Bryozoans are small invertebrates that expand from a party of one to a colony of thousands, which might encrust an entire kelp blade. The individual bryozoan—called a zooid—lives within a box-shaped compartment made of calcium carbonate and chitin, a material found in crab shells. Zooids are tiny, perhaps no taller than 1/32 of an inch.
The tiny larval bryozoan is a clamlike swimmer in a bivalve shell. Opening its shell like an umbrella, it parachutes down onto a clean kelp blade. Alert for chemical cues, the bryozoan tests the surface, then cements itself to the blade with a sticky glue. The youngster settles in place and changes to its adult form, a captive within its own shelled rectangular fort. Once established on the kelp, the lone settler begins to multiply. Budding off clones in neat rows, a colony fans out to frost the blade with a crust of the tiny animals. Bryozoan colonies are important food sources for some sea slugs and fish.
Bryozoans possess a unique feeding structure called a lophophore. The lophophore is a U-shaped or circular ring of ciliated tentacles used for filter feeding. Extending a crown of tentacles above its shell, the bryozoan flicks its tentacles through the water to catch bits of food.
Bryozoans are studied intensively by biochemical scientists. Producing a large variety of chemical compounds, some bryozoans may have medicinal use.
There are approximately 4,000 species in the phylum Bryozoa, making it one of the major phyla. Colony forms vary among the different species, ranging from flat, encrusted sheets to folded, leaflike bushes.
Bryozoans use tiny mobile pincers called "avicularia" to pluck off any settlers that land on them.
If a piece of a bryozoan colony breaks off, the piece can continue to grow and form a new colony.