Placozoa

Abstract

The most primitive metazoan animal phylum Placozoa presently harbors a single named species, the enigmatic Trichoplax adhaerens. In 1883, the German zoologist Franz Eilhard Schulze discovered this microscopic marine animal on the glass walls of a seawater aquarium at the University of Graz, Austria (Schulze 1883). The animal, usually measuring less than 5 mm in diameter and less than 20 μm in thickness, looked like an irregular hairy plate sticking to the glass surface (Fig. 5.1) and was thus named Trichoplax adhaerens (Greek for “sticky hairy plate”) (see Schierwater 2005 for historical overview). Recent genetic analysis of placozoan specimens from different ocean waters around the world, including the Mediterranean Sea, revealed the presence of several cryptic species (Eitel et al. 2013), i.e., species, which are morphologically cum grano salis undistinguishable. The real placozoan biodiversity is estimated to include several dozen genetically, developmentally, and ecologically distinguishable species.