The cilia on each polyp, they found, are arranged in hexagonal units that keep ciliary movement streamlined, which helps explain how cilia can coordinate their movement to produce predictable swirls. “It demonstrates that coral skeletal architecture and living tissue are functionally integrated,” Pacherres said.
Combined, the studies recontextualize how ciliary movement, previously overlooked, gives corals some stability and buffers them against environmental change, he said.
The relationship between cilia and bleaching remains ambiguous, said Alderdice, who was not involved in the studies. But testing cilia under bleaching conditions without heat — by using red light, for example, which can also trigger bleaching — would help answer this question.
Playing Catch-Up
Scientists knew from the 2014 work that cilia help corals self-ventilate when water flow is low, said David Suggett, a marine biologist at King Abdullah University of Science and Technology who wasn’t involved in the research. “But until this point, we had been focusing on molecular and metabolic machinery” to understand how corals evolved to deal with low-oxygen conditions, he said. “We hadn’t really appreciated that there are these behavioral-physiological mechanisms at play.”
For Suggett, the research raises a critical question: What does this say about how corals will deal with future climates?
Sometimes, corals of a single reef don’t bleach evenly, with some individuals dying off while their neighbors persist. In new research led by Suggett’s colleague Tadd Truscott, marine biologists are finding that these patchy bleaching patterns are correlated with areas of reduced water flow — areas therefore receiving less oxygen.
Next, Kühl and Pacherres’ team wants to test corals under different conditions — especially under normal light-dark cycles — and to better understand the mechanism driving ciliary beating. Is it a molecular process that activates their movement? The physics of warmer seawater? A bit of both, or something else?
It’s only in the past decade that scientists have realized the role of deoxygenation in coral health, Suggett said. Ocean acidification took center stage as the primary concern for corals in the early 2000s. In hindsight, deoxygenation was a much bigger issue, he said. The new research suggests that deoxygenation, not just bleaching, is a fatal threat to corals resulting from a hotter climate.
“We’re playing massive catch-up,” Suggett said. “The amount of information we’re gathering quickly is demonstrating just what a problem for corals it is — so much so that we’re starting to really revisit long-standing paradigms of the role of other environmental factors, like temperature and light, where in fact, it could be oxygen that’s been the smoking gun all along.”
