Discarded feeding structures of larvaceans
contribute food to the deep seafloor
How do deep sea animals get enough to eat? Over the years, researchers have suggested a number of possible additional food sources for deep-sea organisms that might make up for the lack of food observed in sediment traps. Some researchers have theorized that additional food washes into the deep sea from shallow coastal areas or river plumes. Others have suggested that algal blooms or the sunken carcasses of whales and other large animals could provide the “missing” food. Marine biologists at the Monterey Bay Aquarium Research Institute (MBARI) addressed this question, at least for animals that live on the deep seafloor off the coast of Central California. After analyzing hundreds of hours of deep-sea video, MBARI scientist Bruce Robison and his colleagues found that "sinkers"—the cast-off mucus nets of small midwater animals called larvaceans—are a significant source of food for deep-sea organisms.
Larvaceans are small, tadpole-like animals related to the tunicates or "sea squirts" found in tide pools. The "giant larvacean" of the genus Bathochordaeus is about 50 millimeters (two inches) long, and is widely distributed in both the Pacific and Atlantic Oceans. Like most larvaceans, it feeds on tiny food particles in the surrounding seawater. Bathochordaeus lives inside two net-like mucus filters, which are collectively called its "house." The outer filter, which can be up to one meter (three feet) across, traps coarse particles in its mesh. The inner filter has slightly tighter weave, and traps small particles that the animals eats. The larvacean constantly pumps water through both filters, which typically become clogged after about 24 hours of use. At that point, the larvacean abandons its house and swims off to create a new one. The cast-off larvacean house deflates like a punctured parachute and sinks rapidly toward the seafloor, with its accumulation of debris and the tiny animals that colonize the mucus.
Scientist Robison had observed hundreds of these cast-off larvacean houses (called "sinkers") while exploring the waters of Monterey Bay using MBARI's remotely operated vehicles (ROVs). After seeing how common sinkers were, Robison wondered if they might be a significant source of food in the deep sea. As he explained, "When it became apparent that sinkers might be important carbon sources, we went around asking other oceanographers if they had seen these things [sinkers] in their sediment traps. It turns out that, although sinkers are relatively common, the odds of a sinker even hitting a sediment trap in the open ocean are extraordinarily small. In addition, sinkers often simply disintegrate when they contact a solid object. So either the scientists were not seeing sinkers at all, or if they did see them, it was in the form of a big glob in the bottom of the sediment trap, which they would typically throw out, assuming it was contamination."
To estimate how much carbon these sinkers might be delivering to the seafloor, Robison first needed to find out how common they were. For 10 years (from 1994-2003), he and his research team conducted monthly surveys of the midwater at ten different depths in Monterey Bay using MBARI's ROV Ventana. As part of their study, they pored over hundreds of hours of video taken during these surveys, counting both inhabited larvacean houses and sinkers themselves.
Knowing the volume of water that was observed during each midwater survey, Robison, Rob Sherlock, and Kim Reisenbichler were able to estimate the overall abundance of sinkers in Monterey Bay. Over the 10-year period, they observed an average of about four sinkers per day for every square meter of deep seafloor. In other words, a patch of seafloor the size of a large dinner plate might receive carbon from about 100 sinkers over a year's time.
To complete their study, the scientists also needed to know how much carbon each sinker transported to the seafloor. First they collected some sinkers, relying on MBARI's skilled ROV pilots to do the job. Catching bits of drifting mucus using a three-ton underwater robot was no easy feat. As Robison put it, "We collected more than a hundred of these things, and every one of them was a major challenge. About one in four attempts was successful. The patience and skill of those pilots was just amazing."
Back in the lab, the researchers carefully measured the amount of organic carbon in each sinker. Finally, by multiplying the number of sinkers reaching the seafloor times the average amount of carbon per sinker, they were able to estimate how much carbon the sinkers were carrying to the seafloor. To their surprise, Robison and his colleagues found that sinkers were delivering almost as much carbon as was the detritus being collected in sediment traps. They had found an additional food source that was more than adequate to feed all those hungry deep-sea animals.
These findings may seem esoteric, but they have global implications. The inability to account for all the carbon reaching the seafloor has been a major concern not only to oceanographers but also to some climate modelers who are trying to understand global warming. The global carbon cycle is like a complex jigsaw puzzle with many interlocking pieces. Robison's research may supply a significant piece of the puzzle that has long been missing.
MBARI participants: Bruce Robison, Kim Reisenbichler, and Rob Sherlock
- "Sinkers" provide missing piece in deep-sea puzzle
- Robison, B.H., K.R. Reisenbichler, and R.E.Sherlock (2005). Giant larvacean houses: Rapid carbon transport to the deep sea floor. Science, 308: 1,609-1,611