Freshwater bryozoans of Lithuania (Bryozoa)

Abstract Nine species of freshwater bryozoans were recorded in Lithuania in a survey of 18 various types of freshwater bodies. Eight species were assigned to the Class Phylactolaemata and families Plumatellidae and Cristatellidae (Plumatella repens, Plumatella fungosa, Plumatella fruticosa, Plumatella casmiana, Plumatella emarginata, Plumatella geimermassardi, Hyalinella punctata and Cristatella mucedo). The ninth species, Paludicella articulata, represented the Class Gymnolaemata. Plumatella geimermassardi and P. casmiana were recorded for the first time in Lithuania. For the plumatellids, species identification was achieved partly by analysing statoblasts’ morphological ultrastructures by scanning electron microscopy.


Introduction
Freshwater bryozoans grow in colonies of minute tentacle-bearing clones (zooids) that feed upon microscopic plankton. They are often found in ponds, lakes, and rivers, forming a cryptic but often a significant part of the aquatic fauna (Bushnell 1966).
Bryozoans are important to ecosystems as filter feeders (Wood et al. 2006), extracting phytoplankton from the water and producing faecal pellets that nourish benthic mei-ofauna (Bushnell and Rao 1974). The colony structure also creates important habitat and shelter for other organisms: protozoans, rotifers, ostracods, nematodes and chironomids (Ricciardi and Reiswig 1994). Bryozoans hosting certain myxozoan parasites can spread proliferative kidney disease in fish, which is often fatal in farmed and wild fish populations (Grabner and El-Matbouli 2008;Bartošová-Sojková et al. 2014).
In their natural habitat freshwater bryozoans are easily overlooked and, in many areas, there is little information on the identity or distribution of species.
Fundamental studies of freshwater bryozoans in Europe were launched with a pioneering monograph by Allman (1856), which established the Class Phylactolaemata as an exclusively freshwater group and named Plumatellidae as the largest family within that class. A second monograph by Jullien (1885) described species from France following Allman's taxonomy. Shortly afterwards a third monograph appeared from Germany (Kraepelin 1887) proposing a new taxonomic scheme for the plumatellids which has by now fallen out of use. Since those early years there have been bryozoan surveys from a number of European countries, including the Netherlands (Lacourt 1949), Sweden (Borg 1941), Italy (Viganó 1965, Luxembourg (Geimer andMassard 1986), Belgium (Loppens 1906), Bulgaria (Gruncharova 1968), Ireland (Smyth 1994), and Britain (Mundy 1980). In most of these works only 5-8 species were documented.
In recent years the number of freshwater bryozoans documented from Europe has grown to 19 Geimer 2005, 2008a;Wood and Okamura 2004). Meanwhile from seven to eleven freshwater bryozoan species have been reported in countries neighbouring Lithuania: Latvia, Poland, and Belarus (Kaminski 1984, Fauna Euro-pea 2013. Based on the species diversity in other European countries, we can expect a more diverse list of bryozoans in Lithuania as well.
The present work describes freshwater bryozoans studied in 18 freshwater bodies in Lithuania.

Climate of Lithuania
Lithuania is distinguished by a highly diverse geography: plains, hills, abundant forests, lakes, wetlands, and Baltic Sea. The climate of the Lithuania can be described as typical European with strong continental influence providing warm summers and fairly severe winters. The weather is often windy and humid due to the proximity of the Baltic Sea.
The average air temperature is 7.2 °C. July is the warmest month with an average temperature of 18 °C. January and February are the coldest months with average temperatures around -3.35 °C, but sometimes winter days can be much colder with temperatures about -32.4 °C. Annual precipitation ranges from 560 to 700 mm. Snow cover can last from 60 to 90 days. The flat landscape retains much of the precipitation, which leads to a relatively high water level (Lithuanian Hydrometeorological Service under the Ministry of Environment).

Characteristic of sampling sites
Our bryozoan survey was conducted during April through October 2015-2017. We investigated localities that included different types of water bodies: lakes, ponds, lagoons and lotic habitats (streams and rivers). Figure 1 shows regions in Lithuania that were surveyed. Geographical details and descriptions of collecting sites are listed below and summarized in Table 1 Table 1).
Water pH varies from 7.51-7.85 (Rokai and Tribalė ponds respectively) to pH 8.35 in Raudondvaris pond. Lampėdžiai lake (located in Kaunas city). Relative large (1,252 km²), semi-artificial lake. The bottom is sandy. Shorelines are without the trees, only Phragmites sp. occurs occasionally in the littoral. Water pH is 8.06. Maišia stream (located in the outskirts of Kaunas). One side of shore is overgrown by deciduous trees, Phragmites sp. and Typha angustipholia. Water is polluted by sewage. Water pH is 7.48. Veršvio stream (located in western part of Kaunas city). Small and shallow stream, that dries up in the summer. Shore is lined by trees and shrubs. Bottom is sandy. Water pH is 7.79.

Region 2
Šventupė pond (Ukmergė district). The shoreline is overgrown by shrubs and other vegetation. Phragmites sp., Acorus calamus, Lemna minor dominates in littoral. A small stream enters in one end of pond. Another end of the pond is connected with Šventoji River. Water pH 7.40. Mūša pond (Ukmergė district). Mūšia stream enters in this artificial pond. Phragmites sp., Acorus calamus and Nymphaea lutea occur in the littoral. Water pH is 7.81.

Region 3
Plateliai Lake (Plungė district) is the large lake covering about 12 km² with a maximum depth of 47 m. Water is contributed by seventeen small streams. The Bottom is sandy in the collecting sites. Skyplaičiai Lake (Plungė district) covers 0.068 km² and is surrounded by a mixed deciduous forest. The bottom is muddy; shorelines are overgrown by Phragmites sp. According to the EU Habitats Directive, this lake is notable for its Charophyta communities.

Region 4
Škilietai Lake (Trakai district) covers about 0.033 km² with maximum depth of 12 m. The lake is surrounded by pine forest. Elektrėnai Reservoir (Vilnius district and Trakai district) is the third largest artificial lake in Lithuania. The reservoir measures about 0.0126 km². The lake is fed by inflows from the Strėva River, and nine other rivulets. Strėva River (Trakai and Kaunas district). Average current velocity is 0.1-0.3 m/s. The bottom is sandy mixed with silt. Phragmites sp. and Nymphaea lutea grow at the edges of the river. Water pH was 7.80 in the sampling site.

Region 5
Saterečius Pond (Utena district). The pond is surrounded by marsh and mixed deciduous forest dominated by Alnus sp. In summer the pond is almost overgrown with macro-algae and such macrophytes as Nymphaea lutea and Stratiotes aloides.
Water pH is 6.78. Žvirgždelis Lake (Utena district) covers an area of 0.027 km²; the bottom is silt and Phragmites sp. dominates in littoral. Water pH is 7.03.

Region 6
Snaigynas Lake (Lazdijai district). The lake covers an area of 2 km², with an average depth of 3 m. The shores are low and overgrown by shrubs and trees. The lake bottom is sandy in littoral. A small shallow stream flows out from this lake into Trikojis Lake.

Sampling and observations
Statoblasts were taken by net from the surface of water and aquatic plants. Bryozoan colonies were collected from submerged branches, stones, and aquatic plants in the littoral of the water bodies. Statoblasts and bryozoan colonies were stored in 70% ethanol. Identification of most species was based on morphology of statoblasts and colony (when colonies were available) using light and scanning electron microscopy (SEM) (Hitachi S-3400N).
Statoblasts characters included overall length and width, length and width of the fenestrae, and surface micro-sculpture of statoblasts. Abbreviations used for measurements are as follows: L/W ratio of the statoblast, VfL ventral fenestra length; VfW ventral fenestra width; DfL dorsal fenestra length; DfW dorsal fenestra width.
Measurements were taken from SEM images with software Original Hitachi S-3400N Scanning Electron Microscope software ver 7.3.
Statoblasts were rinsed with distilled water several times then treated by KOH in order to remove any debris and cleaned using vortex for a few minutes. Statoblasts were prepared for scanning electron microscopy by simple drying without sputtering. The identification keys by Wood and Okamura (2005) were used.

Material examined
The authors collected specimens during April through October 2015-2017. In total, 53 statoblasts and 8 colonies collected from 18 localities in Lithuania were examined. In addition, some data collected by Bronė Pajiedaitė (1932-1934 were included in this study for comparison. The representative specimens are deposited in the zoological collection of Biology Department of Vytautas Magnus University.

Results
The survey of 18 water bodies yielded nine species of freshwater bryozoans (  Description. Colonies were about 5-8 cm size. The transparent branches of colonies were attached to the substratum for almost whole of their length. Floatoblasts were identified by the broadly oval shape and the absence of tubercles on the statoblast  (n=10); DfL 135-258 (178±14) μm; DfW126-212 (163± 9) μm (n=10). Fenestra of floatoblasts circular, covered with rounded tubercles that become less prominent towards the centre of fenestra. The annular nodules often described for this species have not yet been observed in Lithuanian material.
Distribution in Europe. According to Økland and Økland (2005), Wood and Okamura (2005), P. repens is common in Britain, Ireland and Europe. Kaminski (1984) described P. repens as most common species in the studied lakes in Poland.
Remarks on habitat and ecology in Lithuania. Plumatella repens has been the most commonly encountered species, with floatoblasts occurring in all surveyed sites, include lentic and stagnant habitats. Although colonies were found in only two ponds from listed sites, we have since become aware of colonies occurring in other lakes and ponds not listed here. Pajiedaitė (1933) described P. repens as most common species in Lithuania, which can grow in various freshwater bodies. On the other hand, the exact locations of her collecting sites were not listed in her thesis. In addition, because of early difficulty in identifying this species, distribution reports prior to the mid-1980s are not necessarily reliable (Wood and Okamura 2005).
Remarks. Lacourt (1968) postulated close relationship between Plumatella repens and P. fungosa based on a "short oval statoblasts" and molecular studies confirmed a close relationship between these species . Plumatella repens can be confused with young colonies of P. fungosa (Wood and Okamura 2005). In addition, statoblasts of P. repens are similar to those of its congeners P. nitens Wood, 1996, P. nodulosa Wood, 2001, P. orbisperma (Kellicott, 1882, P. recluse Smith, 1992, and P. rugosa Wood, Wood, Geimer & Massard, 1998(Massard and Geimer 2008a. Fig. 3 Material examined. A few floatoblasts from Lampėdžiai Lake in April 2016. P. geimermassardi were recorded in Lithuania for the first time. However, the species is so far represented only by statoblasts.

Plumatella geimermassardi Wood & Okamura, 2004
Description. Floatoblasts were identified by the large dorsal fenestra with tubercles and narrow annulus. The annulus at the poles is mostly as large as laterally and is covered by weakly visible tubercles (Fig. 3) Distribution in Europe. Plumatella geimermassardi is known from England, Ireland, Belgium, southern Norway, northern Germany, Italy and Finland (Wood and Okamura 2005).
Remarks. Floatoblasts of P. geimermassardi are among the smallest floatoblasts among all European plumatellids with an average length of around 320 μm (Wood and Okamura 2004). The uniformly narrow annulus offers an easy identifying feature characteristic for broad floatoblasts in this species. The relatively large area of dorsal and ventral fenestrae is matched only by those of P. nitens or Stephanella hina on other continents (Wood 1996;Toriumi 1955).  Fig. 4 Material examined. A floatoblasts collected from Linksmakalnis pond (June 2015) and Maišia stream (April 2015). Colony from Aristava pond (locates in Kėdainiai district 55°17'07.1"N, 24°04'28.6"E and it is not included in general list of studied sites during this survey) was taken in June 2017 (Fig. 4).
Distribution in Europe. According to Wood and Okamura (2005) P. fungosa is widespread in Europe. It has been recorded from several places in southern Sweden and Finland; it is common in Denmark and has been reported from Iceland (Økland and Økland 2005) and Poland (Kaminski 1984).
Remarks on habitat and ecology in Lithuania. During this survey, floatoblasts of P. fungosa were found in seven water bodies from 18 surveyed, with prevalence in stagnant water, with neutral to slightly alkaline pH 7.01-8.15 (Table 1). Pajiedaitė (1933) described the colonies in Kaunas Lagoon, Nevėžis River (Kaunas district) and Dubysa River (Šiauliai district). She noted that P. fungosa often occurred in polluted water and described colonies, found in old port of Kaunas city, where water was polluted by oil of ships and trash. A similar observation has been made by other authors (Bushnell 1966;Geimer and Massard 1986). Based on the available data, we consider P. fungosa to be prevalent in Lithuania.
Remarks. Large bulky colonies of P. fungosa are easily recognizable freshwater bryozoan species in Europe (Wood and Okamura 2004). Floatoblasts of P. fungosa are lateralyasymmetrical and distinctfrom the symmetrical floatoblasts of P. repens and P. rugosa. Molecular studies showed a close relationship between P. repens and P. fungosa . Distribution in Europe. Geimer and Massard (1986) defined the range of this species to include most of Europe. Økland and Økland (2005) considered P. emarginata to be a southern species, with limited distribution in Norway and Sweden.
In our survey only a few statoblasts were found in Šventupės pond (Table 1). Wood and Okamura (2005) noted that P. emarginata is particularly tolerant of rapidlyflowing water. The occurrence of floatoblasts in the Neries River (Kaunas district -not included in this study) is consistent with this observation, although colonies were not found. From our data P. emarginata would be considered uncommon in Lithuania, although this should be verified through further surveys.
Remarks. The species is widely distributed throughout the Holarctic (Wood and Okamura 2005), although some reports may have confused it with similar species, P. mukaii or P. reticulata (Massard and Geimer 2008a).  Description. Colony was about 5-6 cm long. Branches of colony are short, almost entirely attached to the substrate. The terminal parts of branches are semi-transparent and whitish. The floatoblasts were recognized by the distinctly elongated shape of the fenestra on both valves. Both capsuled floatoblasts and the distinctive leptoblasts were found, along with associated colonies (Fig. 6A, B)  ( Fig. 6B, right side) have a uniformly narrow annulus and extensive oval fenestrae; which length was at least 1.5 times its width.
Distribution in Europe. Plumatella casmiana is currently known throughout most of Europe (Massard and Geimer 1995b).
Remarks on habitat and ecology in Lithuania. This is the first reported occurrence of P. casmiana in Lithuania. Floatoblasts of P. casmiana were recorded in almost half of the investigated water bodies (Table 1). However, colonies were found in Linksmakalnis Pond only.

Distribution in Europe.
Plumatella fruticosa is considered to be widespread, especially in northern portion of Europe (Økland and Økland 2005). It is considered common in Poland (Kaminski 1984).
Remarks on habitat and ecology on Lithuania. Pajiedaitė collected colonies in Dubysa river (Šiauliai district) and Satarečius pond (Utena district) (Pajiedaitė 1933) At first glance we could state that P. fruticosa is common in Lithuania, since during this survey statoblasts were found in most water bodies. However, we found colonies only in Rokai pond with sandy-mud bottom and stones in the littoral (Table 1, Fig. 7). Thus, it is possible, that statoblasts are spread by waterfowl among various ponds and lakes, but these may not be the preferred environment for growing colonies (Økland and Økland 2005).
Remarks. The combined statoblast characteristics (large length/width ratio, strong asymmetry of floatoblast and sessoblast, narrow fenestra on dorsal floatoblast valve) distinguish P. fruticosa from all other plumatellid species (Ricciardi and Reiswig 1994). Molecular results provided by Hartikainen (Hartikainen et al. 2013) imply that P. fruticosa is not a plumatellid and provide evidence for polyphyly in Plumatella. However, the position of P. fruticosa remains unresolved (Hartikainen et al. 2013). Fig. 8 Material examined. A few statoblasts from Veršvio stream were found in August 2015 Unfortunately, these were later lost before critical dimensions could be taken.

Hyalinella punctata (Hancock, 1850)
Description. Colonies were not observed, and species was identified according floatoblasts. The statoblasts are larger than any other plumatellid species and show crowded tubercles on the fenestrae of both valves. Pajiedaitė (1933) described floatoblasts by oval shape, with length 440 μm and width 230 μm. These dimensions were slightly smaller than 500 μm and 350 μm suggested by Wood and Okamura (2005).
Distribution in Europe. Hyalinella punctata has been widely reported worldwide, including neighbouring Poland (Kaminski 1984), but verified specimens are known only from Britain, Ireland, Europe, North America and northern Asia (Wood and Okamura 2005).
Remarks on habitat and ecology in Lithuania. Few small colonies of H. punctata were described on Nymphaea lutea leaves in small lakes in the Zarasai district (55°44'50"N, 25°50'4"E) and Dubysa river (Šiauliai district; 55°51'29"N, 23°08'31"E) by Pajiedaitė (1933). During the present survey, floatoblasts of H. punctata were recorded only in the Veršvio stream (Table 1). The available data are not sufficient to estimate the prevalence and frequency of this species in Lithuania.
Remarks. Hancock (1850) described colonies of H. punctata as "thick and transparent with less profuse branching than in Plumatella and produce only floatoblasts, while individual zooids are indistinct, usually arranged linearly and lack interzooidal septa". In fact, features distinguishing Hyalinella from Plumatella are not clear-cut (Hirose and Mawatari 2011), because the diagnosis of Hyalinella is based on the transparency and thickness of the colony wall (ectocyst), but the condition of the ectocyst depends to some extent on environmental factors Okamura 2005, Hirose and. Generic placement of some species between Plumatella and Hyalinella has remained unstable (Hirose and Mawatari 2011).
Description. Colonies of C. mucedo are recognized by their elongated shape and colourless, transparent body wall. The length of colonies found varied from 5 to 10 cm (Pajiedaitė 1933;this study). The large statoblasts are easily recognized by circular form with hooked spines radiating from the edges of the fenestrae on both valves (Fig. 9). Diameter of statoblasts was about 1 mm.
Distribution in Europe. Cristatella mucedo is a common species in Lithuania, with a Holarctic distribution, occurring in Britain, Ireland, Europe, Asia and North America (Økland and Økland 2000;Wood and Okamura 2005).
Remarks on habitat and ecology in Lithuania. During this survey a few colonies of C. mucedo occurred in South Lithuania (Snaigynas lake), but statoblasts were found in various water bodies of different regions of the country (Table 1). Pajiedaitė (1933) noted that C. mucedo more often occurred in South Lithuania. She found colonies of C. mucedo without statoblasts in June/July and noted that statoblasts inside colonies appeared in first part of August. Numerous colonies with statoblasts were found at the end of September and they died late autumn once the water temperature dropped to 3 °C in November 1932 (Pajiedaitė 1933).
Remarks. A more detailed discussion of the ecology and life history of C. mucedo can be found in Okamura (1997).
Description. The species was recognized by the slender colony branches forking at wide angles and often growing free from the substratum. Colonies were small, about 2-3 cm. Branches of colony were transparent and shiny. Zooids 1.0-1.5 mm in length with 16 tentacles on a circular lophophore were described by Pajiedaitė (1933).
Distribution in Europe. Paludicella articulata is known worldwide (Wood and Okamura 2005). However, the species has not been found in Poland (Kaminski 1984). Remarks on habitat and ecology in Lithuania. Paludicella articulata was recorded by Pajiedaite (1933) in only two localities: Paštys Lake (Utena district) (55°42'36"N, 25°41'48"E) and Satarečius pond (Utena district). Since P. articulata tolerates cold temperatures (Økland and Økland 2005) and prefers flowing water (Wood and Okamura 2005) it was surprising finding of this species in stagnant Satarečius pond together with C. mucedo. Coexistence of the two species was also noted by Pajiedaitė (1933), who explained it by different local conditions in the same pond; colonies of P. articulate were observed only near a small stream flowing into the pond. Otherwise, she noted that C. mucedo was mostly observed in the warmer waters of Central and South Lithuania. Økland and Økland (2005) showed positive co-occurrence of these two species in Norway.
During this survey P. articulata was found in the outlet of Snaigynas lake, which is of glacial origin and characterised by low temperature.
Remarks. Colonies of P. articulata consist of sometimes creeping but more often elongated, mostly erect, slender zooids. There are normally three adjacent zooids: one distal and two lateral ones (Davenport 1891) The contiguous arrangement of the zooids and the subterminal 4-sided zooecial orifice readily distinguish the species from its closest relative, Pottsiella erecta (Ricciardi and Reiswig 1994).

Discussion
Overall nine species of freshwater bryozoans are now known from Lithuania. This contrasts with about 19 species reported from Europe and about 13 species from the Baltic area Nikulina 2006). Given the fact that only a relatively few water bodies of Lithuania have been investigated so far it is likely that the final tally of species will be higher.
The majority of surveyed pools were stagnant, neutral or slightly alkaline (Table 1) and should have been suitable for bryozoans to grow successfully. However, while intact colonies were found only in few sites, statoblasts were widely distributed. The rarity of colonies in water pools might be explained by fluctuating climatic conditions, especially the alternation of drought and rainfall. The similar process under Lithuanian conditions was described by Pajiedaitė (1933), who found P. articulata and P. repens colonies in Paštys Lake in 1931, but completely absent the following year. Pajiedaitė (1933) suggested this disappearance may have been due to rising of water level after rainfall in Paštys Lake. She wrote that bryozoans are sensitive "creatures" and cannot survive such drastic environmental change. She went on to describe a similar situation in Nevėžis River in 1932, where after week of rainfall, nearly all bryozoan colonies had died (Pajiedaitė 1933). Jong-Yun- Choi et al. (2015) documented the negative effects of heavy rainfall in Korea on colonies of Pectinatella magnifica (Leidy, 1851).
Another possible reason of finding small number of colonies could be the lack of suitable substratum for the attachment of colonies. Because bryozoans are sessile organisms, they absolutely require a solid, inert substratum on which to grow (Ryland 1970). For example, we have found statoblasts of five bryozoan species in a pond at the Kaunas botanical garden (Table 1). However, colonies have never been observed there, possibly due to lack of solid substratum to which bryozoan colonies can attach. The presence of statoblasts could be the result of waterfowl, which are known to transport them from one site to another (Wood and Okamura 2005).
With this study, we have now recorded 13 species of freshwater bryozoans recorded in Baltic area: C. mucedo, P. magnifica, Fredericella indica Annandale, 1909, Fredericella sultana (Blumenbach, 1779, Lophopus crystallinus (Pallas, 1768), H. punctata, P. casmiana, P. emarginata, P. fruticosa, P. fungosa, P. geimermassardi, P. repens, Stolella indica Annandale, 1909, and P. articulata Nikulina 2006) All bryozoan species documented in Lithuania are common and widely distributed through Europe. The composition of species found through this survey was similar to that recorded 86 years ago, with the exception of finding two additional species for Lithuania. The status of P. repens and P. fungosa as common freshwater bryozoan species, and P. articulata as rare, have not changed for almost a century.
Curiously, this survey did not encounter F. sultana, which is otherwise well known in northern Europe, Britain and Ireland (Geimer and Massard 1986;Wood and Okamura 2005). Also absent was the large gelatinous species, P. magnifica, which is currently expanding its range across Europe and is already reported from areas including Hungary (Szekeres et al. 2013), Germany (Grabow 2005), Czech Republic (Rodriguez and Vergon 2002;Balounova et al. 2011), andPoland (Balounová et al. 2011). Additional bryozoan species that might be expected in Lithuania include L. crystallinus, which is recorded in neighbouring countries as Belarus, Kaliningrad and Poland (Nikulina 2006); also, P. rugosa, P. reticulata Wood, 1988, and F. indica. Since the brackish species, Victorella pavida Saville Kent, 1870, is known in neighbouring Latvia, it is likely to be found also in Lithuania.
In summary, we believe that the list of freshwater bryozoa presented here is not final. Lithuania is an extremely watery region; there are more than 3000 lakes of a wide variety of sizes and many rivers flow across the country. Therefore, it is very likely, that further research will reveal additional species.