Essay by Eric Worrall

Following the embarrassment of record coral abundance in their endangered reefs, Aussie academics now claim their reefs are suffering “cryptic” diversity loss.

Coral reefs: How climate change threatens the hidden diversity of marine ecosystems

Published: August 24, 2023 7.22am AEST

Samuel Starko Forrest Research Fellow, The University of Western Australia
Julia K. Baum Professor of Biology, University of Victoria

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As climate change pushes corals beyond their limits, a key question is why different corals vary in their sensitivity to warm waters.

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Using genomic tools, we identified three distinct types of Porites lobata on Kiritimati. These lineages, which may represent distinct species, are indistinguishable by eye but genetically different. 

Such biodiversity is known as “cryptic diversity” or “hidden diversity.” Although cryptic diversity is widespread across corals, its ecological implications remain unclear. 

Marine heat waves threaten cryptic diversity

We found that one genetic lineage of Porites was highly sensitive to the heat wave: only 15 per cent of its colonies survived compared to 50-60 per cent in the other lineages. Thus, even in a coral widely considered to be stress tolerant, heat waves can have hidden impacts, threatening diversity that is invisible to the naked eye.

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Because interbreeding between cryptic lineages and species can offer a potential avenue for future adaptation, losses of genetic diversity could make a bad problem even worse by limiting future adaptation to changing environments.

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Read more: https://theconversation.com/coral-reefs-how-climate-change-threatens-the-hidden-diversity-of-marine-ecosystems-211007

The abstract of the study;

Marine heatwaves threaten cryptic coral diversity and erode associations among coevolving partners

SAMUEL STARKO HTTPS://ORCID.ORG/0000-0002-9604-9188 , JAMES E. FIFER, DANIELLE C. CLAAR HTTPS://ORCID.ORG/0000-0003-2389-111X, SARAH W. DAVIES HTTPS://ORCID.ORG/0000-0002-1620-2278, ROSS CUNNING HTTPS://ORCID.ORG/0000-0001-7241-1181, ANDREW C. BAKER HTTPS://ORCID.ORG/0000-0002-7866-2587, AND JULIA K. BAUM HTTPS://ORCID.ORG/0000-0002-9827-1612 Authors Info & Affiliations

SCIENCE ADVANCES

11 Aug 2023
Vol 9, Issue 32

Abstract

Climate change–amplified marine heatwaves can drive extensive mortality in foundation species. However, a paucity of longitudinal genomic datasets has impeded understanding of how these rapid selection events alter cryptic genetic structure. Heatwave impacts may be exacerbated in species that engage in obligate symbioses, where the genetics of multiple coevolving taxa may be affected. Here, we tracked the symbiotic associations of reef-building corals for 6 years through a prolonged heatwave, including known survivorship for 79 of 315 colonies. Coral genetics strongly predicted survival of the ubiquitous coral, Porites (massive growth form), with variable survival (15 to 61%) across three morphologically indistinguishable—but genetically distinct—lineages. The heatwave also disrupted strong associations between these coral lineages and their algal symbionts (family Symbiodiniaceae), with symbiotic turnover in some colonies, resulting in reduced specificity across lineages. These results highlight how heatwaves can threaten cryptic genotypes and decouple otherwise tightly coevolved relationships between hosts and symbionts.

Read more: https://www.science.org/doi/10.1126/sciadv.adf0954

The researchers are suggesting heatwaves are killing corals and symbionts which don’t have heat stress genes, potentially leading to loss of the genetic diversity vital to the future survival of the coral.

In my opinion, the paleo evidence does not support the assertion that observed temporary shifts in genetic abundance are any kind of threat to the world’s coral reefs.

Corals are one of the most ridiculously resilient organisms on the planet. They sailed straight through the Cretaceous–Paleogene extinction event which killed the dinosaurs. Although adult corals are immobile, coral colonies regularly spawn countless microscopic and highly mobile larvae, through sexual reproduction, which seek out and colonise new sites. No doubt many of those larvae contain mutations or novel genetic combinations, which sometimes assist the larvae in colonising new sites.

Coral’s shield against adversity is their immense genetic diversity, and the mobility of their spawn – neither of which is threatened by temporary shifts in the relative abundance of different genetic alleles.