The disposable mansion
In the midwater depths of the world's oceans, a tadpole-shaped animal builds and discards a new, elaborate house for itself on a continuous cycle. This creature is a larvacean, a type of tunicate. Despite their simple appearance, larvaceans are chordates, placing them among the closest living invertebrate relatives of vertebrates like humans. Species like Oikopleura dioica keep their tadpole-like shape throughout their lives. The "giant" larvacean, in the genus Bathochordaeus, is only about 10 centimeters long. Its disposable home, however, is more than a meter across.
The house is not built from collected materials but is secreted entirely from the animal's own body as mucus. The larvacean inflates this secreted blob into a filtering structure, a process that can take less than an hour. This "snot palace" has two main parts: a coarse outer filter to block large debris and a fine-meshed inner filter to concentrate food particles of the correct size. The animal lives inside this structure, beating its tail to pump vast amounts of seawater through the filters. A single giant larvacean can filter up to 76.2 liters per hour. When the filters become clogged, typically after just a few hours for smaller species, the larvacean abandons the entire structure and secretes a new one.
A carbon-sequestering snowfall
The constant cycle of building and abandoning these mucus houses has a significant effect on the ocean's carbon cycle. Discarded houses are clogged with particles of organic matter, including carbon-rich food. These abandoned structures collapse and sink rapidly, becoming a component of what is known as "marine snow", a continuous shower of organic detritus falling from the upper waters to the deep ocean.
The sinking rate of these houses is remarkably fast, estimated at around 800 meters (2,600 feet) per day. This speed means the carbon-rich material is quickly transported to the deep seafloor, bypassing microbial degradation that would otherwise occur in the upper water column. This process helps the ocean's biological carbon pump, which moves carbon from the atmosphere into long-term storage in the deep sea. In regions like Monterey Bay, giant larvaceans may be responsible for transporting as much as one-third of the total carbon that reaches the deep seafloor. The fecal pellets of larvaceans also sink rapidly, further contributing to this carbon transport. Unfortunately, the highly efficient filters also trap microplastics, which are then transported to the deep sea along with the discarded houses.