A bryozoan is a tiny aquatic animal that lives in colonies, sometimes containing thousands of identical units called zooids. Each zooid is typically less than a millimeter long, but together they build structures that can look like crusty patches on rocks, branching coral-like fans, or even basketball-sized gelatinous blobs. There are roughly 6,000 known living species, found in oceans, lakes, and rivers worldwide.
Colonial Animals, Not Plants
Bryozoans are often mistaken for plants, corals, or algae because they don’t move and they grow on surfaces like rocks, shells, and submerged wood. Their name literally means “moss animal,” a nod to the fuzzy, plant-like appearance of some species. But each zooid in a colony is a distinct animal with its own digestive system, muscles, and nervous tissue.
A colony starts from a single zooid and expands through budding, with new zooids cloning off existing ones. The result is a connected structure where individual animals share a common body wall but each maintain their own feeding and digestive organs. Some colonies stay flat, encrusting a surface like a thin sheet. Others grow upright in branching, fan-like forms. Freshwater species can produce soft, gelatinous masses with no hard skeleton at all.
How a Single Zooid Works
Each zooid has two main parts: a protective outer casing (often hardened with calcium carbonate) and a soft inner body that handles feeding and digestion. The soft body includes a ring of ciliated tentacles called a lophophore, which is the zooid’s feeding apparatus. When conditions are safe, the lophophore extends out of the casing like a tiny flower. Hair-like cilia on the tentacles beat in coordinated waves, creating a current that pulls water inward and traps microscopic food particles, mainly microalgae and other organic debris floating in the water column.
Food captured by the tentacles is directed down toward the mouth at the center of the ring, then passes through a U-shaped gut that loops back so the waste exits near the tentacle crown rather than deep inside the colony. This keeps things tidy in a densely packed group of animals. When threatened, the entire soft body retracts into the protective casing in a fraction of a second.
Specialized Roles Within a Colony
Not every zooid in a bryozoan colony does the same job. Most colonies show a degree of polymorphism, meaning some zooids have evolved specialized shapes for specific tasks. The standard feeding zooids (autozooids) handle nutrition for the colony, but modified zooids may serve as defensive structures with snapping jaw-like appendages, or as brood chambers that protect developing embryos. This division of labor is one reason bryozoan colonies can be so successful. It resembles the way cells in your body specialize into different tissues, except here each “cell” is a complete animal.
Where Bryozoans Live
The vast majority of bryozoan species are marine, living in oceans from shallow tidal zones to deep-sea floors. They attach to hard surfaces: rocks, shells, pier pilings, kelp blades, even the hulls of ships. Marine species typically build skeletons from calcium carbonate, using either calcite or aragonite (two crystal forms of the same mineral), and some species use both. These mineralized skeletons give marine bryozoan colonies a stony or coral-like texture.
A smaller number of species live in freshwater lakes, ponds, and rivers. These freshwater bryozoans belong to a separate class called Phylactolaemata, and they look dramatically different from their ocean relatives. Instead of hard, crusty colonies, freshwater species often produce soft, jelly-like masses.
The Mysterious Freshwater “Blob”
If you’ve ever seen news stories about a strange, slimy blob pulled from a lake, there’s a good chance it was a freshwater bryozoan called Pectinatella magnifica. This species forms large, gelatinous, transparent-to-brownish-purple colonies that can reach 30 to 50 centimeters in diameter. Living colonies have a bubbly surface texture, with rosette-like clusters of zooids visible across the surface. Each zooid sports a lophophore with 50 to 84 tentacles and a distinctive patch of red pigment around its mouth.
Pectinatella magnifica attaches to rocks, submerged wood, aquatic vegetation, freshwater mussels, and even other bryozoans. It tolerates a wide range of conditions, thriving in water from 4 to 32°C. In autumn, dying colonies lose their structure and become smooth, slimy, transparent masses of gelatin peppered with small black survival capsules called statoblasts. These statoblasts are roughly circular with a ring of hooked spines around the edge, designed to latch onto surfaces or passing animals and survive winter conditions until they can sprout a new colony the following year.
How Bryozoans Reproduce
Bryozoan colonies grow primarily through asexual budding. One zooid produces a genetically identical neighbor, which produces another, and so on. This is how a colony expands from a single founder into a structure containing hundreds or thousands of zooids. Many species also reproduce sexually, releasing larvae that swim briefly before settling on a surface and becoming the founding zooid of a new colony.
Freshwater species have an additional trick: statoblasts. These are tough, seed-like capsules that can withstand drying, freezing, and other harsh conditions. When the parent colony dies in cold weather, statoblasts persist and germinate when conditions improve. Some statoblasts have hooks or spines that help them hitch rides on animals or float to new locations, which is one way freshwater bryozoans colonize isolated ponds and lakes.
An Ancient Lineage
Bryozoans have a deep evolutionary history. For decades, the oldest confirmed fossils came from early Ordovician rocks, roughly 480 million years old. But a 2021 study published in Nature identified a tiny, erect, branching fossil called Protomelission gatehousei from early Cambrian deposits in Australia and South China, pushing the origin of the group back by about 35 million years. This places bryozoans alongside most other major animal phyla during the Cambrian explosion, roughly 515 million years ago.
Their big moment came later, during the Great Ordovician Biodiversification Event, when ocean chemistry shifted to favor calcite formation and hard seafloor surfaces became more common. Bryozoan colonies grew larger, mineralized more heavily, and diversified rapidly. By that point, six major orders with advanced polymorphism were already present, suggesting the group had been quietly evolving for tens of millions of years before conditions allowed them to leave a robust fossil record. Today, bryozoans remain one of the most diverse groups of colonial animals in the ocean, even if most people have never heard of them.