Coral reefs can extend more than 1,000 miles, but they are made by coral polyps as small as one sixteenth of an inch.

These creatures don’t move about as adults. “Think upside down jellyfish stuck to a rock,” said Andrew D. Mullen, a graduate student at the Scripps Institution of Oceanography at the University of California, San Diego.

Mr. Mullen, Tali Treibitz and other American and Israeli scientists built a new microscope to study corals in nature while he and Dr. Treibitz, now running a lab at the University of Haifa, were both working in the lab of Jules S. Jaffe at Scripps.

As they report in Nature Communications, this is the first microscope made for use on the seafloor that is powerful enough to show details, almost as small as one micron, of living corals in their natural state.