Larvaceans are, with stiff competition, among the most bizarre inhabitants of the deep ocean, but we know little about them because their most interesting features are destroyed when brought to the surface. Scientists have now made a breakthrough in studying these creatures, something of increased significance since their role in combating climate change was revealed.
Larvaceans themselves are 1 to 10 centimeters (0.4 to 4 inches) long, but they construct mucus structures that can have a thousand times their volume. Sometimes known as houses, these structures focus the flow of up to 80 liters of water an hour to help the animals feed. Three years ago, Dr Bruce Robison of the Monterey Bay Aquarium Research Institute revealed that giant larvaceans known as Bathochordaeans play an important role in removing mid-ocean carbon that might otherwise eventually reach the atmosphere.
To learn more, we need to overcome the challenges of studying these “mucoid structures”. Now Robison has described in Nature a way to address this with a laser scanner mounted on a submarine that can create high-resolution 3D reconstructions of what marine biologists affectionately call snot balls.
Known as DeepPIV, the system emits a laser sheet that “penetrates cleanly through gelatinous and mucus structures,” the paper says. As the submersible moves backwards and forwards, it gives a cross-sectional view. These have been used to produce the first three-dimensional reconstructions of larvaceans' membranes.
The paper reports observing 71 giant larvaceans at depths between 100 and 400 meters (330 and 1,320 feet). For something so temporary and insubstantial, the structures are astonishingly complex, with the paper listing “upper cushion chambers, ramp valves, escape chambers, and… food-concentrating filters and supply chambers.”
The team mapped the movement of water through the structures and revealed giant larvacean's houses are not just bigger versions of their smaller cousins but also have inlet channels and oversized outer layers. Besides food collection, Robison thinks the houses protect giant larvaceans from stinging jellies and disguise their motions from predatory fish.
Larvaceans feed on everything from bacteria to tiny animal plankton in the water they filter. When their filters become clogged or damaged, they abandon them before building replacements. The discarded structures sink surprisingly quickly to the ocean floor, preventing other life forms from consuming them. The quantities of carbon removed in an individual house are tiny compared to a whale. However, larvaceans are so abundant, and they replace their filters so frequently, it amounts to a non-trivial counter-weight to the enriched carbon cycle caused by burning fossil fuels. One unanswered question is how these astonishing structures are built.
“Whereas a spider builds a complicated web one silky strand at a time, the house of a larvacean is extruded all at once as a rudiment and is then inflated,” the paper notes. We have no idea how they can make such intricate structures in this way.
If we can solve this, the authors think the extraordinary efficiency of larvacean structures could provide design ideas for pumping and filtration systems or even 3D printers
“Mucus is ubiquitous in the ocean, and complex mucous structures are made by animals for feeding, health, and protection,” said lead author Dr Kakani Katija noted in a statement. DeepPIV may help us understand many other species as well.