New Laser System to Study Living Deep-Sea Creatures

Living in an essentially zero-gravity environment, many deep-sea animals have evolved soft, gelatinous bodies and collect food using elaborate mucous filters. Until now, studying these delicate structures has been virtually impossible. A new study describes a unique laser-based system for constructing 3D models of diaphanous marine animals and the mucous structures they secrete.

This close-up view of a “giant larvacean” (blue, tadpole-like animal) shows the inner filter of its complex, two-tiered feeding system. (Source: MBARI)

According to Kakani Katija, Monterey Bay Aquarium Research Institute principal engineer, “mucus is ubiquitous in the ocean, and complex mucous structures are made by animals for feeding, health, and protection. Now that we have a way to visualize these structures deep below the surface we can finally understand how they function and what roles they play in the ocean.”

In this study, the researchers focused on one of the most prolific mucus architects, deep-sea animals called larvaceans. They are abundant throughout the world’s ocean basins and range from less than one centimeter to about ten centimeters in length. So-called “giant” larvaceans create balloon-like mucous webs that can be up to a meter across. Inside these outer filters are smaller, fist-sized inner filters that the animals use to feed on tiny particles and organisms, ranging from less than a micron to a few millimeters in size.

These illustrations show how a sheet of laser light illuminates the inside of a larvacean filter, revealing internal structures. (Source: MBARI)

Despite their insubstantial bodies, larvaceans remove vast amounts of carbon-rich food out of the surrounding water. When their mucous filters become clogged the animals release the mucus, which sinks rapidly to the seafloor. This helps the ocean remove carbon dioxide from the atmosphere and carries microplastics from the water column down to the seafloor.

This is the first study to provide quantitative data about these mucus structures in the open ocean. To gather these data, Katija, who heads MBARI’s Bioinspiration Lab, worked with a team of engineers, scientists, and submersible pilots to develop an instrument called DeepPIV – PIV stands for particle imaging velocimetry. Mounted on a remotely-operated vehicle (ROV), the instrument projects a sheet of laser light that illuminates particles in the water. By recording the movement of these particles, researchers can quantify tiny currents around marine animals as well as water flowing through their filters and their transparent bodies.

This illustration shows MBARI’s MiniROV carrying a DeepPIV system that illuminates a larvacean filter (lit in red by a sheet of laser light) in the depths of the ocean. (Source: MBARI, K. Fulton-Bennett)

During field deployments of the DeepPIV system, Katija and her colleagues discovered that, as the ROV moved back and forth, the sheet of laser light revealed a series of cross sections through the transparent, gelatinous bodies and the mucus filters of giant larvaceans. By assembling a series of these cross-sectional images, the team was able to create three-dimensional reconstructions of individual larvaceans and their filters, much as radiologists do following a CAT scan of a human body.

ROV chief pilot Knute Brekke: “We were using a 12,000 pound robot to move a millimeter-thick laser sheet back and forth through a larvacean and its fist-sized mucous filter that was drifting hundreds of meters below the ocean surface.” Combining three-dimensional models of larvacean filters with observations of flow patterns through the filters, Katija and her collaborators were able, for the first time, to identify the shape and function of different parts of the larvacean’s inner filter.

The first ever 3D reconstruction of a giant larvacean is revealing the complex structure of its inner filter. The animation was made in collaboration with the Digital Life Project at the University of Massachusetts. (Source: MBARI)

Using 3D rendering software, they were able to virtually “fly through” the inner filter and study the flow of fluid and particles through different parts of the filter. “Among other things, we’re hoping to understand how larvaceans build and inflate these structures,” she continued. “This could help us design better 3D printers or build complex inflatable structures that could be used in a number of environments,” including underwater and in outer space.

Expanding on this work, members of the Bioinspiration Lab are experimenting with new 3D plenoptic imaging systems that can capture highly-precise information about the intensity, color, and direction of light in a scene. They are also collaborating on the development of new underwater robots that will be able to follow gelatinous animals through the water for hours or days at a time. (Source: MBARI, K. Fulton-Bennett)

Reference: K. Katija et al.: Revealing enigmatic mucus structures in the deep sea using DeepPIV, Nature, onlin June 3, 2020; DOI: 10.1038/s41586-020-2345-2

Link: Deep particle image velocimetry, Monterey Bay Aquarium Research Institute, Moss Landing, California, USA

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