dinoflagellates. Existing criteria are based on overall skeletal morphology and bio/geo-chemical indicators-none of them being particularly robust. Here we explore another skeletal feature, namely fine-scale growth banding, which differs between these two groups of corals. Using various ultra-structural imaging techniques (e.g., TEM, SEM, and NanoSIMS) we have characterized skeletal growth increments, composed of doublets of optically light and dark bands, in a broad selection of extant symbiotic and asymbiotic corals. Skeletons of zooxanthellate corals are characterized by regular growth banding, whereas in skeletons of azooxanthellate corals the growth banding is irregular. Importantly, the regularity of growth bands can be easily quantified with a coefficient of variation obtained by measuring bandwidths on SEM images of polished and etched skeletal surfaces of septa and/or walls. We find that this coefficient of variation (lower values indicate higher regularity) ranges from ~40 to ~90% in azooxanthellate corals and from ~5 to ~15% in symbiotic species. With more than 90% (28 out of 31) of the studied corals conforming to this microstructural criterion, it represents an easy and robust method to discriminate between zooxanthellate and azooxanthellate corals. This microstructural criterion has been applied to the exceptionally preserved skeleton of the Triassic (Norian, ca. 215 Ma) scleractinian Volzeia sp., which contains the first example of regular, fine-scale banding of thickening deposits in a fossil coral of this age. The regularity of its growth banding strongly suggests that the coral was symbiotic with zooxanthellates.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Anthozoa / physiology*
  • Biological Evolution
  • Coral Reefs*
  • Dinoflagellida / physiology*
  • Ecosystem
  • Fossils*
  • Geography
  • Geology
  • Microscopy, Electron, Scanning
  • Microscopy, Electron, Transmission
  • Species Specificity
  • Symbiosis*

Grant support

This work was partially supported by the European Union within the European Regional Development Fund, through the Innovative Economy Operational Program POIG.02.02.00-00-025/09 (NanoFun; cathodolumienscence microscopy) and POIG.02.01-00-14-032/08 (transmission electron microscopy), European Research Council (ERC) Advanced Grant 246749 BIOCARB (to AM), and the National Science Centre (Poland) research grant DEC-2011/03/N/ST10/06470 (to KF). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.