Coral reefs, built over thousands of years out of the exoskeletons of tiny sessile animals called coral polyps, host some of the most diverse ecosystems on Earth—and the most threatened. Reef health depends on the symbiosis between the millimeter-sized polyps and single-celled algae called zooxanthellae; that relationship can be thrown out of balance by climate change, pollution, and a host of other factors.

Perhaps surprisingly, much remains unknown about the natural behaviors of corals and zooxanthellae because of the difficulty of imaging the minute organisms in situ. Optical microscopes with the requisite few-micron resolution can necessarily focus only a thin depth of field at a time. In the dynamic ocean environment, keeping an instrument in focus by mechanically repositioning lenses is infeasible.

Now Jules Jaffe and his colleagues at the Scripps Institution of Oceanography at the University of California, San Diego, have designed an underwater microscope based on a deformable, electrically tunable lens that can automatically refocus in less than 2.5 ms. The lens—like the microscope objectives, CCD camera, and other elements of the device—was purchased commercially. Lead authors Andrew Mullen and Tali Treibitz, both experienced divers, engineered the components into an instrument that’s practical to use underwater.

With their microscope, the researchers studied corals in a lab tank, in the Red Sea, and off the coast of Hawaii. Shown here is one of their lab images, taken under fluorescent illumination, of a single coral polyp; the red dots are individual zooxanthellae. In Hawaii, they monitored a coral bleaching event, in which corals expelled their zooxanthellae en masse and were subsequently colonized by harmful algae. And in the Red Sea, they observed a previously unrecorded behavior of coral polyps “kissing” their neighbors. (A. D. Mullen et al., Nat. Commun. 7, 12093, 2016.) Image credit: Jaffe Lab, Scripps Institution.