A Plate of Hairs
In the warm, shallow waters of the Red Sea near Eilat, you can find the phylum Placozoa, arguably the simplest animal on Earth. The most studied species, Trichoplax adhaerens, has a name that translates to "hairy sticky plate," which accurately describes its appearance and behavior. First discovered by German zoologist Franz Eilhard Schulze in 1883 inside a European saltwater aquarium, this organism baffled scientists for decades. Some thought it was the larval stage of a jellyfish, but in the 1970s it was confirmed to be a unique adult animal and placed in its own phylum.
A Placozoan is a flat, disk-shaped creature, often only a millimeter or two in diameter and thinner than a sheet of paper. Its body is a simple three-layered sandwich of cells. An upper layer provides protection, while a ciliated lower layer allows for movement and feeding. In between lies a loose network of fiber cells. It has no front or back, no organs, no muscles, and no neurons. Despite this, its cells show a division of labor, an important characteristic of all animals. The entire organism consists of only a few thousand cells of just six to eight different types. For comparison, a fruit fly has about 50 cell types, and humans have hundreds.
Deceptively Simple Genetics
Despite its physical simplicity, the genome of Trichoplax is surprisingly complex. Its nuclear genome contains about 98 million base pairs and an estimated 11,500 genes. This genetic toolkit includes many genes that, in more complex animals, are responsible for forming organs and tissues. The presence of these genes suggests that the simple body plan of Placozoa might be a case of secondary simplification from a more complex ancestor, though this is still debated. Their mitochondrial genome is also unusual; at 43,079 base pairs, it is the largest known among animals.
Feeding is straightforward. The animal glides over mats of algae and bacteria using the cilia on its ventral surface. When it finds a food source, it lowers its body to form a temporary external stomach, secreting digestive enzymes from gland cells to break down the food before absorbing the nutrients. This process is known as phagocytosis.
Reproduction is primarily asexual, with the animal simply splitting into two smaller, genetically identical individuals through binary fission. They can also reproduce by budding off small "swarmers." Evidence of sexual reproduction exists in their genetics, but the process has not been directly observed in the wild.