The ocean remains vastly unexplored. Although researchers have completely mapped the surfaces of the Moon and Mars, they have resolved less than 10% of the ocean seafloor. Yet the ocean is the largest habitable space on Earth. The midwater—the vast region between the seafloor and the sunlit, shallower depths—makes up most of that living space and is a particularly difficult place to explore. Far out of reach of scuba divers, the cold, dark, and high-pressure environment requires specialized technology outfitted with cameras and other sensors. GPS does not work underwater, and options for tracking wildlife are much more limited than on the ocean surface or on land. Much of what we know about midwater animals is based on sparse samplings gathered in nets dragged behind ships. However, many species are too fragile to be studied that way: The soft, gelatinous tissues that make up many of their bodies are easily broken, as are the mucus structures some animals create.
One such soft-bodied, deep-sea creature that builds intricate living structures—essentially houses made out of mucus, sometimes called snot palaces—is the giant larvacean (genus Bathochordaeus), shown in figure 1. With a translucent, tadpole-like body barely 10 cm long, the animal primarily lives at depths down to 500 meters. The larvacean secretes its house from cells lining its head and then inflates it by beating its tail. The mucus house serves as a protective shield from passing predators and filters small organic particles suspended in the water column. The animal feeds on those particles, known as marine snow. By using its tail to pump water through its mucus filters, the larvacean concentrates the particles it feeds on. Eventually the particles clog the filter and the animal abandons its house, which sinks to the seafloor. The abandoned houses may account for one-third of the carbon transported to the seafloor in Monterey Bay.
A giant larvacean inside its mucus house, which consists of a coarse outer filter up to a meter in diameter and a fine, inner filter. The inset shows the tadpole-like animal, whose roughly 10 cm length is dominated by its tail, which beats to pump water and food particles through the house’s intricate cavities. The oblong-shaped blob in the upper left is an old, abandoned mucus house that has not yet sunk to the seafloor. (Courtesy of MBARI.)
A giant larvacean inside its mucus house, which consists of a coarse outer filter up to a meter in diameter and a fine, inner filter. The inset shows the tadpole-like animal, whose roughly 10 cm length is dominated by its tail, which beats to pump water and food particles through the house’s intricate cavities. The oblong-shaped blob in the upper left is an old, abandoned mucus house that has not yet sunk to the seafloor. (Courtesy of MBARI.)
Details of that process and the larvacean life cycle remain mysterious, including the way the animals construct their houses, why the houses have such a characteristic shape, how often the animals rebuild their houses, and how much water they filter. More research is needed to understand those critical processes, which are an important part of Earth’s carbon cycle.
Robots in the deep
Studying such fragile deep-sea animals requires observing and measuring them noninvasively. With Paul Roberts and Denis Klimov, two other members of our group at the Monterey Bay Aquarium Research Institute, we use tethered underwater robots, known as remotely operated vehicles (ROVs), which are ideal for the job. Shown in figure 2a, each is operated by pilots in a control room on a ship. For deep-sea research, an ROV can be outfitted with a host of cameras, sensors, collection containers, and manipulator arms that can operate equipment and conduct experiments several kilometers deep in the animals’ natural environment. Imaging provides views of animals underwater at the spatial and temporal scales needed for studies of their form and function, and careful selection of lighting in an otherwise dark environment reveals a plethora of information.
Tethered to a ship on the ocean surface, (a) this remotely operated vehicle (ROV) controls the position of the DeepPIV instrument mounted to its frame. Using the ROV camera and a laser extended by a half-meter rigid arm, the instrument collects video data for particle velocimetry experiments and three-dimensional reconstructions. The laser projects a sheet of light that illuminates the motion of suspended particles and the interior structure of any sea creature nearby. The camera captures the lit objects. (b) Extending from the ROV, the laser illuminates air bubbles in a laboratory tank. (Courtesy of MBARI.)
Tethered to a ship on the ocean surface, (a) this remotely operated vehicle (ROV) controls the position of the DeepPIV instrument mounted to its frame. Using the ROV camera and a laser extended by a half-meter rigid arm, the instrument collects video data for particle velocimetry experiments and three-dimensional reconstructions. The laser projects a sheet of light that illuminates the motion of suspended particles and the interior structure of any sea creature nearby. The camera captures the lit objects. (b) Extending from the ROV, the laser illuminates air bubbles in a laboratory tank. (Courtesy of MBARI.)
One specialized imaging and illumination system is the newly developed DeepPIV (particle image velocimetry) instrument, which uses a red laser projected into a 1-mm-thick sheet of light, as shown in figure 2b. A camera captures light scattered by objects in the laser sheet. DeepPIV is used in tandem with an ROV, which allows the pilot to carefully position instruments around an animal of interest without disturbing it. That’s no easy feat: The meters-long vehicle has to be positioned with subcentimeter accuracy, which is complicated by currents that affect both the ROV and the animal. The researchers can then either hold the instrument stationary relative to a target or scan the laser sheet through a volume. The two options yield very different perspectives of an animal.
The DeepPIV laser sheet reveals any objects or suspended particles present in the illuminated slice of water. The laser light even propagates inside the translucent bodies of many midwater animals, so the technique effectively reveals water flow as well as body structures. The image recorded by DeepPIV when the ROV is held stationary relative to an object can be analyzed with either particle tracking or particle image velocimetry—methods commonly used to characterize fluid flow in laboratory environments. Both methods visualize a two-dimensional slice of the animal and the surrounding particle field.
But if the laser sheet is moved through the target of interest while recording, a stack of images is generated that represent different planes in a volume. From that stack, researchers can create a 3D model in a fashion akin to a medical computed tomography (CT) or magnetic resonance imaging scan. Recording the image stack takes only a few seconds but requires moving the ROV with exquisite precision. Different pixel intensity levels distinguish the animal’s features in much the same way that a CT scan distinguishes bones and tissues from internal organs (see the article by John Boone and Cynthia McCollough on page 34 of this issue).
A look inside
Imaging results from DeepPIV offer a close look at water flow in and around the larvacean house. By zooming in on specific locations while the moving particles are illuminated by the laser, one can measure water-filtration rates and discern water-transport pathways through the house. A single giant larvacean can filter up to 76 liters of water per hour—an impressive achievement for an animal less than 10 cm long.
In comparison with other, larger animals in Monterey Bay, larvaceans have a profound impact on the movement of organic matter: Collectively, they filter all the water in their depth range every 13 days. Every time a larvacean abandons its house—which may happen as often as once a day—the captured particles and carbon sink to the ocean floor, where they provide nutrients for bottom dwellers or become part of the sediment. Moved far away from surface waters and the atmosphere, the carbon is effectively sequestered in the seafloor.
How is such a small invertebrate able to filter so much water? The answer may lie in the structure of its house. For a closer look, our group used DeepPIV to scan larvaceans inside their mucus structures at depths between 200 and 400 meters. The images and 3D visualizations, coupled with flow data collected by DeepPIV, allowed us to navigate through the house’s chambers and flutes to better appreciate how they function. We discovered that water enters the inner house through two inlet channels, and a coarse, mesh-like filter prevents large objects from entering. The larvacean’s tail pumps the water through the tail chamber and into the bifurcated inner chamber, and downstream tapered channels help transport food particles to the animal’s mouth. Although the 3D models provide an unprecedented view of structure and connectivity inside the mucus house, it’s still unclear exactly how the animal filters water at such a high rate.
Significant contributions to our work were made by other engineers, ROV pilots, and ship’s crew at MBARI.
Additional resources
Joost Daniels is a senior research engineering technician, Alana Sherman is an electrical engineering group lead, and Kakani Katija is a principal engineer, all at the Monterey Bay Aquarium Research Institute (MBARI) in Moss Landing, California.
