First Underwater Microscope Sees Coral "Kissing"

Researchers from Scripps Institution of Oceanography at the University of California San Diego have designed and built a diver-operated underwater microscope to study millimeter-scale processes as they naturally occur on the seafloor.

The research team observed coral turf wars, coral polyp “kissing,” and much more using the new microscopic technology.

Many important biological processes in the ocean take place at microscopic scales, but when scientists remove organisms from their native habitats to study them in the lab, much of the information and its context are lost. In a quest to overcome this challenge, Scripps oceanographer Jules Jaffe and his team developed the new microscope to image marine microorganisms in their natural settings without disturbing them.

The Benthic Underwater Microscope, or BUM, is an underwater computer with a diver interface tethered to a microscopic imaging unit to study marine subjects at nearly micron 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.

The microscope is the first instrument to image the seafloor at such small scales, and is capable of seeing features as small as single cells underwater.

To test the new technology’s ability to capture small-scale processes taking place underwater, the researchers used the imaging system to view millimeter-sized coral polyps off the coast of Israel in the Red Sea, and off Maui, Hawaii.

During experiments in the Red Sea, the researchers set up the microscope to capture the interactions of two corals of different species placed close to each other. The images revealed micro-scale processes in which corals emit string-like filaments that secrete enzymes from their stomach cavity to wage a chemical turf battle to destroy the tissue of other species in a competition for seafloor space. Yet, when the researchers placed corals of the same species next to each other, they did not eject these gastric fluids, as they can recognize friend versus foe.

The researchers also captured video of neighboring individual polyps on a single coral colony taking turns embracing one another, an unknown phenomenon the researchers call coral polyp “kissing.”

Next, the researchers deployed the instrument off Maui following one of the largest coral-bleaching events on record, which occurs when single-celled algae that live inside the coral polyp eject themselves during high ocean temperature events. Recently bleached corals are still alive, but in their weakened state can be rapidly invaded and overgrown by filamentous turf algae.

Using the microscope, the research team observed a previously unreported honeycomb pattern of initial algal colonization and growth in areas between the individual coral polyps during coral bleaching. These findings provide insight into a process that Scripps marine ecologist Jennifer Smith calls the “succession of algae” where small filamentous algae initially settle on the ridges between coral polyps and eventually smother the living tissue. The images showed that algae are able to actively overgrow living corals during a bleaching event.