Study Methods for Freshwater Bryozoans 103-110 © Biologiezentrum Linz/Austria; Download Unter

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Zeitschrift/Journal: Denisia

Jahr/Year: 2005

Band/Volume: 0016

Autor(en)/Author(s): Wood Timothy S.

Artikel/Article: Study methods for freshwater bryozoans 103-110 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at

Study methods for freshwater bryozoans

T.S. WOOD

Abstract: Bryozoans are found in a wide variety of aquatic habitats. For most species proper collection requires their removal along with the substratum to which they are attached. Specimens are narcotized in menthol or chloral hydrate, then fixed and preserved in 70 % ethanol. Statoblast valves may be separated with hot KOH for examination with light microscopy. For laboratory culture the colonies are grown upside down in water conditioned by the presence of fish. In situ culturing can be achieved on artificial substrata. Standard techniques are used for histological work and scanning electron microscopy. Practical methods for chromosome and molecular studies also are available. Several good reference books review the biology of freshwater bryozoans and offer identification keys.

Key words: Bryozoa, Phylactolaemata, culturing, methods.

Introduction All bryozoans grow on submerged surfa- ces, including plants, wood, rocks, glass, alu- Bryozoans are among the most fascina- minum, and a wide range of synthetic mate- ting invertebrate animals in fresh water. Alt- rials such as automobile tires, plastics (in- hough similar in structure to their marine re- cluding plastic bags) and fiberglass. Most latives, the freshwater species are larger, bol- species occur on the undersides of objects der, and easier to study. There is much to be where they are protected from settling parti- learned about the entire group, since many cles, although this is less true in turbulent aspects of their ecology, physiology, and de- water. Although cold adapted species can be velopment are still poorly understood. More- found at great depths, all species normally over, like corals, bryozoans represent a fine occur in water less than 1 m deep, and so example of modular organization, in which they are easily collected from shore. Warm, the entire animal is composed of many indi- eutrophic waters generally support greater vidual units capable carrying out basic func- growth and species diversity than habitats tions of life. Studying these minute creatures were the water is clear and cold. Places whe- offers rewarding opportunities to scientists, re bryozoans are usually not found include: students, and other interested persons. This paper describes a range of techniques and • Oily, rotting, or actively corroding sub- procedures currently being used to investiga- strata; te freshwater bryozoans. • Habitats with low oxygen or a pH below 6; • Sites lacking firm substrata; Finding bryozoans • Rivers and streams where smooth, roun- Bryozoan colonies are often seen but sel- ded rocks shift easily during high water. dom recognized. Their name, meaning Quiet backwaters of lakes and rivers of- "moss animal," refers to the appearance of ten harbor bryozoan colonies. Other good certain species: inert brown crusts and places to look are the undersides of floating spindly tendrils that are easily mistaken for or dangling objects that are out of reach to dead moss. Other species, however, are gela- benthic predators such as snails and flat- tinous blobs ranging from the size of a pea to worms. Floating buoys and docks, old dang- Denisia 16, zugleich Kataloge der OÖ. Landesmuseen that of a football. ling rope or branches, padded boat fenders Neue Serie 28 (2005), 103-110 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at

crystallinus by examining flood and floating debris. JONES et al. (2000) describe a method for recovering statoblasts from lake se- diments by passing material through a stack of standard sieves with mesh openings of 1.0 mm, 500 um, and 150 urn. Organic particles are separated from heavier material by wash- ing and swirling, and the final product is examined on P8 coarse filter paper.

Collecting bryozoans

When collecting bryozoans it is desira- ble to keep the specimens intact. For branching colonies this can be done only by re- moving them together with the underlying substratum. With wood and other soft substrata this is easily done with a sturdy, sharp knife. For reasons of safety a fixed blade knife is recommended over a folding blade. For Fig. 1: Basic tools for collecting bryozoans: even boat hulls are all good potential sub- many years 1 have used a 9-inch Buck knife 14x magnifying lens, fixed blade knife, and strata. Additional good sites include streams (El Cajon, California), which is perfect for wide-mouth polyethylene jar. The knife is almost any situation (Fig. 1). safely transported inside the jar, especially below reservoir impoundments where there may be abundant suspended organic parti- when the collector is clambering around Hard, brittle substrata, such as stone, re- submerged substrata. cles. Certain aquatic vegetation offers better quire a hammer and cold chisel. With prac- substrata than others. For example, bryozo- tice, it is possible to chip off the specimen ans grow well on floating leaves of Ameri- intact and then trim away excess rock with can lotus {Nelumbo luted), but are seldom the chisel or a small pair of pliers. Some seen on leaves of the white or yellow water rocks, such as granite or basalt, do not chip lily (Nymphaea virginiana, Nuphar luted). well and are best avoided. In temperate regions of North America Other useful equipment for collecting bryozoans are abundant in the late spring bryozoans (Fig. 1) includes: when water temperatures rise above 20 °C. There may be a period during July and Au- Small magnifier (or loup), usually worn gust when colonies are scarce, but this is fol- on a lanyard around the neck, magnifying lowed by renewed growth in the fall. Some lOx to 14x. Some people like an elasticized species, such as Fredericella and Lophopus are lanyard so the lens is not flopping about or well adapted to cold temperatures and may dipping into the water. A Coddington-type even be collected under the ice in winter. magnifier with solid lens is preferred because it is always fully submersible. To use the One useful technique for finding bryozo- lens properly, bring it to your eye and then ans is to search first for their statoblasts. raise the object into view. The hand holding While this may not lead directly to bryozo- the lens should reach out with the small fin- an colonies, it can at least indicate which ger to stabilize the object. species are occurring in the general area. A simple method is to use a magnifying lens to Specimen container, preferably one that examine the small particles that cling to flo- is buoyant and unbreakable. I use a standard ating objects at the waterline. Free stato- 1-liter widemouth polyethylene bottle. Both blasts adhere particularly well to plastic fo- the bottle and the lid will float, and the am (expanded polystyrene, or Styrofoam®), bottle will also hold my knife snugly. (Note, and they show up well against the white however, that when the bottle contains the background. HlLL & OKAMURA (2005) have knife or heavy specimens it will sink unless detected the presence of the rare Lophopus some air is trapped inside).

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Preparing specimens together. Bryozoan specimens to be used for DNA analysis require additional treatment Collected specimens will normally keep and should be stored in 100 % ethyl alcohol in a 1-liter jar for up to 12 hours, depending (see Subcellular studies below). on temperature and the degree of crowding. Colonies are then sorted in a shallow tray and processed as soon as possible. Identifying specimens

Normally the first step in processing is Several good identification keys are to anaesthetize the zooids so the lophopho- available (see Further information below). res will remain extended. Among the most Genera can often be determined from colo- effective and convenient narcotizing agents ny morphology alone, but identification to are menthol and chloral hydrate, both avail- species almost always requires the presence able in crystalline form from chemical sup- of statoblasts. Searching for statoblasts is ply companies. In tropical regions chloral best done with a dissecting microscope and hydrate has the advantage of a higher mel- fine forceps. If statoblasts are not immedia- ting point, but it is often difficult to obtain. tely apparent it may be necessary to dig into Crystals of menthol can be powdery and the colony, especially in older, central re- messy to work with, so you may want to regions. Plumatellid sessoblasts are sometimes form them into larger chunks. Melt them to- best found by peeling a portion of the colo- gether over low heat, pour into a shallow ny away from the substrate. Try to disregard pan of aluminum foil and cool to make a statoblasts found outside the colony since small slab 1.5-3 mm thick. Break the slab in- these may have come from a different sour- to smaller pieces and store in a tight contai- ce. ner in a cool place. To anaesthetize bryozoans place colonies in a small amount of wa- Species identification in the large genus ter, add enough menthol to cover at least Plumatella may involve measuring various half the surface, and cover the container. parts of the floatoblast using an ocular mi- Leave undisturbed for at least 45 minutes at crometer. It is useful to know that each floa- room temperature. The menthol diffuses toblast is composed of two halves, or valves, slowly into the water and relaxes the mus- which normally separate at the time of ger- cles. To achieve uniform effect you may mination. By convention, the valve desig- swirl the water once very gently after about nated as "dorsal" is the one with the small- 30 minutes. This procedure is finished when est central area covered by large chambers of zooids no longer respond to gentle probing. the annulus (Fig. 2). If the two valves are The more tentacles on the lophophore the unequal in size the dorsal valve is always the more quickly menthol takes effect, so for smaller one. example Cristatella is immobilized more quickly than Fredericella species. If left too Separating and cleaning the two valves long in menthol the tentacles start to curl, is a simple process devised by WlEBACH then slowly disintegrate. (1974) to reveal some of the pattern and texture of the central fenestra. Statoblasts Ethyl alcohol at 70 % is a good general are placed in a 1 M solution of potassium fixative and preservative. Colonies contai- hydroxide and heated in a spoon over a fla- ning significant amounts of water, such as Pecünateüa or Asajirella, should have a least me for 30-60 seconds. Care should be taken one change of alcohol after 24 hours. In the not to let the solution boil and spatter sta- past, some workers injected full strength for- toblasts in all directions. Valves may separa- malin over colonies to harden the lopho- te immediately in the KOH solution, or phore and keep it nicely expanded. A 10 % they may open spontaneously after being solution of formalin is still sometimes used transferred back to clean water. In some spe- as a fixative and preservative, sometimes cies the capsule can be isolated from the ou- with sucrose added (BUSHNELL 1965). How- ter periblast. If the statoblast fails to open it ever, because of the toxic vapors formalin may be given additional time in hot KOH. should be used only with excellent ventila- Roughened or scalloped edges are indica- tion, and most workers choose to avoid it al- tions of overcooking.

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Culturing bryozoans if a portion of the substratum can be positioned against a glass plate with wedges or Growing bryozoans in the laboratory is rubber bands the colony may grow out onto seldom easy, and for Cristatella and Pecrina- the glass. At that point the old substratum tella species it has never been achieved for can be removed. more than a few days. ODA (1980) was able to maintain colonies of Lophopodella carteri Bryozoans may also be cultured in situ on pure cultures of Chlamydomonas reinhard- on artificial substrata. WÖSS (1996, 2000, tii. WAYSS (1968) reported using various 2002) achieved recruitment of colonies on unicellular green algae in 1:1 Knop's solu- panels of wood and plexiglass hung vertical- tion and soil extract. Mukai (pers. comm.) ly below a floating raft. Others have success- simply employed frequent changes of water fully used glass and corrugated polyethylene from a eutrophic lake. However, one of the substrata, but found little colonization on most effective and trouble-free culture sys- vinyl or rubber sheeting (WOOD 1973; Ma- tem uses aged water conditioned by the pre- huchariyawong, pers. comm.). For most stu- sence offish. In my lab we keep a number of dies I prefer growing colonies on inverted large goldfish (Carassius auratus L.) in water glass petri plate lids held inside customized that is never filtered and seldom changed. sections of corrugated HDPE drainage pipe Their 200-liter aquaria are well lit and the (Fig. 5). Suspended in the water under te- fish are well fed. Water from these tanks is thered, floating logs or other support the circulated continuously through smaller, black plastic devices are almost inconspicu- darkened chambers in which bryozoans ous. The inverted Petri plate covers are ea- grow (Fig. 3). sily removed without disturbing the atta- It is important that the bryozoans be ched colonies, and with transmitted illumi- protected from settling particles. The easiest nation the transparent substratum affords an way to achieve this is to grow the colonies excellent microscopic view. To make the on the underside of artificial substrata, such plastic holders, a pipe (diameter 4 in or 110 as inverted dishes. For example, individual mm) is cut into 8 cm lengths. Four tabs, 2 plastic petri plates containing bryozoans can cm long and 2 cm wide, are cut into one end have plastic toothpicks applied with ther- of each unit and bent 90° inwards to sup- mal glue and inserted into glass tubes atta- port the petri lid. To hold their shape, the ched to the inside wall of the culture cham- tabs are heated at their base with a flame, ber (Fig. 4). Or, glass petri plates containing then held in bent position while cooling wa- colonies can be inverted on racks. Yet a ter is applied. third possibility, good for mass culturing, is to grow colonies on the lower sides of glass Germinating statoblasts plates that incline in parallel series like ho- neycomb in a beehive (Fig. 3). Bryozoan statoblasts undergo an obligate period of dormancy that normally lasts 3- It is relatively easy to transfer colonies of 5 weeks or more. In principle, it is possible Lophopodella, Lophopus, and Cristatella. They to germinate many statoblasts in a predicta- are simply nudged off their original substra- ble time by first storing them under unfavo- tum and placed directly onto a new one rable conditions until the dormancy period where they should attach within 3-12 hours. is exceeded. Then, upon the return of favo- Branching colonies, however, will attach to rable conditions they should germinate si- new substrata only by the actively growing multaneously with 2-7 days. In practice, the tips. Free branches of FredericeUa and certain germination of statoblasts can often seem Plumatella species may be left undisturbed capricious, and it is obvious we still have for 1-2 days so the branch tips are in contact much to learn about this process. with the desired substratum; or the branches may be gently weighted with short lengths Statoblasts from temperate regions easi- of fine glass rods until they adhere. In gene- ly survive desiccation and freezing in their ral, branches that are firmly attached natural habitat. ROGICK (1940) offers a sum- throughout their length are seldom success- mary of data on the viability of dried stato- fully transferred to a new location. However, blasts. However I find that statoblasts ger-

106 © Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at minate more quickly and reliably after being stained with Delafield's hematoxylin, coun- stored in water at 4 °C. Statoblasts from tro- terstained with eosin and sectioned at 7 pical species seldom tolerate refrigeration um. Methods for preparing chromosomes and are best kept at room temperature in have been nicely worked out by BACKUS 1-2 % saline solution. (1989; see also BACKUS & MUKAI 1987).

The following points are relevant to sta- To prepare statoblasts for scanning elec- toblast germination: tron microscopy it may first be necessary to 1. Deionized water or rainwater enhances remove a closely applied outer membrane. If germination in all species. present, this membrane is visible with trans- 2. Plumatella statoblasts crowded together mitted illumination as a thin halo around germinate faster than isolated statob- the statoblast. Usually it is easily removed lasts (OWEN, personal communication). with fine forceps and needles. TATICCHI et 3. Lophopodid statoblasts may be stored at al. (2004) accomplish the same thing with a room temperature if they are kept in 30 second exposure to 40 % KOH. To avo- complete darkness. Exposure to strong id distortion of the fenestra upon drying the light will then trigger germination (ODA statoblast can be freeze dried or frozen in a 1959). small drop of water and the ice allowed to 4. Cristatella statoblasts appear to have a sublimate in a household freezer. Critical longer obligate dormancy than other point drying is also an obvious option. species. 5. Fredericella statoblasts stored at cold temperatures may rupture if warmed too Subcellular studies rapidly. Available descriptions of subcellular Here is a convenient methods for germi- procedures include RADP analysis (OKA- nating buoyant statoblasts: A glass Petri MURA et al. 1993), identification of poly- dish is inverted in a pan of water with edges morphic microsatellite loci (FREELAND et al. resting on supports so the rim is about 1 cm 1999) and analysis of 18S ribosomal DNA above the bottom of the pan. (Alternative- (WOOD & LORE 2005). Whenever a very ly, an inverted plastic Petri dish can be floa- clean specimen is required without contami- ted from the surface film of water). All trap- nation from other organisms it is important ped air is removed. Buoyant floatoblasts to to clear the gut. This is accomplished by be germinated are picked up with an eye- holding living colonies in filtered or uncar- dropper pipet and propelled with a squirt of bonated bottled water for at least 10-12 water under the petri dish. Care is taken not hours. Although seldom necessary, a very to overshoot the dish or to insert air bubbles small amount of unscented talcum powder under the dish. It takes a little practice to suspended in the water will speed this pro- jostle the floatoblasts inside the pipet so cess, with inert talc replacing the original they will exit with the jet of water. Once the gut contents. Colonies are then narcotized floatoblasts have been placed, a Petri lid can as usual and fixed in 100 % ethanol. Tissue be gently positioned under the inverted dish for DNA analysis should include only the and the entire unit can then be lifted from polypides (lophophore, gut, and other tis- the water. In this way the germinating sta- sues) plucked from the colony and stored in toblasts can be examined with a microscope 100 % ethanol. without being disturbed. However, evapora- tion works quickly, and the petri dish should not be kept out of the pan of water for more than a few hours.

Microscopy

Standard methods are sufficient for histological work. For their studies on non- branching colonies, for example, MUKAI & ODA (1980) embedded tissue in paraffin,

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Further information graphs, a good taxonomic key, and the most complete background information to date on the eco- The following is a selection of useful re- logy of freshwater bryozoans. sources for additional information on freshwater bryozoans: International Bryozoology Association. www.nhm.ac.uk/hosted_sites/iba, and BRIEN P. (1960): Classe des Bryozoaires. — In: GRAS- SE P. (Ed.): Traite de Zoologie 5(2): 1053-1335. www.civgeo.rmit.edu.au/bryozoa/iba.html. An outstanding summary of bryozoan biology, Founded in 1965, the 1BA is composed of with emphasis on anatomy, physiology, and deve- scientists and students from nearly 40 coun- lopment, well illustrated with exceptionally clear tries. They include paleontologists, marine drawings. and freshwater biologists, and specialists in GEIMER G. & J.A. MASSARO (1986): Les Bryozoaires du systematics, ecology, genetics, physiology, Grand-Duche de Luxembourg et des Regions embryology, and other aspects of bryozoolo- Limitrophes. — Musee de l'Histoire Naturel. Marche-aux-Poissons, Luxembourg: 1-188. gy. A conference is held every three years, An excellent treatment of most European spe- and published proceedings are widely distri- cies, including Paludicella articulate, with descrip- buted. New members are always welcome. tions and scanning electron micrographs, good ecological information, and more.

LACOURT A.W. (1968): A Monograph of the Fresh- References water Bryozoa - Phylactolaemata. — Zoologi- BACKUS B.T. (1989): Chromosomal and Genetic In- sche Verhandelingen 93, Leiden, E.J. Brill: 1- vestigations of the Phylactolaemate Bryozoa 159. — Ph.D. Thesis, The George Washington Uni- A good but aging compilation of information versity: Washington, D.C.: 1-131. with emphasis on systematics, including an ex- BACKUS B.T. S H. MUKAI (1987): Chromosomal haustive bibliography through about 1964. Spe- heteromorphism in a Japanese population of cies descriptions and distribution data beyond Pectinatella gelatinosa and karyotypic com- Europe and North America are unreliable, and parison with some other phylactolaemate the identification key is useless. bryozoans. — Genetica 73: 189-196. NIELSEN C. (1989): Entoprocts: Keys and notes for BUSHNELL J.H. (1965): On the taxonomy and distri- the identification of the species. — Synopses bution of freshwater Ectoprocta in Michigan. of the British Fauna (New Sen), Leiden, E.J. Part I. — Transact. Amer. Microsc. Soc. 84: Brill 41: 1-131. 231-244. An excellent taxonomic treatment of entoproct bryozoans, all of which are marine except Urna- FREELAND J.R., JONES C.S., NOBLE L.R. & B. OKAMURA tella graalis, including basic information about (1999): Polymorphic microsatellite loci identi- the phylum. fied in the highly clonal freshwater bryozoan, Cristatella mucedo. — Molecular Ecology 9: PRENENT M. & G. BOBIN (1956): Bryozoaires, premie- 341-342. re partie, Entoprotes, Phylactolemates, Cte- nostomes.— In: LECHEVAUER P. (Ed.): Faune de HILL S. & B. OKAMURA (2005): A novel technique for France 60: 1-398. the assessment of the distribution of Lopho- A detailed but somewhat dated review of bryozo- pus crystallinus, a rare phylactolaemate wit- an biology illustrated with many line drawings, hin the U.K. — In: WYSE-JACKSON P. (Ed.): Bryo- zoan Studies 2004. Proc. 13* Intern. Bryozool. with descriptions of major taxonomic groups and Assoc. Balkema, Rotterdam & Brookfield (in a limited identification key. press). WOOD T.S. (2001): Bryozoans. — In: THORP J. & A. JONES K.E., MARSH T.G. & WOOD T.S. (2000): Survey- COVICH (Eds.): Ecology and Classification of ing for phylactolaemate bryozoans by sieving North American Freshwater Invertebrates. Academic Press, San Diego: 505-525. lentic sites for their statoblasts. — In: HERRERA- CUBILLA A. & JACKSON J.B.C. (Eds.): Proc. 11th In- A good summary of bryozoan biology, including tern. Bryozool. Assoc. Conf. Smithsonian Tro- Entoprocta, with a well illustrated key for most pical Research Institute, Panama: 259-264. North American species. MUKAI H. & S. ODA (1980): Histological and histo- WOOD T.S. & B. OKAMURA (2005): A new key to the chemical studies on the epidermal system of freshwater bryozoans of Britain, Ireland and higher phylactolaemate bryozoans. — Anno- Continental Europe, with notes on their eco- tationes Zoologicae Japonenses 53: 117. logy. — Freshwater Biol. Ass. Publ., Amblesi- de, UK 63: 1-111. ODA S. (1959): Germination of the statoblasts in The newest regional monograph, including spe- freshwater Bryozoa. — Sei. Rep. Tokyo Kyoiku cies descriptions, many drawings and photo- DaigakuB 9 (185): 90-131.

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Fig. 2: Paired valves of the floatoblast of Plumatella fungosa, separated with hot KOH, showing the pattern of tubercles in the central "fenestra" area. The valve with the smaller fenestra (right) is considered dorsal. Scale bar = 100 urn.

Fig. 3: Aquaria for culturing freshwater bryozoans. The larger aquarium holds only goldfish; the smaller one, normally shielded from light, has bryozoans (not seen) growing on the lower side of inclined glass plates. The foreground tubing is an airlift pump: bubbles of air released at the inside bottom of the long vertical shaft carry siphoned water from the small aquarium into the larger one (WOOD 1996).

Fig. 4: Culturing bryozoans in the laboratory. Inverted plastic petri dishes with plastic half-toothpicks attached are hung from pieces of glass tubing cemented to the inside wall of a culture aquarium. The bryozoans are Plumatella vaihiriae on the left and P. fungosa on the right. Photo is taken from outside the aquarium. Scale bar = 2 cm.

Fig. 5: Culturing bryozoans in situ. Plastic holder for glass petri dish is made from HDPE drainage pipe. The petri plate is inverted with bryozoans facing down. The device is normally suspended below a fixed or tethered floating object.

ODA S. (1980): Effects of light on the germination of statoblasts in freshwater Bryozoa. — An- notationes Zoologicae Japonenses S3: 128- 153.

OKAMURA B.C., JONES S. & L.R. NOBLE (1993): Ran- domly amplified polymorphic DNA analysis of clonal population structure and geographic variation in a freshwater bryozoan. — Proc. Royal Soc. London 253: 147-154.

ROGICK M.D. (1940): The viability of dried statoblasts of several species. — Growth 4(3): 315- 322.

TATKCHI M.I., GUSTTINELU A., FlORAVANTI M L-, CAfFARA M., PIERONI G. & M. PREARO (2004): Is the worm- like organism of Plumatella fungosa (Bryo- zoa, Phylactolaemata) the vermiform phase of Tetracapsuloides bryosalmonae (Myxozoa, Malacosporea)? — Italian J. Zool. 71: 143-146.

WAYSS K. (1968): Quantitative Untersuchungen über Wachstum und Regeneration bei Pluma- tella repens (L.). — Zool. Jahrb. Abt. Anat. Ontogenie der Tiere 85: 1-50.

WIEBACH F. (1974): Specific structures of sessoblasts (Bryozoa: Phylactolaemata). — In: POUYET S. (Ed.): Bryozoa 1974. Proc. 3rd Conf. Intern. Bryozool. Assoc. Documents des Laboratoires de Geologie de la Faculte des Sciences de Ly- on HS 3 (fasc. 1, 2). Universite Claude Ber- nard, Lyon: 149-154.

WOOD T. (1973): Colony development in species of Plumatella and Fredericaila (Ectoprocta). — In: BOARDMAN R.S., CHEETHAM A.H. & W.A. OLIVER (Eds.): Development and Function of Animal Colonies Through Time. Dowden, Hutchinson & Ross, Inc., Stroudsburg, Pennsylvania: 395- 432.

WOOD T. & M. LORE (2005): The higher phylogeny of phylactolaemate bryozoans inferred from 18S ribosomal DNA sequences. — In: WYSE- JACKSON P. (Ed.): Bryozoan Studies 2004. Proc. 13th Intern. Bryozool. Assoc. Balkema, Rotter- dam & Brookfield (in press).

Wöss E. (1996): Life history variation in freshwater bryozoans. — In: GODRON D., SMITH A. & J. GRANT-MACKIE (Eds.): Bryozoans in Space and Time. National Institute of Water & Atmos- pheric Research, Wellington, New Zealand: 391-399.

Wöss E. (2000): Colonization and development of freshwater bryozoan communities on artificial substrates in the Laxenburg Pond (Lower Austria). In: HERRERA-CUBILLA A. & J.B.C. JACKSON (Eds.): Proc. 11th Intern. Bryozool. Assoc. Conf. Smithsonian Tropical Research Institute, Pan- ama: 431-438. Woss E.R. (2002): The reproductive cycle of Pluma- Address of author: tella casmiana (Phylactolaemata: Plumatelli- Dr. Timothy S. WOOD dae). — In: WYSE JACKSON P.N., BUTTLER C.J. & M. SPENCER JONES (Eds.): Bryozoan Studies 2001. Department of Biological Sciences Proc. 12th Intern. Bryozool. Assoc. Balkema, Wright State University Rotterdam & Brookfield: 347-352. 3640 Colonel Glenn Highway Dayton, OH 45435 USA E-Mail: [email protected]

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