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What is a Bryozoan?

Bryozoans, sometimes referred to as moss animals or ectoprocts are tiny, colonial organisms. Their development does not follow either a true protostome or true deuterostome pattern. They, along with the Phoronids (worm-like animals) and the Brachiopods (bivalve-like animals sometimes referred to as lampshells) are thus classified based on the presence of a specialized feeding structure called a lophophore, an extension of the body wall into a tentacled structure that surrounds the mouth and is either horseshoe-shaped or circular. Bryozoan colonies can be encrusting, arborescent (branching, and tree-like), or even free living. Individuals within colonies may be referred to as either zooids, or polypides.  The term polypide refers to the contents of each zooid (gut, lophophore, muscles, etc.) within the body wall (Barnes 1980).  

Zooids of most species are enclosed in a protective tunic made from either chitin (a tough protein also found in insect exoskeletons) or calcium carbonate. This exoskeleton has an orifice, or opening, through which the lophophore is extended into the water column for feeding.  In some species, the orifice is covered by an operculum.

Sketch of 3 types of individuals in a colony of Bowerbankia, an arborescent type of bryozoan.  Individuals in arborescent colonies live attached to a common stolon and have chitinous body walls.  A.  Individual zooid with its lophophore extended for feeding.  B.  Retracted individual.  C.  A degrading individual that is brooding an embryo.  Redrawn from Barnes, 1980.

Sketch of a colony of Electra, an encrusting type of bryozoan.  In encrusting bryozoans, an individual zooid is housed in a calcified structure called a zooecium.  Redrawn from Barnes, 1980.

Bryozoan Biodiversity:
Bryozoan diversity (as measured by number of species) within the Indian River Lagoon is approximately 1/3 that of bryozoa in coastal and offshore habitats (Winston 1995). Approximately 36 species of bryozoa are known to inhabit the Indian River Lagoon, Florida. The highest level of diversity among ectoprocts occurs in areas where salinities are above 30 . Only 12 species have been found in the less saline areas of the lagoon. Winston (1995) reported that bryozoan species composition within the lagoon has remained fairly stable for the last 20 years, however, there are often large seasonal or year-to-year changes in population structure. Further, many of the less common species have been observed in only one location within the lagoon. This observation has important ecosystem management implications because, if many species do, in fact, exist within the lagoon at only one to a few locations, then any degradation in water quality in these critical habitat zones, could quickly reduce ectoproct diversity in the IRL (Winston 1995). Balanced against this finding is the fact that approximately 97% of species in the IRL also occur at other sites: estuarine, coastal and offshore (Winston 1995). Thus, most species could be repopulated following restoration of the habitat.

Reproduction:
All freshwater and most marine bryozoans are hermaphroditic (Barnes 1980) with some species being simultaneous hermaphrodites, (producing both sperm and eggs at the same time), and others being protandric hermaphrodites. In dioecious species, the entire colony may consist of same-sex zooids, or both male and female individuals can be present. In hermaphroditic species, the ovaries are typically located distally, while the testes are located basally. Commonly, one to a few ovaries and many testes are active. These structures usually consist of masses of eggs or sperm covered by peritoneum. The masses eventually bulge into the coelom and the peritoneum ruptures. Some species release eggs and sperm directly to the water column, while others brood their eggs either within the coelom or externally in the cavity of the tentacular sheath or in the atrial wall. Brooded eggs are generally large, few in number, and are heavily yolked (Barnes 1980).

Embryology:
Following fertilization, larvae are produced which show wide variation in body form from species to species. The larvae of non-brooding bryozoans feed during the larval stage, while the larvae of brooding bryozoans do not, since these larvae tend to settle soon after release. The most common larval type in bryozoans is the cyphonautes larva which is somewhat triangular in shape and has an apical tuft of cilia. Upon settling, larvae attach via adhesive sacs and undergo metamorphosis to the adult form. The first zooid in a colony is called the ancestrula. It is from this individual that the rest of the colony will grow asexually from budding.


The cyphonautes larva, the most common larval form in the bryozoa.

 



Another type of bryozoan larva.  This one is from Bugula, an arborescent type.

Temperature:
On a local scale, temperature controls all aspects of bryozoan life. In spring, rising water temperatures and increased intensity of sunlight stimulate phytoplankton growth, which initiates active budding in bryozoans, and, to some degree, sexual reproduction.

Salinity:
Three IRL species (Victorella pavida, Conopeum seurati, and C. tenuissimum) are truly estuarine and found almost entirely in brackish water. The remaining 33 species are warm water, euryhaline species which inhabit harbors, bays, river mouths, and coastal areas where salinity is somewhat variable, yet generally above 30 (Winston 1995).

Trophic mode:
All bryozoans are suspension feeders. Each individual zooid in a colony has ciliated tentacles which are extended to filter phytoplankton less than 0.045 mm in size (about 1/1800 of an inch) from the water column. Bullivant (1967, 1968) showed that the average individual zooid in a colony can clear 8.8 ml of water per day. 

Habitats:
Seagrasses as well as floating macroalgae, mangrove roots, shells, docks, pilings and other structures provide support for bryozoan colonies. In turn, bryozoans provide habitat for many species of juvenile fishes and their invertebrate prey such as polychaete worms, amphipods and copepods. (Winston 1995).

Importance in the Indian River Lagoon:
Bryozoans are ecologically important in the Indian River Lagoon due to their feeding method. As suspension feeders, they act as living filters in the marine environment. Using Bullivant's (1967, 1968) calculation that individual zooids may filter an average of 8.8 ml of water per day, Winston (1995) reported that colonies of Zoobotryon verticillatum located in 1 square meter of seagrass bed could potentially filter and recirculate an average of 48,600 gallons of seawater per day.