Placozoans are among the most structurally simple multicellular animals known to science, often described as the “simplest animals on Earth”. These tiny, flat, disk-like organisms, typically measuring about 0.55 mm to 3 mm in diameter and 15 to 25 micrometers thick, possess an amorphous shape that constantly changes as they move. Superficially resembling large amoebas, placozoans are free-living marine invertebrates found globally in tropical and subtropical waters. Despite their unassuming appearance, they represent a fundamental form of animal life, holding clues to the early evolution of multicellularity.
A Simple Body Plan
The physical structure of placozoans is straightforward. They possess no organs, no nervous system, no muscles, and no digestive tract. Their body is composed of only a few thousand cells, organized into a flattened, two-layered structure. This “sandwich-like” body plan consists of an upper epithelium facing the open water and a lower epithelium facing the substrate.
Between these two epithelial layers lies a loose network of fiber cells, which are arranged in three layers. The lower epithelium contains ciliated cells, responsible for movement, and gland cells, involved in digestion. The upper epithelium consists of different cell types, including “shiny spheres.” Placozoans lack any fixed symmetry, meaning there is no distinct front or back, left or right, or consistent top or bottom side.
Feeding and Reproduction
Placozoans move by two primary mechanisms: gliding along surfaces using the cilia on their lower surface and by constantly changing their overall body shape, similar to an amoeba. Their movement is not accompanied by changes in body shape when relying on ciliary beats, but shape changes are a mode of locomotion on their own. This allows them to explore their environment and locate food sources. Their feeding method is unique among animals, involving external digestion.
When a placozoan encounters a food source, such as algae or organic detritus, it crawls over it and forms a temporary digestive cavity by contracting part of its ventral surface. Gland cells within the lower epithelium then secrete digestive enzymes directly onto the food. The liquefied nutrients are subsequently absorbed by the cells of the lower epithelium. Multiple placozoans may aggregate to form clusters for feeding on dense food substrates.
Reproduction in placozoans primarily occurs asexually through two methods: budding and fission. In budding, small individuals detach from the parent organism. Fission involves the parent body constricting in the center and splitting into two roughly equal-sized daughter individuals. While sexual reproduction is thought to occur, it is rarely observed in laboratory settings. Evidence for sexual reproduction includes the development of egg-like cells.
A Window into Early Animal Evolution
Placozoans hold scientific importance due to their position as basal metazoans, representing one of the earliest branches of the animal kingdom. The most studied species, Trichoplax adhaerens, was first discovered in 1883. Its simple body plan has led scientists to consider it a model organism for understanding the transition from single-celled organisms to multicellular animals.
Despite their morphological simplicity, the genome of Trichoplax adhaerens presents a paradox. Its mitochondrial DNA, at 43,079 base pairs, is the largest known among metazoans, more than double the typical size, due to numerous noncoding regions and introns. Its nuclear genome, though compact at 98 million base pairs, encodes a rich array of genes, including transcription factors and signaling pathway genes, similar to those found in more complex animals, including humans.
This genetic complexity, contrasted with their basic body organization, makes placozoans an important subject for studying the origins of multicellularity and the evolution of fundamental animal traits. Their primitive features and the presence of genes associated with advanced animal development provide insights into how complex body plans and specialized cell types may have evolved from simpler ancestral forms. The study of placozoans continues to offer a unique perspective on the foundational steps in animal evolution.