The bryozoans possess an indistinct head and a U-shaped gut, and secrete a protective covering. The class Phylactolaemata has no marine representatives and includes only 50 species world wide. Though this group is species poor, it is one of the most common groups of sessile invertebrates in the freshwaters of Canada. However, they generally go unnoticed since each individual zooid only reaches the size of 0.5 mm.
Bryozoans exist as aggregated colonies of zooids. Each zooid consists of a basal segment which is cuboidal in shape. This area houses the bulk of the animal including the gonads and viscera. It is cemented to the substrate, or to other zooids and is protected by a chitinous exoskeleton which is often transparent. The top portion of each zooid is called the lophophore, which is an extension of the body wall that is expanded into a horseshoe shape which bears many tentacles. The whole structure has the appearance of a leaf without the material between the veins. The function of the lophophore is for food acquisition.
The morphology of the colonies vary among the species. Some exist in clumps with most of the bases cemented in the same area, while others extend as strands into the water column.
Bryozoans, as a group, can endure a wide range of pH (though they seem to prefer alkaline waters) and low oxygen levels. They flourish in both lentic and lotic environments; and have adapted to levels of industrial waste and sewage that are hazardous to many other organisms.
Gas exchange occurs by simple diffusion across the body wall, aided by the large surface area on the lophophore. The coelomic fluid serves as the circulation system for food particles, wastes and internal transport of gases. Wastes are collected and disposed of in two separate ways. Coelomocytes found in the coelom capture and store wastes for ejection from the body. Nitrogenous wastes (e.g. ammonia or urea) simply diffuse through the body wall or are stored in the tissues there. There is a complete absence of nephridia which is decidedly odd for metazoans of this size.
In southern Canada, bryozoans have two or three generations per growing season. As the water temperature drops, these colonies become dormant or inactive. At this point colonies form statoblasts which are modified unopened zooid buds. They are cold resistant and survive the freezing waters when the rest of the zooid colony dies off. When the water warms past 8C, each statoblast germinates into a functional zooid with the ability to form between one and five newer buds. This immediate second generation of buds also has budding power. The new colony grows faster and larger as the temperature of the water rises.
Bryozoans are also capable of sexual reproduction. This usually occurs in the second summer generation. The sperm is formed in the funiculus (a thin tube of tissue that connects the middle section of the gut to the base of the colony) and released into the coelom. There they swim around for 1-2 weeks as they develop. Soon they contact the eggs that are formed on the inner lining of the coelom, fertilizing them, and the resulting larvae begin development. The larvae are released into the water column. If released into running waters, the current will carry the larvae far away from the parent colony, where they will settle on a substrate, attach and form a new colony of zooids.
Bryozoans filter feed on small particles with each zooid collecting its own food. There are tiny runways of cilia on either side of each tentacle of the lophophore which leads towards the mouth. The cilia on either side create a current, pushing water between the opened tentacles. The inner runway captures food particles and passes them from cilia to cilia into the open mouth. For larger particles the tentacles will actually fold inwards to capture the food and hold it to the mouth. No one is really sure what these animals actually eat. Algae, bacteria, rotifers, protozoa, and tiny worms have all been found digesting in the gut contents of dissected bryozoans. At other times, rotifers and algae have been observed as passing through the gut unharmed.
Colonies (called zooarium) are typically attached to solid surfaces, including rocks, plants, and even boat bottoms. Some colonies can grow to a weight of several kilograms in a single summer. A few species occur on every continent except Antarctica. The distributions of individual species are difficult to determine as there is much uncertainty concerning the taxonomy of many species.
Since bryozoans are sessile they are exposed to grazing by other invertebrates such as worms, insect larvae, snails and mites. Chironimid larvae often burrow into dead portions of the colony and often damage the attached, living zooids. Fish don't often feed on bryozoan colonies as some species have a chemical in their coelomic fluid that is toxic to vertebrates. The nutritional value of these animals is uncertain, as even many crustaceans avoid consuming bryozoans unless there is no alternate source of food.
Individual zooids only actually survive for a few weeks. The gut and lophophore then degenerate and turn brown. Nutritious bits are salvaged by surrounding colony members via phagocytosis. The rest of the dead cells containing stored wastes remain as a brown body. In some species a new zooid refills the space, expunging the brown body so it can move in. In other species the brown body remains but is compacted or absorbed into the gut of the new resident to be expelled as fecal matter.