Freshwater bryozoans or moss animals
Moss animals use a crown of tentacles to filter algae, detritus and microscopic organisms from the water.
Bryozoans occur in both still and running waters. Their presence indicates good water quality.
Most bryozoans are sessile and immobile, but some colonies are able to slowly glide on the substrate.
Individual zooids are about 0.5 mm long.
The life cycle includes both sexual and asexual reproduction. Sexually produced “larvae” undergo metamorphosis into adults, which grow the new colonies by budding clones of themselves. Moss animals also reproduce asexually by creating stratoblasts (capable of surviving freezing or desiccation). Stratoblasts are produced by adult zooids in the fall, after which the colony dies.
The basic body plan of bryozoans superficially resembles marine corals or freshwater hydra. In fact, the similarity is only apparent. They are unrelated and each one belongs to different phyla. What seems to be an individual animal is actually a colony of zooids (not polyps as in corals and hydra).
The bryozoan zooid is a complex animal with cell layers, tissues and organs. They are capable of independent feeding, digestion and reproduction. Zooids share certain tissues and fluids which unify the colony. Therefore it is impossible to distinguish precisely, where one zooid ends and the adjacent one begins.
Bryozoans use a food-gathering structure (lophophore) bearing the crown of ciliated tentacles to filter the water and trap algae, small particles of detritus, diatoms and other microscopic organisms. Mouth is situated at the base of tentacles. Food is processed in a stomach, intestine, and passes out of the body through anus. Digestive tract is U-shaped, with anus located below the ring of tentacles.
All members of the colony are genetically identical clones, produced by asexual reproduction. The colony grows and expands by budding new zooids from parental tissues. If a piece of bryozoan colony breaks off, the part (with at least one living zooid) drifts in the current until it encounters a solid object, to which the zooid may adhere. If conditions permit, zooid will continue to grow by creating buds and establishes a new colony.
Fission is similar process of reproduction, which occurs in gelatinous colonies. The body wall is constricted by muscular contractions and divided parts of the colony move slowly in different directions.
Another method of asexual reproduction is forming small encapsulated structures called statoblasts. Dormant statoblasts remain viable for long periods and can survive variable conditions of freshwater environments. These seed-like masses of cells can be transported across the land by animals, wind, or currents. When they reach appropriate environment (or suitable conditions return into the habitat), stratoblasts develop into zooids and start to form new colonies.
Bryozoans are also capable of sexual reproduction. All members of the colony are hermaphrodites producing both eggs and sperm.