Bryozoaires

One could easily miss the bryozoans (entoprocts) or mistake them as an alga or coral. Bryozoans are a phylum of microscopic, aquatic invertebrates that live in sessile colonies of genetically identical members. The individuals are not autonomous and are termed zooids. They grow as calcified or gelatinous encrusting masses or branching tree-like structures. Having said that, there are notable exceptions, including a genus of solitary species (Monobryozoon), a mobile species (Cristatella mucedo), and a recently found planktonic species (in genus Alcyonidium) that floats as a ball (Peck et al. 1995). Like the phoronids and the brachiopods they feed using a specialized horseshoe-shaped structure called a lophophore. Known also as “moss animals,” there are somewhere between 4000-6000 living species, some estimate that number closer to 8000 species (Ryland 2005). Most bryozans are marine or brackish, fewer than 100 species live in freshwater (Massard and Geimer 2007). About 15,000 fossil species have been found, dating from the early Ordovician/late Cambrian. Molecular phylogenetic analyses indicate that bryozoans originated earlier in the Cambrian period (along with almost all other invertebrate phyla) and that the earliest bryozoans were non-calcified, thus did not fossilize (Fuchs et al. 2009).

Marine bryozoans are bountiful world wide, especially in tropic zones, but are found in all latitudes and depths, even in the cold waters of Antarctica (Brusca and Brusca 2003; Kozloff 1990).

The bryozoans are divided into three very distinct monophyletic classes (Fuchs et al. 2009). Members of the class Phylactolaemata are entirely freshwater species; the Stenolaemata are exclusively marine, and Gymnolaemata, the largest class, containing 75% of living bryozoan species, is primarily marine, although some species inhabit brackish water (Brusca and Brusca 2003; Kozloff 1990).

The fertilized egges in some bryozoans in the class Stenolaemata divide so that up to one hundred identical eggs are brooded at a time in specialized zooids. There is great diversity in the types of bryozoan larva, some feed, some are flattened, some have a shell, some are zooid-like, but all form a ciliated, free-swimming larva for some length of time, then settle and undergo dramatic reorganization to reach their mature form.

A bryozoan colony begins with an ancestrula (the primary zooid), which is formed sexually. The colony then grows by asexual budding, in a pattern dictated by the particular taxon. Bryozoan colonies are found in a wide array of colony formations. Encrusting forms (most common) can cover large areas of rocks, algae, shells or exoskeletons of other invertebrates, ship hulls, and other hard substrates. Other forms include arboristic, branching, discus, amorphous blob shapes or (especially in freshwater taxa) the zooids can grow as buds along a cord-like stolon. There is one genus of mobile bryozoans, Cristatella, which, in the shape of a caterpillar, crawls along substrates at very slow speed! Some freshwater taxa also form new colonies by asexually producing statoblasts, which drop to the bottom if the parent colony does not survive and survive harsh conditions in a dormant mode. The statoblast then generates a new zooid when conditions are more optimal. (Brusca and Brusca 2003; Kozloff 1990)

The individual zooid each live in a box shaped or bud-shaped exoskeleton (zoecium) which can be mineralized, gelatinous or chitinous, and in some taxa may have an operculum over its little opening at the top. Typically suspension feeders, the zooid protracts through this opening a special feathery feeding organ called the lophophore, which is composed of a circle or horseshoe of tentacles. Cilia on the lophophore tentacles create water currents to carry appropriate sized food particles (including protists and invertebrate larvae) along food grooves on the lophophore which lead to the mouth.

Within a colony, individual zooids may be more or less connected to one another; many taxa have pores or a cord (funiculus) linking individuals in a colony, through which the individuals share coelomic fluids. In some kinds of colonies zooids function together to create more powerful water currents to bring in more food. All colonies contain autozooids, which feed and excrete wastes, some colonies also have non-feeding heterozooids, individuals specialized for gamete production, protection, or other functions and are supported with nutrients shared by surrounding zooids. Zooids may have spines on their zoecium, some that produce toxins, to ward off predators. Protective zooids may have their operculum modified into a protective structure, either an avicularium – a movable beak-like structure to rid the colony of pests, or a vibraculum – a long, movable setae-like structure thought to help in cleaning off the colony. Grazing by nudibranchs, snails, sea urchins and crustaceans is a common threat to bryozoans (Brusca and Brusca 2003; Kozloff 1990).

Bryozoans do not have nephridia or a circulatory system, instead gas exchange and nitrogenous excretions occurs passively by diffusion in the tiny zooids. When more complex wastes build up, the zooid forms a “brown body”, in which the soft tissue and lophophore (together called the polypide) degenerate within their casing (called the cystid). The cystid can then regenerate a new polypide, with the old brown body in its gut. The brown body in some taxa is then excreted through the anus (located near the mouth, but on the outside of the lophophore). In taxa with zooids arranged on stolons, the brown body simply falls off the shoot and a new zooid is regenerated. The nervous system in bryozoans is minimal, including a ganglion, nerve ring around the pharynx, and nerve net that extends into the tentacles and vicera. Sensory structures are limited to tactile cells on the lophophore (Brusca and Brusca 2003; Kozloff 1990).

Bryozoans are generally hermaphroditic. Rather than having discrete gonads, transient germ tissues on the zooid’s body wall peritoneum or on the funiculus (which connects the gut to the body wall) produce gametes. While sperm is spawned through pores in lophophore tentacles, eggs are usually harbored inside the body wall, and are internally fertilized by sperm, coming in on lophophore feeding currents (Brusca and Brusca 2003; Kozloff 1990).

Individual bryozoan zooids are typically about 0.5 mm long. The colonies can reach sizes up to a meter across (Brusca and Brusca 2003).

The distinct lophophore organ of the bryozoans is also found in the brachiopods and phoronids, and these three phyla have long been associated as close relatives. However recent phylogenetic work now places the bryozoans quite distinct from the brachiopods and phoronids, as a more basal group in the containing superphylum Lophotrochozoa (Halanych 2004).