From Science:
When it comes to churning up the world's oceans, Mastigias jellyfish are quite the little blenders. New research suggests that large groups of the small, placid creatures–along with all of the sea's other motile beings–can mix as much heat, gases, and nutrients through the water column as the winds and tides do.
On the surface, the sea is a roiling mass. But dip 100 meters below and the water is calm. How, then, do the world's oceans distribute heat and food throughout their depths? Currents driven by salinity and temperature differences can transport a lot. But another part of the answer comes from an idea conceived by the grandson of Charles Darwin. About a half-century ago, the famed naturalist's descendant–also named Charles–proposed that a body moving through a fluid would tend to drag some of that fluid with it. Applied to the oceans, the hypothesis means that the churning action created when aquatic creatures swim–even the smallest and slowest–might stir a significant amount of water.
Most scientists have remained skeptical, however, arguing that small marine creatures in particular could not overcome water's viscosity enough to circulate much of anything. Now, it turns out, the idea first posed by Darwin's grandson may be right.
More here.