Shiny spheres of placozoans (Trichoplax) function in anti-predator defense

When individual placozoans, Trichoplax adhaerens, were fed to polyps of the hydroid Podocoryna carnea, the polyps became paralyzed in all 27 trials. In one instance the hydroid polyp died and in five instances a fragment of the placozoan survived. The time to recovery from paralysis varied with the relative size of the polyp and the placozoan. Placozoans were disassociated into separate cells and then reaggregated by centrifugation to produce pellets of tissue lacking the vesicles called shiny spheres, normally abundant in the upper epithelium. We fed pellets of two size classes to the hydroid polyps. When the placozoan pellet was smaller than the hydroid, paralysis was eliminated in nine of ten trials. When the placozoan pellet was comparable in size to the polyp, paralysis occurred but the recovery time was substantially reduced, compared with trials in which polyps were fed intact placozoans possessing shiny spheres. These results support a function of shiny spheres in anti-predator defense.