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Mechanical properties, spectral vibrational response, and flow-field analysis of the aragonite skeleton of the staghorn coral (Acropora cervicornis)

Abstract

Understanding the structural and mechanical properties of coral skeletons is important to assess their responses to natural and anthropogenic challenges and to predict the long-term viability of hermatypic corals in a changing ocean. Here, we describe the microstructure of the critically endangered staghorn coral (Acropora cervicornis) skeleton and its mechanical properties, spectral and fluidic behavior, including uniaxial compressive strength, resistance to plastic deformation, spectral vibrational response, and flow-field analysis. We evaluated skeletons of A. cervicornis retrieved from a nursery off Broward County, Florida, USA. Optical micrographs and X-ray computed topography revealed a complex system of canals and pores that allow rapid skeletal elongation while retaining sufficient strength to withstand currents, waves, and other physical forces. Compressive loading of the aragonite skeleton resulted in complex stress–strain deformation behavior; the unique pore arrangement resisted catastrophic cracks and prevented instantaneous failure. Vickers microhardness was 3.56 ± 0.31 GPa, which is typical for soft aragonite materials yet sufficient to withstand the hydraulic pressure of ocean waves. Impressions made by the diamond indenter had almost no cracks radiating from their corners, which again demonstrated the ability of the complex skeleton microstructure to suppress crack formation and growth (e.g., from the bites of grazers). Maps of the ν1 mode Raman peak of identation surfaces showed evidence of residual strain. However, the ν1 peak’s position barely changed (from 1083.6 cm−1 outside the impression to 1083.9 cm−1 in the center), indicating weak stress sensitivity. Flow-field analysis revealed small-scale, counter-rotating vortices formed in the skeleton’s wake, which can entrain food particles within range of polyp tentacles and facilitate transport of respiratory gases and wastes. Considered together, our results demonstrate that the perforate skeleton of A. cervicornis is well-adapted to withstand physical forces normally encountered in its shallow-water habitat, but may be susceptible to anthropogenic stressors that alter its architecture.

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Acknowledgements

This research was supported in part by NSF MRI Award #133775. The GE Phoenix Nanotom-M™ X-ray CT was acquired through NSF Award #0959511. Manuscript revision was supported in part by US Environmental Protection Agency Cooperative Agreement X701D00720 UCFL to JE Fauth. Harvest of staghorn coral fragments to establish the NSU coral nursery was authorized under Florida Fish and Wildlife Conservation Commission Special Activity License #SAL-10-1086-SCRP. The authors also provide many thanks to an anonymous reviewer for their detailed review, which helped to improve our paper tremendously.

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JF and NO planned the study; DSG provided the chemically cleaned coral skeletons; AC-P, BM, and ZS performed the uniaxial compression and Vickers indentation tests; AC-P photographed the coral skeleton; MO performed optical microscopy and collected Raman maps of the indented skeleton surfaces; TS conducted flow-field experiments and analyzed the flow-field data; BEC and SNY performed CT scans and analyzed those data; and AC-P, MO, GS, SNY, JS, SB, JF, and NO all contributed to synthesis and manuscript writing.

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Correspondence to Nina Orlovskaya.

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Carrasco-Pena, A., Omer, M., Masa, B. et al. Mechanical properties, spectral vibrational response, and flow-field analysis of the aragonite skeleton of the staghorn coral (Acropora cervicornis). Coral Reefs 39, 1779–1792 (2020). https://doi.org/10.1007/s00338-020-02003-8

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Keywords

  • Compressive strength
  • Coral nursery
  • Failure analysis
  • Microstructure
  • Raman spectroscopy
  • Vickers hardness