Most people mistake bryozoans for seaweed--and it's no wonder. These tiny animals often colonize by branching out into shapes that look more like spaghetti than living animals.
Bryozoans are made up of colonies of individuals, called zooids. If you look at colonies through a magnifying glass, you can see openings in the geometric patterns they form. It is inside each of those openings that an individual zooid lives. Bryozoan colonies can get very large--containing about two million zooids and stretching a foot or more across.
Zooids are very tiny (less than one thirty-second of an inch), and come in shapes ranging from oval and box-like to vase-like and tubular. They eat using a food-snaring organ called the lophophore--an "O" or "U" shaped fold in the body surrounded by cilia-covered tentacles. These tentacles sweep the water, circulating water through the lophophore, and capturing the bacteria and plankton on which they feed.
Zooids live together in attached colonies that can be encrusting, like the white lacework of Membranipora, or branching, like the tufted erect bryozoan (this type looks just like a seaweed, except it is pink to tan in color). The "crust" is formed by a protective limestone covering secreted by the colony. Some colonies are only lightly encrusted, making them slightly stiff; others secrete a heavier crust, giving them a hard, crunchy covering. A few types of bryozoans secrete a flexible protein cuticle instead.
After a short larval stage, bryozoans attach themselves to a hard surface. They can be typically found on the stems of Irish moss.
Bryozoans feed on plankton and bacteria by sweeping the surrounding water with their lophophore. They are mainly eaten by nudibranchs (sea slugs) and sea spiders.