A new type of underwater microscope that can produce high resolution images of living marine organisms in their natural environment is described in a study published online in Nature Communications. In this study, video footage of living corals is recorded with a previously unattainable resolution.

Underwater microscopy is challenging because precise and rapid imaging techniques are required in order to adapt to the intrinsic instabilities of the marine environment, such as water currents. Non-invasive imaging also requires a long working distance to avoid disturbing the imaged subjects. Although corals and algae can be imaged in laboratories, a lot of information is lost outside their natural environment: for instance, details about the organisms’ responses to acidity or temperature, and their interactions with neighbouring organisms. Previous resolution underwater has been up to about 20-50 micrometers.

Using a long working distance microscope objective and specific types of lenses and illumination, Andrew Mullen and colleagues develop a type of optical microscope that can reach a near micrometer-scale resolution. They demonstrate that the microscope can be handheld by divers or deployed for hours-long filming of biological systems on the sea floor.

The authors illustrate the capabilities of the microscope by filming how different corals compete for space, for example by quickly emitting gut-like filaments when threatened. They also quantify how, during bleaching, algae cover and colonize corals. This underwater microscope could help to shed light on ecological interactions in a changing oceanic environment.