TINY OCEAN POWERHOUSES KEEP EARTH COOL

(A colony of salps with diver. Photo by Lars Plougmann, courtesy Wikimedia Commons)

Meet the salp, a small barrel-shaped organism that also happens to be an energy-efficient self-propelling wonder, whose waste material helps remove carbon dioxide from the upper ocean and the atmosphere. 

Technically salps are tunicates, interesting animals that straddle the vertberate-invertebrate divide. They possess primitive notochords (embryolike backbones) in their larval stages, which they lose in their adult forms. Hence salps and tunicates offer a notable example of the old biological adage: ontogeny recapitulates phylogeny. (Trans: the the embryonal development of an individual organism [ontogeny] follows the same path as its evolutionary history [phylogeny].) Though they look like jellyfish, salps are more closely related to you and me than to jellyfish.

In this week's Proceedings of the National Academy of Sciences (PNAS), researchers at WHOI and MIT report that mid-ocean-dwelling salps are capable of capturing and eating organisms of many different sizes. From the National Science Foundation:

"We had long thought that salps were about the most efficient filter-feeders in the ocean," said Larry Madin, WHOI Director of Research and one of the paper's authors. "But these results extend their impact down to the smallest available size fraction, showing they consume particles spanning four orders of magnitude in size. This is like eating everything from a mouse to a horse."

(A colony of connected salps. Photo: Kelly Sutherland and Larry Madin, WHOI)

Salps capture food particles, mostly phytoplankton, with an internal mucus filter net. Until now it was thought that included only particles larger than the 1.5-micron-wide holes in the mesh. But a mathematical model suggested salps could capture food particles smaller than that. When offered food particles of three sizes in the laboratory, salps ate prey both smaller and larger than the mesh openings—implying they eat the most numerous and widely distributed organisms in the ocean, including bacteria and the smallest phytoplankton.

"We found that more small particles were captured than expected," said lead author Kelly Sutherland. "When exposed to ocean-like particle concentrations, 80 percent of the particles that were captured were the smallest particles offered in the experiment."

The finding explains how salps, which exist either alone or in chains containing a hundred or more animals linked together, are able to survive in the open ocean where the supply of larger food particles is low. By filtering the smallest particles, they can survive where other grazers can't. The new findings also recognize the salps' enhanced role in the carbon cycle, since they consume the entire 'microbial loop' from small to large. 

(A colony of salps with divers. Photo from here.)

Salps excrete large, dense, carbon-containing fecal pellets. The larger and denser the pellets the faster they sink to the ocean bottom where they remain sequestered for decades or even centuries. Sequestering carbon on the seafloor frees the upper ocean to accumulate more carbon from the atmosphere.  

Useful poop.

In the video you can watch salps swimming and eating in rhythmic pulses—the tireless mobile heartbeats of the deep blue home—drawing seawater in through an opening at the front end, capturing food particles, rolling them into strands for passage through the gut for digestion. The amazing performance relies on a feat of bioengineering—the production of a nanometer-scale mucus net—the biomechanics of which we don't understand and can't recreate. 

(Video credit: Kelly Sutherland; Larry Madin; Roman Stocker, WHOI/MIT)

Kelly R. Sutherland, L.P. Madin, and R. Stocker. Filtration of submicrometer particles by pelagic tunicates. PNAS. DOI:10.1073/pnas.1003599107