Feeding and Symbionts
Trichoplax adhaerens feeds on small algae, particularly on green algae (Chlorophyta) of the genus Chlorella, cryptomonads (Cryptophyta) of the genera Cryptomonas and Rhodomonas, and blue-green bacteria (Cyanobacteria) such as Phormidium inundatum, but also on detritus from other organisms. In feeding, one or several small pockets form around particles of nutrients on the ventral side, into which digestive enzymes are released by the gland cells; the organisms thus develop a temporary "external stomach", so to speak. The enclosed nutrients are then taken up by pinocytosis ("cell-drinking") by the ciliated cells located on the ventral surface.
Entire single-celled organisms can also be ingested through the upper epitheloid (that is, the "dorsal surface" of the animal). This mode of feeding could be unique in the animal kingdom: the particles, collected in a slime layer, are drawn through the intercellular gaps (cellular interstices) of the epitheloid by the fiber cells and then digested by phagocytosis ("cell-eating"). Such "collecting" of nutrient particles through an intact tegument is only possible because some "insulating" elements (specifically, a basal lamina under the epitheloid and certain types of cell-cell junctions) are not present in the Placozoa.
Not all bacteria in the interior of Placozoa are digested as food: in the endoplasmic reticulum, an organelle of the fiber syncytium, bacteria are frequently found that appear to live in symbiosis with Trichoplax adhaerens. In particular it has been proposed that there is a ricketsial endosymbiont.
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