New underwater microscope provides ringside seat to coral turf wars

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Marine researcher studying coral micro-organisms with the Benthic Underwater Microscope developed by scientists at Scripps Institution of Oceanography, UC San Diego

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Marine scientists are getting an up-close view of never-before-seen coral sea life thanks to a newly-developed microscopic imaging system built for underwater use. The diver-operated microscope from Scripps Institution of Oceanography at the University of California San Diego (UCSD) allows researchers to observe millimeter-scale interactions and processes as they happen live in the wild, including "kissing" behavior between coral polyps and battles over turf.

While coral reefs can span up to hundreds of kilometers, they're built by tiny individual polyps around one millimeter in size, which makes studying these small life forms and the processes that occur at the micro level vital for monitoring the health of larger ecosystems like reefs. But when scientists bring these organisms out of the ocean and into the lab to study them, the larger context is lost and the information gleaned is limited, while fragile features might be destroyed.

In response, Scripps oceanographer Jules Jaffe and his team developed the Benthic Underwater Microscope (BUM). It's the first instrument to capture underwater images of seafloor organisms with near micrometer resolution which can study reef microorganisms in their native habitat without disturbing them.

A cousin to a previously developed plankton imaging system, the BUM consists of an underwater computer/diver interface attached to a microscopic imaging unit that's able to study organisms at a maximum resolution of around two micrometers. It includes a high-magnification lens, a ring of bright LEDs that focus on a single point for fast exposures, and fluorescent imaging capabilities. A flexible, electrically tunable lens brings subjects into focus quickly and precisely when viewing in 3-D, changing shape in a similar manner to the human eye.

"With the current housings, the microscope can be taken to depths below 100 ft (30 m), however normal dive operations only require the system to be taken to depths of around 30 ft (9 m)," Andrew Mullen, Scripps research team member, told Gizmag. "Additionally the instrument's depth rating can easily be further increased by using thicker ports."

So far the BUM has enabled several new discoveries. "We have used the instrument thus far primarily to study corals," says Mullen. "We have looked at coral polyp behavior during competition, coordinated behavior between polyps of the same colony, and the colonization of bleached corals by algae."

In the Red Sea off the coast of Israel, the BUM captured the interactions between two separate coral species placed near each other. In a competition for seafloor real estate, the micro-images recorded string-like filaments from the coral secreting harsh chemical enzymes from their stomach cavity to destroy the tissue of opposing species. When coral of the same species where paired close to each other, they didn't show the same aggressive behaviors.

While studying the aftermath of one of the largest coral-bleaching events on record off the coast of Maui, the BUM discovered a previously unreported honeycomb pattern of algal colonization and growth between individual coral polyps. Single cell algae that live inside polyps and provide the coral with most of its energy, eject themselves during high ocean temperatures, which leads to the bleaching. The filamentous turf algae that subsequently colonize the coral settle on the ridges between the polyps, which eventually smothers the living tissue.

The BUM system can also save images for later study, which is key for studying the activity of slow moving polyps. "It can record images and video," says Mullen. "It can also be left on the reef for up to about eight hours to make time-series recordings."

Coral "polyp kissing" was observed in an overnight time-lapse recording, and found neighboring polyps periodically embrace throughout the night. The researchers were surprised at the behavior and unsure of the purpose, but believe it's a way for the polyps to exchange organic material.

Mullen and Jaffe will next use the BUM to capture images of microscopic particles in the water near the coral's surface in an attemot to understand how water flows allow them to exchange the gases necessary for breathing.

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