Some of the world's weirdest and most fascinating creatures live in the deep sea, but frustratingly for marine biologists, these creatures are also some of the hardest to study. The remoteness, darkness, and high pressures of the deep make human observation difficult, and many deep-sea animals are too fragile to study in captivity. This means most people have never heard of these marvelous creatures, let alone understand how they make their living. Giant larvaceans are one of these amazing, but largely unknown, animals. The worm-like invertebrates float around the deep sea encased in a pair of large (up to 1 m diameter) mucous houses, with one tucked within the other. The outer house is thought to be a protective structure, while the smaller inner house is used for filter feeding on tiny plankton. However, the details of the larvacean filtering mechanism have been only superficially understood because of the immense technical limitations of studying a large and fragile ball of mucus drifting through the deep ocean.
A new study, led by Kakani Katija at the Monterey Bay Aquarium Research Institute, USA, used a sophisticated laser imaging system attached to a remotely operated vehicle to study wild giant larvaceans and their mucous homes at depths up to 400 m. By shining a laser at the animals while the robot carefully maneuvered around them, the edges, folds and other intricacies of the mucous house could be illuminated and photographed. Then, back on shore, the team used these images to generate a complete 3D model of the filter feeding mechanism. At sea, the team also used their underwater robot to release small amounts of fluorescent dye into the water, which allowed them to observe flow patterns throughout the mucous houses without otherwise disturbing the animals.
The team discovered that the larvacean house is an amazingly complex structure, especially considering it's made entirely from mucus and may be discarded and replaced daily. Sea water, pumped by the beating tail of the larvacean, enters the inner house via two long tubes that extend to the periphery of the outer house, passing through a protective pre-filter on the way. The flowing water is then divided into a symmetrical pair of food-concentrating filters, which allow water to exit while food particles are trapped and then passed directly to the larvacean's mouth. A complex series of valves, extra chambers and connecting threads further regulate water flow and internal pressure, enabling the mucous house to remain properly inflated.
Thanks to this cutting-edge imaging technology, the structure and function of the inner mucous house is now well understood. Getting good images of the thin mucous walls of the larger, outer larvacean house remains a challenge, however, and so the details of its structure and function remain unknown. One possibility is that the mucus serves as a coarse filter that prevents large particles from clogging the delicate inner feeding apparatus. The outer house may also deter predators by acting as a physical barrier, or even by functioning as a cloaking device that muffles the turbulence produced within the inner house during filter feeding. Finding the answer will require further advances in deep-sea robotics, laser-assisted video recording, and other technologies, but given the recent achievements of Katija and her crew, finding answers to these sorts of questions finally seems within the realm of possibility.
