Placozoa, meaning “flat animals,” are simple multicellular organisms. These tiny, blob-like creatures, often no larger than a grain of sand, were first discovered in 1883 by German zoologist Franz Eilhard Schulze. For decades, the phylum was thought to contain only Trichoplax adhaerens. However, recent DNA research has revealed greater genetic diversity, leading to additional species classifications since 2018. Their simplicity prompts scientists to investigate how such basic forms can exhibit complex biological functions.
Simple Body, Complex Actions
Placozoans have a simple body plan, lacking true organs, muscles, or a nervous system. Their transparent, flattened bodies, typically 1 millimeter across and 25 micrometers thick, consist primarily of two epithelial layers: a dorsal (upper) and a ventral (lower) surface. The dorsal layer has flattened, ciliated cover cells, some with shiny spheres possibly involved in predator defense.
The ventral surface, facing the substrate, has columnar cylinder cells with cilia, and gland cells. These cilia enable placozoans to glide along surfaces and change their shape, similar to an amoeba. Between these layers, star-shaped fiber cells form a network thought to coordinate movement.
Placozoans use external digestion for feeding on small algal cells and detritus. When encountering food, the ventral surface forms temporary pockets or folds around the particles. Digestive enzymes are then released into this temporary “external stomach” to break down food before nutrients are absorbed.
How Placozoa Reproduce
Placozoans primarily reproduce asexually. The most frequent method is binary fission, where the animal divides into two or sometimes three roughly equal parts. They can also reproduce by budding, where small fragments or multicellular “swarmers” break off from the main body, particularly from the upper surface, and develop into new individuals.
While asexual reproduction is prevalent, sexual reproduction has also been observed, though less frequently. This process involves the formation of eggs and sperm within the degenerating parent organism, typically a single egg and non-flagellated sperm. Following fertilization, which has not been directly documented in nature, embryonic development has been observed in laboratory settings.
Where Placozoa Live
Placozoans are free-living marine invertebrates found globally, predominantly in shallow, warm marine waters. They inhabit tropical and subtropical regions but have also been documented in temperate zones. These creatures are typically found clinging to solid surfaces within benthic habitats, such as rocks, coral reefs, shells, glass in aquariums, and mangrove roots.
Their diet primarily consists of small algal cells, including green algae like Chlorella and cryptomonads such as Cryptomonas, along with organic detritus. While known to inhabit the epibenthos, recent findings suggest they can also be found within seafloor sediments, expanding their ecological niche. This adaptability allows them to thrive in diverse coastal marine environments.
Placozoa and the Tree of Life
The phylogenetic position of Placozoa is important, as their simplicity offers insights into the early evolution of animal life. Many analyses place Placozoa as one of the earliest branching lineages within the animal kingdom (Metazoa), often considered a sister group to all other animals or to a clade including cnidarians and bilaterians. Studying these organisms can illuminate the origins of multicellularity and the development of body plans before more complex animal features emerged.
Placozoans possess a compact genome, which is among the smallest known for free-living animals. Despite this small genome and their morphological simplicity, their genetic makeup encodes a rich array of transcription factors and signaling pathways. These genes are commonly associated with diverse cell types and developmental processes in more complex animals, suggesting that placozoans may retain ancestral genomic features or possess cryptic cellular complexity yet to be fully understood.