In February 2020, the first bleaching event in 14 years occurred in the Keppel Islands.

The corals mainly affected were the fast-growing branching corals that live in the shallow waters fringing the islands.

While many see this as a devastating event, a new study published in Nature suggests that the ancestors of today’s large-polyp coral species survived the last great extinction 66 million years ago (the K-T extinction event) and therefore large-polyp coral species might be better equipped to survive sudden and severe climate change than we think (Dishon, Grossowicz et al. 2020).

An artist’s rendering of an asteroid a few kilometers across colliding with the Earth. Such an impact can release the equivalent energy of several million nuclear weapons detonating simultaneously.

Large-polyp coral species can eat plankton

Boulder, brain and solitary corals don’t rely on the easy energy produced by zooxanthellae to survive—they can consume plankton for energy, though this makes them slower growing. They are also generally fleshier than branching corals and often prefer deeper waters (although not always).

Because of this capacity to consume plankton, and because of their size and shape, they are more likely to survive bleaching events caused by warming seas and light when other corals have lost their zooxanthellae.

These corals are called large-polyp species.

 

Tubastrea or sun coral is a large-polyp species found in the Keppel Islands. Photo R. Berkelmans.

Small-polyp coral species rely more on autotrophy

Branching corals that rely almost entirely on the tiny algae (zooxanthellae) that live inside them to produce energy through photosynthesis (autotrophy) are called small-polyp species. The readily available energy source from photosynthesis means they can grow faster than large-polyp species that have to rely entirely on catching their prey. But too much light and high water temperatures can be toxic, causing the algae to be expelled so that the coral appears pure white, hence the term ‘bleaching’. This can result in the coral slowly starving.

Bleached small-polyp species, Acropora tennuis, Monkey reef February 2020. Photo Max Allen Freedom Fast Cats.

According to the study, these types of corals are more likely to:

  • be found across all or most of the world in appropriate habitats
  • be small (<20 cm), or solitary
  • be slower growing (>0.5 cm/yr
  • have larger corallites (>3 mm) – a corallite is a single polyp skeleton
  • have no zooxanthellae – they can eat plankton
  • their growth form means that their polyps can potentially interact to coordinate their response to stress

Scanning Electron microscope image of corallites of Pseudosiderastrea tayami. IRD – Benzoni, F. / CC BY.

Some coral species survived the K-T extinction

The study also suggests that today, around the world, corals with these same characteristics (generally the large-polyp species) have relatively stable populations, while small-polyp species are decreasing in abundance and diversity (IUCN 2020).

They are exhibiting a similar dynamic survival response as seen at the last major extinction, the K-T (Carpenter, Abrar et al. 2008), which killed reef-building corals after a meteorite measuring 10 km in diameter hit the area we now know as Mexico and caused the rapid onset of a severe winter. Blocking out the sunlight, the dust storm from the meteorite strike caused a rapid 6 °C cooling of sea surface temperature. Nutrients in the ocean increased because of lower marine productivity; phosphorous was released into the ocean by chemical weathering from acid rain; and the ocean became more acidic.

After the K-T extinction, it took 2–10 million years for coral reefs to recover, beginning with coralline algae and other colonisers. Its a totally different timeframe to the way we think of coral reefs and their survival in terms of human life spans!

While many small-polyp, reef-building species of corals had become extinct, at least some must have survived to become our present day important zoooxanthellate reef-builders. But it was the large-polyp species, with no zooxanthellae, that could eat plankton that survived.

Coral nematocyst firing sequene. When potential prey makes contact with the tentacles of a polyp, the nematocyst cell is stimulated. The barbs at the end of the nematocyst are designed to stick into the polyp’s victim and inject a poisonous liquid. When subdued, the polyp’s tentacles move the prey toward its mouth and the nematocysts recoil back into their capsules.

Can the large-polyp corals of the Keppels survive climate change?

The Keppel Islands have an abundance of both small- and large-polyp species. And it is the small-polyp species that are always hit hardest by bleaching events.

Interestingly, some of the large-polyp species found in the Keppels that have withstood past bleaching events have similar characteristics to coral fossils that survived the K-T extinction (Bambach 2006).

Imagine if over those 66+ million years coral populations have evolved to be able to tolerate extreme changes in temperature all over the world! And may still be evolving ready for the next big change…

 

Sources

Bambach, R. K. (2006). “Phanerozoic Biodiversity Mass Extinctions.” Annual Review of Earth and Planetary Sciences 34(1): 127-155.

Carpenter, K. E., M. Abrar, G. Aeby, R. B. Aronson, S. Banks, A. Bruckner, A. Chiriboga, J. Cortés, J. C. Delbeek, L. DeVantier, G. J. Edgar, A. J. Edwards, D. Fenner, H. M. Guzmán, B. W. Hoeksema, G. Hodgson, O. Johan, W. Y. Licuanan, S. R. Livingstone, E. R. Lovell, J. A. Moore, D. O. Obura, D. Ochavillo, B. A. Polidoro, W. F. Precht, M. C. Quibilan, C. Reboton, Z. T. Richards, A. D. Rogers, J. Sanciangco, A. Sheppard, C. Sheppard, J. Smith, S. Stuart, E. Turak, J. E. N. Veron, C. Wallace, E. Weil and E. Wood (2008). “One-Third of Reef-Building Corals Face Elevated Extinction Risk from Climate Change and Local Impacts.” Science: 560-563.

Dishon, G., M. Grossowicz, M. Krom, G. Guy, D. F. Gruber and D. Tchernov (2020). “Evolutionary traits that enable scleractinian corals to survive mass extinction events.” Nature10(1): 3903.

IUCN (2020). The IUCN Red List of Threatened Species. Version 2020-1.