Phylactolaemata

(Freshwater bryozoans)

Phylum Ectoprocta

Class Phylactolaemata

Number of families 5

Thumbnail description
Filter-feeding invertebrate animals composed of many identical zooids joined seamlessly as a colony, each zooid capable of independent feeding, digestion, and reproduction

Evolution and systematics

Class Phylactolaemata includes five families and approximately 80 known species. Families include Cristatellidae, Fredericellidae, Lophopodidae, Pectinatellidae, and Plumatellidae.

Physical characteristics

Freshwater bryozoans appear in a variety of forms. The most conspicuous colonies are large, gelatinous masses, but other species are small, moss-like growths or patches of branching tubules. At the microscopic level the basic design is the same: every colony is composed of many identical zooids the anatomical features of which are devoted mostly to feeding and digestion. A horseshoe-shaped lophophore of ciliated tentacles (lophophore shape is circular in Fredericellidae) generates a current of water that draws suspended food particles toward the mouth. An elongated U-shaped gut mixes and digests the food then packs undigested particles into small pellets, which are expelled through the anus. Circular muscles within the colony wall maintain slight hydrostatic pressure, which enables the feeding apparatus to project into the water while long retractor muscles pull the lophophore unit back into the colony interior. Each zooid has a single nerve ganglion between mouth and anus, and nerve tracts extend into the lophophore and to the gut. No nerves pass between zooids. A cord of tissue called a funiculus connects the tip of the gut to the inner colony wall and functions in sexual and asexual reproduction. Each zooid is capable of budding additional zooids and forming dormant statoblasts.

Distribution

Every continent except Antarctica. Common in lakes, ponds, rivers, and streams growing on a variety of solid, submerged surfaces. Colonies and dormant statoblasts are easily carried to new sites by waterfowl or transported on aquatic vegetation.

Habitat

All species can be found in shallow water, where they grow on the sides or undersides of submerged rocks, wood, vegetation, rubber tires, plastic trash, and other solid, stable materials. These animals avoid rotting, oily, or marly substrata, and generally do not occur on wave-tossed materials. Most species thrive at a pH greater than 6 and a temperature range of 41–86°F (5–30°C).

Behavior

Nothing is known.

Feeding ecology and diet

Phylactolaemate bryozoans appear to be indiscriminate in the materials they eat, rejecting only particles that are too bulky or active. Ingested materials are reported to include a variety of plankton and detritus. Retention time in the gut varies with the rate of ingestion, which depends on the concentration of suspended particles in the water. During active feeding many of the living organisms taken into the gut emerge from the anus still alive. Studies have shown ingested bacteria to be an important source of nutrition, and it is possible that many ingested particles are simply carriers of bacterial food.

Reproductive biology

The life cycle includes both sexual and asexual reproduction. During the brief season of sexual activity, sperm are formed on the funiculus and then detach to circulate freely within the colony. Grape-like clusters of eggs appear on the inner colony wall near the tip of certain zooids. Internal fertilization appears possible, but there is also evidence of outcrossing. The mechanism for sperm exchange between colonies is unknown. The fertilized egg develops within a special embryo sac, becoming a motile stage composed of one or two fully formed zooids surrounded by a ciliated mantle. Once released from the colony, this larva-like structure can swim actively for several hours before settling on a substrate. The mantle pulls back, and the new zooids begin feeding almost immediately.

In some species asexual reproduction may be as simple as fragments of the colony lodging in a new location. In some globular species the colonies may pinch off daughter colonies, which then glide slowly away. However, all species also engage in asexual production of dormant statoblasts capable of withstanding drought, freezing temperatures, and other unfavorable conditions. Many statoblasts contain air chambers that provide buoyancy, so on release from the colony, they can drift considerable distances or adhere to feathers of waterfowl. Another type of statoblast is attached directly to the substratum, where it remains long after the colony has disintegrated. The period of obligate dormancy can last several months, and some statoblasts remain viable for years. When favorable conditions resume, the statoblast germinates to produce a single zooid capable of forming a new colony.

Conservation status

Freshwater bryozoans in general are common and abundant, although certain species are considered rare, especially in tropical regions. No species are listed by the IUCN.

Significance to humans

In their natural habitat freshwater bryozoans contribute to nutrient cycling, and they are grazed upon by fish and certain invertebrates. However, they also are a nuisance when they grow inside pipelines and filters, blocking or seriously disrupting the flow of water in irrigation, wastewater, and cooling water systems. Any pipeline that carries untreated water from a lake or river is at risk of becoming fouled with bryozoan colonies.

Species accounts

List of Species

No common name

Fredericella sultana

order

Plumatellida

family

Fredericellidae

taxonomy

Fredericella sultana Blumenbach, 1779, canals in Göttingen, Germany.

other common names

None known.

physical characteristics

Colony composed of thin, brown, branching tubules, some attached to a submerged object and others extending freely into the water, easily mistaken for small plant roots. Lophophore has 18–25 tentacles arranged in a circle around the mouth. Statoblast resembles a tiny bean 0.016 in (0.4 mm) long.

distribution

Every continent except South America and Antarctica, common in Europe and Asia. Two slightly different species also occur in North and South America.

habitat

Clean water in a variety of habitats. Thrives in flowing water. Tolerates a wide range of temperature and pH. Capable of overwintering under ice.

behavior

Nothing is known.

feeding ecology and diet

Feeds continuously, ingesting suspended particles from the water, presumably deriving most nutrition from bacteria and detritus.

reproductive biology

The fertilized egg develops into a motile stage containing a single zooid. Seasonality of sexual reproduction has not been documented. Statoblasts are relatively sparse, occurring mostly in colony branches adjacent to substratum.

conservation status

Not listed by the IUCN.

significance to humans

An important host for Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease in salmonid fish, often resulting in significant economic loss to fish farmers.

No common name

Pectinatella magnifica

order

Plumatellida

family

Pectinatellidae

taxonomy

Pectinatella magnifica Leidy, 1851, stream near Philadelphia, Pennsylvania, United States.

other common names

None known.

physical characteristics

The young colony is flat and compact, eventually forming a small mound that enlarges to the size of a football. Zooids cover the entire outer surface, producing slimy mucus with a distinctive, sharp odor. The interior of the colony is a clear, gelatinous mass consisting mostly of water. Individual zooids have red pigment around the mouth and two conspicuous white spots on the lophophore. Only one type of statoblast is formed: buoyant, discoid, approximately 0.04 in (1 mm) in diameter with hooked spines radiating from the periphery.

distribution

United States and southern Canada. Scattered sites in Germany, Poland, Czechoslovakia, and France. In the 1960s invaded Japan and Korea, where the presence of colonies longer than 6 ft (2 m) has been documented.

habitat

Thrives in warm, productive habitats. Prefers quiet or slowly moving water, often found on submerged logs, tree stumps, and dock pilings.

behavior

Nothing is known.

feeding ecology and diet

Feeds continuously, ingesting suspended particles from the water, presumably deriving most nutrition from bacteria and detritus.

reproductive biology

Throughout most of the range, sexual activity occurs early in the summer, the motile stages released in June or early July. Buoyant statoblasts are formed and released in September and October.

conservation status

Not listed by the IUCN.

significance to humans

Despite frequent assumptions to the contrary, pectinatella is a normal part of the freshwater animal community. Its presence does not indicate pollution or any other water quality problems. However, at the end of the season, large colonies sometimes detach from their substrate and drift toward shore, startling fishermen and clogging water intake pipes.

No common name

Plumatella fungosa

order

Plumatellida

family

Plumatellidae

taxonomy

Plumatella fungosa Pallas, 1768, Stariza River, Russia.

other common names

None known.

physical characteristics

Young colony composed of branched tubules spreading across submerged substratum. Zooids initially are oriented along the substratum but become erect when crowded. Older colonies reach a thickness up to 2 in (50 mm). As in all plumatellids, two types of statoblasts are produced: Floatoblasts have a ring of air-filled chambers for buoyancy and are released freely from the living colony. Sesoblasts lack air chambers and are cemented directly to the substratum.

distribution

North America, Europe, and northern Asia.

habitat

Still or gently flowing water. Thrives in eutrophic conditions and tolerates organic pollution better than other bryozoan species.

behavior

Nothing is known.

feeding ecology and diet

Feeds continuously, ingesting suspended particles from the water, presumably deriving most nutrition from bacteria and detritus.

reproductive biology

Sexual activity begins early in the season. Motile stages are released in late June and early July. Statoblasts are produced throughout the season in great abundance.

conservation status

Not listed by the IUCN.

significance to humans

Colonies can clog pipes and filters in irrigation and water treatment systems.

Resources

Books

Lacourt, Adrianus W. A Monograph of the Freshwater Bryozoa: Phylactolaemata. Leiden, The Netherlands: Zoologische Verhandelingen, 1968.

Ryland, John S. Bryozoans. London: Hutchinson University Library, 1970.

Smith, Douglas G. Pennak's Freshwater Invertebrates of the United States: Porifera to Crustacea. New York: Wiley and Sons, 2001.

Wood, Timothy S. "Bryozoans." In Ecology and Classification of North American Freshwater Bryozoans. 2nd edition, edited by John Thorp and Alan Covich. San Diego, CA: Academic Press, 2001.

——. Ectoproct Bryozoans of Ohio. Columbus: Ohio Biological Survey, 1989.

Wood, Timothy S., and Beth Okamura. A New Key to the Freshwater Bryozoans of Britain, Ireland, and Continental Europe. Ambleside, Cumbria, U.K.: Freshwater Biological Association, 2004.

Organizations

International Bryozoology Association. Web site: <http://petralia.civgeo.rmit.edu.au/bryozoa/iba.html>

Timothy S. Wood, PhD