Bryozoans are small marine and freshwater animals that live on submerged surfaces. They are modular in structure, composed of many similar connecting zooids. Traditionally, such a group of zooids is called a "colony." Each zooid has its independent food gathering structure, mouth, gut, muscles, nervous and reproductive systems (Figure 1). Adjacent zooids share certain tissues and fluids which unify the colony physiologically.
Figure 1: Colony types: (A) Plumatella; (B) Fredericella; (C) Lophopodella
Of the three classes of ectoproct bryozoans, by far the best known is Gymnolaemata, with several thousand species, mostly marine, many documented in an extensive fossil record. In contrast, the Class Phylactolaemata has only about 85 named species, all extant and found exclusively in fresh water. The discussion that follows focuses mainly on the Phylactolaemata.
Among phylactolaemate bryozoans, the individuality of zooids is blurred to the extent that it is impossible to distinguish precisely where one zooid stops and the adjacent one begins. An inventory of zooid structures includes the following (see also Figure 2).
Figure 2: Schematic view of phylactolaemate anatomy. Scale bar = 0.4 mm
Polypide. A partially extendable unit that includes all major organ systems suspended in a common coelomic fluid. The polypide includes these parts:
Cystid. This is the laminated living and nonliving structure that separates the coelom from the external environment. Its outermost layer (ectocyst) consists of secreted material, which is some species is a slimy mucus, while in others it is a chitinous, somewhat brittle or leathery cuticle. Occasionally the cystid is called a zooecium,
In tubular, branching colonies the body is often encrusted with tiny particles from the surrounding water. A distinct raphe, or keel, may extend longitudinally along the side opposite the substrate. A clear, unencrusted line running along the keel and broadining towards the zooid tip is called a furrow.
The cystid also includes strands of circular muscle which help to regulate the internal pressure of the coelomic pressure of the coelomic fluid. Tracts of beating cilia on the peritoneum circulate the fluid in random eddies among polypides throughout the colony.
Asexual reproduction in freshwater bryozoans includes simple fragmentation, fission, and several types of budding.
Fragmentation occurs when a piece of a colony is broken off and establishes a new colony elsewhere. This often happens in colonies with fragile, free branches, such as Fredericella indica or Plumatella fruticosa. A branch with at least one living zooid drifts freely until it encounters a solid object to which the sticky zooid tip adheres.
Fission is a more active process which occurs in such gelatinous colonies as Cristatella mucedo and Lophopodella carteri. The body wall is constricted at a certain point by muscular contractions. Once the coelom is fully divided the colony parts separate and move slowly in different directions.
Budding is the formation of new individuals from parental tissues or body parts. By this process a growing colony produces produce new zooids; it also forms small, encapsulated structures called statoblasts. These develop on the funiculus from material derived from the colony wall. Most statoblasts undergo an obligate period of dormancy, and many remain viable after long periods of desiccation or subfreezing temperatures. Under proper conditions a statoblast may germinate and produce a single zooid, which eventually may bud further to create an entire new colony.
Several general types of statoblasts are recognized on the basis of their morphology and function (Figure 3): Floatoblasts have incorporated in their outer edge an extensive band of enlarged chambers which provide buoyancy when filled with a gas. Sessoblasts are generally larger than floatoblasts, have no gas-filled chambers, and are cemented firmly to the substratum. Piptoblasts also have no gas-filled chambers but they are not cemented to the substratum. They lie within the tubular colony interior and may adhere to the colony wall by means of small, keel-like projections.
Figure 3. Exploded view of two types of plumatellid statoblasts. (a) floatoblast, (b) sessoblast. Scale bar = 0.1 mm.
In many cases, statoblasts are essential for species identification. The outer casing of each statoblast can be separated into two opposing halves, or valves, which fit together like the two halves of a clamshell. Each valve has an inner and an outer layer. The inner layers together constitute a central capsule which encloses germinal tissue and yolky materials. The outer layer, or periblast, varies according to species and sometimes bears ornamentation. In a floatoblast, it is the periblast that contains the band of buoyant cells. This band, or annulus, is wider on one valves than on the other, and it encloses a clear, central area known a the fenestra. Traditionally, that valves with the wide annulus is designated the dorsal valve, while its opposite member is the ventral valve.
A single colony produces both eggs and sperm. The sperm develop in conspicuous clusters along the funiculi of polypides in certain regions of the colony. Upon release into the coelom, large numbers of sperm circulate passively, resembling inert, kinked threads. Eggs appear as tiny grape-like clusters of 20-40 cells attached to the inner colony wall ventral to the polypide and near the next developing bud.
Although no exchange of gametes has been observed among colonies, there is strong genetic evidence of cross-fertilization. Fertilized eggs develop into fully ciliated larva-like structures. This development occurs in a special embryo sac. The so-called larva (actually a specialized motile colony) eventually assumes a wedge shape and is released to the surrounding water through a terminal pore. It contains one or more fully-developed polypides enclosed within a ciliated mantle. The larva may swim from several minutes to more than 24 hours, proving various substrates. Within an hour after settling, the larval mantle pulls back, the tiny lophophores are extended, and feeding begins.