Corals Filmed Kissing, Fighting, Dying

A new microscopic imaging system documented the unusual behavior.

An innovative new microscopic imaging system named BUM (Benthic Underwater Microscope) has just revealed never-before-seen behaviors, including a kind of kissing and fighting among corals in their underwater environments.

The technology, described in the journal Nature Communications, is the first to capture underwater images of seafloor organisms with nearly microinch resolution, allowing scientists to study ocean organisms, such as coral, in their natural settings.

"Thus far we have only looked at corals, but the instrument has potential to be used in a variety of environments, including kelp forests and rocky reefs," co-lead author Andrew Mullen of the Scripps Institution of Oceanography told Discovery News.

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BUM has already been deployed in the Red Sea where it recorded corals competing with each other, releasing stringy filaments -- an extension of their gut -- as they fought over coveted underwater turf. Since coral do not fight with their own kind, the scientists think that they can chemically sense and identify different species, essentially enabling them to distinguish friend from foe.

The system also captured a new behavior named "polyp kissing," where individual coral polyps periodically embrace each other throughout the night. The researchers speculate that the polyps are doing this to exchange organic materials with each other, but the behavior is still largely a mystery.

The researchers next took BUM to Maui, Hawaii, to study bleached corals and the invasion of bleached corals by algae. Recently bleached corals are still alive, but in their weakened state can be quickly invaded and overgrown by algae.

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BUM documented a previously unreported honeycomb pattern of initial algal colonization and growth in areas between individual coral polyps during the bleaching. Co-author Jennifer Smith, also from Scripps, said the findings provide insight into a process called the "succession of algae," where small algae initially settle on the ridges between coral polyps and eventually smother the living tissue.

The idea for BUM was conceived by Scripps oceanographer Jules Jaffe, who co-led the study with Mullen. Tali Treibitz, a former Scripps researcher now at the University of Haifa's Charney School of Marine Science, developed and built the system with Mullen.

BUM is a two-part system that consists of an underwater computer with a diver interface tethered to a microscopic imaging unit. This is what permits marine subjects to be studied at microinch resolution. The instrument has a high magnification lens, a ring of focused LED lights for fast exposures, fluorescence imaging capabilities, and a flexible tunable lens, similar to the human eye, to change focus for viewing structures in 3-D.

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"To understand the evolution of the dynamic processes taking place in the ocean, we need to observe them at the appropriate scale," Jaffe explained.

"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," Treibitz added.

BUM has a maximum underwater resolution of about two micrometers (78.7 microinches). So far, the smallest subjects that the researchers have been able to see with the system are individual zooxanthellae, which are single-celled algae that live inside corals and provide coral with most of its energy. Zooxanthellae are 20 micrometers (787.4 microinches) in diameter. To give you an idea of how small that is, the width of a single strand of human hair is 100 micrometers (.00394 inches).

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Mullen explained that a diver is required to set up the instrument underwater. Once that work is completed, BUM can be left on the reef to image autonomously for approximately 8 hours.

Marine biologist Dimitri Deheyn, who did not work on the project, said that BUM is "a major advancement in our understanding of biological processes in situ." He explained that some key activities and mechanisms can only be observed when the imaging is immersed in the subject's environment. Previously, that was only possible in some cases for larger organisms.

Scripps scientist David Kline, says, "This tool will change our perspective of coral reef science and allow us to observe, measure and quantify processes such as coral-coral competition, coral bleaching, coral feeding and coral-algae interactions."

Jaffe and Mullen are now preparing BUM to take images of microscopic particles in the water near the coral's surface to better determine how the flow of water over corals allows them to exchange the necessary gases to breathe.

So far, the scientists have built two identical BUM systems, but they hope to construct more in the future.

Scripps Oceanography graduate student Andrew Mullen discusses the research being conducted using the Benthic Underwater Microscope (BUM), an instrument recently developed by the Jaffe Laboratory for Underwater Imaging at the Scripps Institution of Oceanography at UC San Diego. Credit: Scripps Institution of Oceanography at UC San Diego.

Coral polyps were imaged by BUM in waters off of Eilat, Israel. Coral reefs consist of individual polyps, which are tiny animals. Credit: Andrew Mullen

The researchers obtained a magnified view of the coral Stylophora. Credit: Andrew Mullen

The corals Stylophora and Platygyra fight by releasing string-like filaments at each other. Credit: Andrew Mullen

The coral Pocillopora damicornis was imaged under high magnification. The polyps that make up this and other corals are related to sea anemones and jellyfish. Credit: Andrew Mullen

BUM captured video of the coral Platygyra as it was exposed to four different stimuli. In each frame, the Platygyra is on the left side. The clips are as follows: top-left -- a small colony of the coral Galaxea brought from the researchers' lab; bottom left -- a mesh net filled with Artemia (brine shrimp); top right -- a loose colony of the coral Stylophora from the reef; and bottom right -- a loose foreign colony of Platygyra from the reef. Credit: Andrew Mullen.

Algae colonize and grow in areas between individual polyps during coral bleaching. Credit: Andrew Mullen

Corals can undergo bleaching when they are stressed, such as when they are adversely affected by changes to nutrients, temperature and/or light. If the stress is prolonged, the coral may eventually die. Credit: Andrew Mullen