A bryozoan colony is a group of tiny, genetically identical animals called zooids that live attached to one another and function as a single organism. Each colony starts from one founding individual and grows by cloning, eventually forming structures that can look like moss, lace, or even small branching trees on rocks, shells, and other underwater surfaces. Bryozoans are found in oceans worldwide and in many freshwater lakes and rivers, with roughly 6,000 living species described so far.
How a Colony Begins
Every bryozoan colony traces back to a single larva. After drifting briefly through the water, the larva settles on a hard surface and transforms into a founder zooid called an ancestrula. This ancestrula then begins producing genetically identical copies of itself through asexual budding, each new zooid physically connected to its neighbors. Over weeks and months, what started as a speck smaller than a pinhead can expand into a colony covering several square centimeters or, in some species, much more.
Sexual reproduction still plays a role, but its purpose is dispersal rather than colony growth. Many zooids within a colony produce eggs or sperm, and the resulting larvae swim off to start brand-new colonies elsewhere. The day-to-day expansion of an existing colony, though, is entirely asexual.
Zooids and Their Specialized Roles
Most bryozoan colonies contain two or more types of zooids, each shaped differently to handle a specific job. The most common type, the autozooid, does the feeding. It has a protective outer casing (the cystid) and, inside, a ring of tentacles called a lophophore surrounding a simple digestive tract. When feeding, the tentacles extend into the water and use tiny hair-like cilia to sweep microscopic food particles, mainly algae and bacteria, toward the mouth.
Other zooids in the colony are modified versions that have given up feeding to serve other purposes. Some are reproductive specialists that brood developing embryos. Others act as defensive structures, snapping shut like tiny jaws to discourage small organisms from settling on the colony’s surface. Still others reinforce the colony’s skeleton or help anchor it to its substrate. This division of labor is one of the features that makes bryozoan colonies more than simple clusters of clones. The colony behaves, in many ways, like a single coordinated organism.
Colony Shapes and Growth Forms
Bryozoan colonies come in several distinct growth forms, and the shape a colony takes depends on both its species and its environment.
- Encrusting colonies grow as thin, sheet-like layers over rocks, shells, or seaweed. These are the most common form and often look like a pale, lacy crust. If you’ve ever turned over a shell at the beach and noticed a fine, patterned film on its underside, there’s a good chance it was a bryozoan.
- Arborescent (tree-like) colonies branch upward into the water column, increasing their exposure to food particles carried by currents. These can resemble small coral fans or bushy tufts.
- Gelatinous colonies lack a rigid skeleton entirely. Instead of calcified walls, their zooids sit in a soft, flexible matrix that gives the colony a blobby or jelly-like appearance. Some freshwater species form translucent, rubbery masses on submerged logs or dock pilings.
Bryozoans vs. Corals
Because both bryozoans and corals build colonies of small individual animals, they’re easy to confuse. The differences, however, are significant. Individual bryozoan zooids are far smaller than coral polyps. They’re also anatomically more complex: bryozoans have a one-way digestive tract (food in one end, waste out the other, like humans), while corals have a single opening that serves as both mouth and waste outlet. Corals capture small prey with stinging cells and often rely on symbiotic algae living in their tissues for energy. Bryozoans are strict filter feeders, sifting bacteria and organic particles from the water with their tentacle crowns.
Freshwater Colonies and Survival Strategies
While the vast majority of bryozoan species are marine, a notable minority live in freshwater. These freshwater species face challenges that ocean bryozoans don’t: lakes can freeze solid in winter, ponds can dry up in summer, and conditions can shift dramatically between seasons.
To cope, freshwater bryozoans produce statoblasts, small seed-like capsules packed with cells that can survive extreme conditions. Research on the freshwater species Cristatella mucedo has shown that statoblasts tolerate near-complete drying out and subzero temperatures. Internal ice formation doesn’t kill them; they simply resume development when conditions improve. Inside the dried statoblast, the cellular contents form a glass-like solid state (a process called vitrification) that protects the living material until water returns. This makes statoblasts remarkably durable, capable of hitching rides on birds’ feet or being carried by wind to colonize new water bodies.
A Deep Fossil Record
Bryozoans have been around for a very long time. Six major groups with advanced zooid specialization already appear in early Ordovician rocks, roughly 480 million years old, which long suggested an even earlier origin. In 2021, a team described exquisitely preserved specimens of a tiny modular fossil called Protomelission gatehousei from early Cambrian deposits in Australia and South China. This discovery pushed the origin of the phylum back by about 35 million years, placing it in Cambrian Age 3, around 521 million years ago. That timeline aligns bryozoans with the explosive diversification of nearly all other animal groups with hard skeletons and matches predictions from molecular clock studies that had long pointed to a Cambrian origin.
Practical Impact on Infrastructure
Bryozoan colonies aren’t just a curiosity for marine biologists. In freshwater settings, they can become a costly nuisance. Colonies growing inside water treatment plants in the UK blocked the small nozzles and pipes in filter beds, leading to uneven cleaning cycles and reduced water quality. In one case, built-up pressure from blocked drains actually blew out a filter bed floor. Between 2005 and 2009, four UK water companies spent £1.49 million refurbishing 35 fouled filter beds. Freshwater bryozoans have also clogged cooling water intakes at power plants, blocked irrigation systems, and fouled aquaculture net cages.
Beyond the physical blockages, bryozoan colonies inside water infrastructure can harbor pathogenic bacteria, creating a potential public health concern that makes removal more than just a maintenance issue.