Placozoa—the simplest multicelled animals ever described—live without muscles, nervous tissues, or digestive systems. To eat, these creeping bundles of cells spit out enzymes that externally digest algae floating in the water, which they then absorb through their transparent bodies. New research finds that when Placozoa eat, they do so in groups of as many as 105 individuals, forming “moving fronts” to collectively devour the algae in their path.
Little is known about the behavior of these flat, sand grain–size animals, even though they live in oceans all over the world. To learn more, researchers set up an aquarium that mimicked the habitat of one strain, complete with rocks and algae transported from their home in the Red Sea. Once the Placozoa were released into the tank, the researchers recorded their movements for a week.
As the amoebalike creatures began to chow down on their generous supply of algae, they congregated in groups of varying sizes (see video, above). Tight junctions among their bodies helped the Placozoa form pockets over their prey, where their digestive enzymes pooled together. They also ate on the go, traveling along the glass walls in unison and collectively consuming the algae in their path, the researchers reported last month in Frontiers in Ecology and Evolution.
Communal feasts might help Placozoa digest their food faster than they would solo, the researchers say. But they point out that social eating doesn’t necessarily mean the creatures are cooperative—freeloaders could also be joining groups and absorbing nutrients without contributing their own digestive enzymes. Nevertheless, the discovery that these simple organisms eat together suggests even the earliest animals may have had social tendencies.