Many deep-sea creatures have evolved soft, gelatinous bodies and collect food using elaborate mucous filters, which makes it almost impossible to study their delicate structures.
However, a new study describes a unique laser-based system for constructing 3D models of diaphanous marine animals and the mucous structures they secrete.
“Mucus is ubiquitous in the ocean, and complex mucous structures are made by animals for feeding, health, and protection,” said Kakani Katija, MBARI Principal Engineer and lead author. “Now that we have a way to visualise these structures deep below the surface we can finally understand how they function and what roles they play in the ocean.”
For this study, the research team focused on larvaceans. Larvaceans are abundant throughout the world’s ocean basins and can range from less than 1cm to approximately 10cm in length. Some larger larvaceans create balloon-like mucous webs that can measure up to a metre across. Inside these outer filters are smaller inner filters that the animals use to feed on tiny particles and organisms.
Larvaceans contribute massively to the ocean’s role as a carbon sink, by removing huge quantities of carbon-rich food out of the surrounding water. When their mucous filters become clogged they release the mucus, which sinks to the seafloor. This also carries microplastics from the water column down to the seafloor.
In order to gather the necessary data, Katija 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 DeepPIV instrument projected a sheet of laser light that illuminated particles in the water. By recording the movement of these particles in video, the researchers could quantify tiny currents around marine animals as well as water flowing through their filters and their transparent bodies.
Combining 3D models of larvacean filters with observations of flow patterns through the filters, Katija and her collaborators were able to accurately identify the shape and function of various parts of the larvacean’s inner filter. 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.
“Now we have a technique for understanding the form of these complex structures, and how they function,” Katija explained. “No one has done in situ 3D reconstructions of mucous forms like this before. Among other things, we’re hoping to understand how larvaceans build and inflate these structures. This could help us design better 3D printers or build complex inflatable structures that could be used in a number of environments.”
Expanding on this work, members of the Bioinspiration Lab are experimenting with new 3D plenoptic (light field camera) imaging systems that can capture extremely precise information about the intensity, colour, and direction of light. 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.
“In this paper, we have demonstrated a new system that operates well with a variety of underwater vehicles and midwater organisms,” said Katija. “Now that we have a tool to study the mucus filtering systems found throughout the ocean, we can finally bring to light some of nature’s most complex structures.”
Media from top to bottom: This video is a first-of-its-kind 3D reconstruction of a giant larvacean, revealing the complex structure of its inner filter. It was made in collaboration with the Digital Life Project at the University of Massachusetts, 2020 MBARI; Reddish laser light from the DeepPIV system illuminates a giant larvacean, 2018 MBARI; Kakani Katija works in the ROV control room on MBARI’s research vessel Western Flyer as the DeepPIV system illuminates a giant larvacean, Kim Reisenbichler / 2015 MBARI.
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