Bryozoa (Moss animals)

The phylum Bryozoa, also called Ectoprocta, is one of three lophophorate phyla. The other two are Phoronida and Brachiopoda (lamp shells), both being entirely marine. A feature of all three phyla is a lophophore, a crown of ciliated tentacles surrounding the mouth and used for feeding ( Fig 11.1 and Plate 12.1). Most are marine; only about 20 species are found in freshwater. Although the freshwater fauna is not diverse, freshwater bryozoans are usually common, but not conspicuous, in a wide range of standing and running water habitats. Only four species have been reported from Alberta: Cristatella mucedo, Fredericella sultana, Plumatella repens, and Plumatella fungosa.

Freshwater bryozoans live in colonies. A complete individual, called a zooid, is enclosed in a case, called a zooecium, which may be hard or gelatinous. The lophophore is either horseshoe-shaped or circular. The beating of the lophophore's cilia moves minute food matter, e.g. algae and small invertebrates, to the mouth region. Most bryozoans are hermaphroditic. The gonads shed the gametes into the coelom of the zooid, and this is probably where fertilization occurs. The fertilized egg develops into a ciliated larva, which will eventually settle on the substratum and develop into the first zooid of a new colony. Possibly in some freshwater bryozoan populations, especially north temperate populations, sexual reproduction is absent or occurs only sporadically.

Asexual reproduction is via budding or statoblast production. A statoblast is a small, about 1 mm in diameter, seed-like structure (Plate 12.1). Statoblasts with spines are called spinoblasts; statoblasts that are filled with gas and float are called floatoblasts; those that do not float are called sessoblasts (Plate 11.1). In some temperate species, statoblasts are produced seasonally, mainly in autumn; in others, statoblasts apparently are produced throughout most of the ice-free season. Statoblasts are produced within the zooid, each zooid producing perhaps about 10 statoblasts (a colony about 1 meter square could produce thousands of statoblasts). When the zooid dies, the statoblasts will be released. If the statoblast receives the correct stimuli, the mass of cells within the sclerotized case will develop, usually the following year, into a zooid, and via subsequent budding a colony will form.

Bryozoans are rarely collected by pond-net sampling. Similar to sponges, the best procedure is to bring rocks, aquatic vegetation, and branches of dead trees out of the water for inspection. If a large amount of material is collected, some can be preserved in the field and some, including the material's substratum, ca be brought to the laboratory in the live condition. Although species known to occur in Alberta can usually be identified by growth forms and types of statoblasts, it is advisable to examine the lophophore of living specimens-or the extended lophophore of preserved specimens. For the latter, the animal must be fixed when the lophophore is extended. One method is to put the living bryozoan in a jar of water; sprinkle a few menthol crystals on the surface of the water and leave overnight. This usually results in a well-extended lophophore. Large parts of colonies can be preserved in 70% alcohol. For keys to the entire North American fauna of bryozoans, see Rogick (1959) and Pennak (1978).

If statoblasts can be associated with the correct individual, Alberta's bryozoans can be separated mainly on the basis of statoblast features as given in a key to European Plumatellidae by Mundy and Thorpe (1980). Namely, if the statoblast has hooks (see ( BRYOZOA pictorial key and (Plate 11.1) it is Cristatella mucedo; if no floatoblasts, it is Fredericella sultana; if floatoblasts with a reticulate pattern (best seen with the floatoblast split into the two valves and then viewing one valve under a compound microscope) it is Plumatella fungosa; if the floatoblast is covered with tubercles (view under compound microscope) and without reticulate pattern it is Plumatella repens.

Cristatella mucedo Cuvier: These atypical-looking bryozoans are often collected on the underside of lily pads and on the submerged stems of emergent vegetation.

Fredericeila sultana (Blumenbach): This species is often found on logs of both standing and running water habitats. It is a common although not conspicuous Alberta species.

Plumatella fungosa (Pallas): Plumatella fungosa populations are often found on logs and large rocks and rock-like substrata from both streams and lakes. In autumn, P. fungosa often forms a massive carpet-like covering on large substrata and is the most conspicuous of Alberta's bryozoans.

Plumatella repens (L.): This species does not appear to be very abundant in Alberta. Populations are found mainly in standing water habitats.

Bryozoans are sometimes called ectoprocts because the anus is located outside the lophophore. There is another phylum of superficially similar-looking animals called Endoprocta, which are mainly marine. There is one freshwater representative in North America, but has never been reported from Alberta. This is Urnatella gracilis Leidy. Since it is an endoproct and not an ectoproct, the anus is located within the lophophore, but finding this feature is difficult if one cannot observe living specimens with the lophophore extended. The lophophore is at the end of an erect, segmented-appearing stalk; the stalk is about 5 mm in length. Urnatella is sporadically distributed mainly in large lakes and rivers.

Hui (1963) pertains to the bryozoan fauna of Alberta.

• Pictorial Key for Bryozoa
  

• Figure 11.1
  • Lophophore of Plumatella fungosa
  
  

• Plate 11.1
  Upper, left to right: statoblast of Cristatella mucedo [1 mm]; statoblast (floatoblast) of Plumatella repens [0.3 mm].
  Lower, left to right: statoblast (sessoblast) of P. repens [0.4 mm]; sessoblast of Plumatella fungosa [0.5mm].