By Allen Collins, US National Museum


Placozoans are often described as the simplest known animals. Originally discovered living on the walls of aquaria in Vienna, Austria, it turns out that these minute animals (1-2 mm across) can readily be found in nearly any tropical or subtropical marine waters if one has the will and knowhow. While just a single species has been described, recent molecular work (involving specimens collected at the Bocas lab) has shown quite clearly that the phylum is comprised of numerous deeply diverged evolutionary lineages.

Placozoans are viewed as simple animals because they have just four different cell types. Their bodies are flattened, roundish, and transparent, with two distinct sides. Between the upper and lower surfaces is a syncitial layer of fibrous cells.

The upper layer has lipid inclusions which may serve as defensive repositories of unpalatable substances. The lower side on which the animal creeps though amoeba-like shape change and ciliary action and is probably responsible for digestion, which takes place underneath the animal. Little is known about placozoan diets in nature. In fact, almost all that is known about placozoans comes from observations of them in culture. They readily reproduce asexually and sex has only been inferred from genetic indications that recombination happens.