Scientists at UC San Diego developed an underwater microscope that can film coral reefs at a near micrometer scale, a resolution never before reached. Watch the footage here. (Monica Akhtar/The Washington Post)

It can be easy to forget that coral are animals. Reefs look more like gorgeous marine gardens drifting in a breeze than they do colonies of invertebrates. Thanks to a new microscope — one that works under up to 100 feet of water, capturing tiny objects at high resolution despite the distortion of the sea — you can see these mysterious creatures in a whole new light.

In a study published Tuesday in Nature Communications, researchers from the Scripps Institution of Oceanography at the University of California at San Diego show off the stunning results of their latest foray into underwater microscopy: Polyps "kiss." Single-celled algae jump ship as the water warms — a devastating event known as coral bleaching. Disparate species of coral come to microscopic blows.

(Andrew Mullen)
(Andrew Mullen)

The Benthic Underwater Microscope (BUM) won't replace work done in labs. It can only capture high-resolution images of objects about a tenth of the width of human hair (about 10 micrometers), and more traditional microscopy can do a lot better than that. But while lab microscopes are great for slicing and staining samples to examine at them in incredible detail, BUM allows researchers to record real-world behavior on a smaller scale. Previous resolution underwater has been up to about 20 to 50 micrometers.

"Normally when you study a coral reef it can be difficult to observe the corals as active animals because the individual polyps are so small and they typically move slowly," Scripps PhD student Andrew Mullen, co-lead author of the study, said in a statement. "With the underwater microscope we can look at very small spatial scales and record activity over several hours."


Stylophora. (Mullen)

And the microscope — which can either be held by a human diver or placed underwater for several hours of time-lapse filming – has already recorded a behavior no one has ever reported before.

In one of the coral colonies they filmed overnight, neighboring polyps took turns embracing or "kissing" one another.

"We were definitely surprised by this interesting behavior, and we are not completely sure of its purpose," Mullen said in a statement. He suspects it may have something to do with the exchange of nutrients, but his team will need to do more research to get to the bottom of these smooches.

In addition to that coral love, the team documented some outright warfare. While filming two different coral species sitting close to each other in the Red Sea, they found that the individuals competed. When two polyps of the same species were in close quarters, everything was peachy keen. But when researchers moved another species into the neighborhood, the polyps sent out microscopic filaments from their guts to digest their competitors.

"We found that different colonies of the same species did not show any aggressive behavior when placed next to each other — the corals were able to determine friend from foe," Mullen said in a statement. "We think they might use some kind of chemical sensing to be able to recognize that their neighbor is of the same species."

Because the researchers moved species into artificially orchestrated territory wars, they can't be sure that this behavior occurs when reefs naturally host different kinds of coral in close proximity. They hope to spot this competition again in a totally natural setting.


Andrew Mullen positions the Benthic Underwater Microscope to study corals. (Jaffe Laboratory for Underwater Imaging/Scripps Institution of Oceanography, UC San Diego)

The team also used BUM to investigate the process of coral bleaching, an event tied to climate change. When ocean waters get too warm, coral reefs expel the algae (zooxanthellae) that usually live symbiotically with them. Some 93 percent of the Great Barrier Reef has been bleached, leaving coral bone-white and prone to starvation. Coral reefs in the United States are experiencing their third straight year of bleaching this summer. After bleaching, coral can be overgrown by filamentous turf algae, which compete with the coral instead of feeding it.

The researchers deployed BUM off the coast of Maui after one of the largest coral-bleaching events on record, observing corals that were bleached but not yet dead. They found that algae was growing in a honeycomb pattern between weakened polyps, "colonizing" the coral before it was dead by weaving around its living tissue — then eventually smothering it.

Like all of the observations reported in the new study, this glimpse into the aftermath of a mass bleaching event will require some follow-up — the researchers need more evidence to support any hypotheses on how or why the invading algae grow the way they do. But they hope that by sharing their new tool with the scientific community, they'll inspire other teams to go looking for the same phenomenon and investigate it further.

"This instrument is a part of a new trend in ocean research to bring the lab to the ocean, instead of bringing the ocean to the lab," study co-lead Tali Treibitz, a former Scripps postdoctoral researcher now at the University of Haifa, said in a statement.

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