Australian Rotifera: Ecology and Biogeography

R. J. ShielA and W. KosteB

ABotany Department, University of Adelaide, Box 498, G.P.O., Adelaide,S.A. 5001, Australia

BLudwig Brill Strasse 5, Quakenbruck D-4570, Federal Republic of Germany

Abstract

The status of studies on Australian Rotifera is reviewed. Approximately 600 species are knownfrom Australian inland waters, with c. 15% endemicity. The ecology of rotifers in four broadhabitat categories (saline lakes, billabongs, reservoirs and rivers) is summarized, and thebiogeography of the known fauna considered.

Introduction

The Rotifera, a small group of metazoans predominantly fresh water in affinity, havebeen known for approximately 300 years. They have not been accorded a comparabledegree of study to other metazoan groups, due in part to their small size (most < 1mm) and consequent difficulty of observation. Nevertheless, with what would now be

considered limited facilities, early naturalists described hundreds of species and providedsystematic drawings that remain basic references (e.g. Hudson and Gosse 1886).

With the exception of the rotifer fauna of New Zealand (e.g. Russell 1960), untilrecently little was known of the Rotifera of the Southern Hemisphere. Early collectorsprovided species lists and some ecological information for the Rotifera of eastern

Australia; approximately 250 species were recorded in these studies (reviewed by Shieland Koste 1979). Since then, we have added a further 350 species to the Australianrecord (cf. Koste and Shiel 1986). It is likely that this represents less than half of theextant fauna; we have collected from only a small area of the continent, with limitedseasonal data. From 1400 to 1600 taxa are known from comparable areas of Europeor North America (cf. Koste 1978).

An interesting feat{ire of the Australian rotifer fauna is the degree of endemicity(c. 15%) in a group long considered to be cosmopolitan. Recent studies (e.g. De Ridder1983; Dumont 1983) suggest that this trend may be more widespread; there is indeeda cosmopolitan component in the rotifer fauna, but there are endemics on all continents,and distinctive zoogeographic associations. The purpose of this contribution is, therefore,to document the rotifer community composition in a range of ephemeral and permanentwaters of mainland Australia and Tasmania.

Ecology

The lack of basic information on rotifer ecology in Australian inland waters stems fromhistorical and geographical factors: low population density (especially of collectors),and large uninhabited areas west of the Dividing Range subject to low and highly variablerainfall, with an abundance of saline lakes and predominantly ephemeral fresh waters.It is, therefore, not surprising that early collectors of Rotifera confined their attention

to smalllentic habitats (garden ponds, dams) in the environs of Brisbane, Sydney andMelbourne.

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Lllnnology in Australia

The first local study to detail population dynamics and ecology of a rotifer was thatof Walker (1973) on the cosmopolitan halophile Brachionus plicatilis in Lake Werowrap,western Victoria. An extensive survey of Murray- Darling waters (Shiel 1981) establishedmarked differences between the rotifer communities of reservoirs, rivers and associatedbillabongs. Recent papers containing information on rotifer ecology include Sudzukiand Timms (1980) (farm dams); Koste (1981), Koste and Shiel (1983), Shiel and Koste(1983), Tait et al. (1984) (Northern Territory billabongs); Shiel et at. (1982), Geddes(1984) (lower River Murray); Shiel and Koste (1983) (River Murray billabongs); Brockand Shiel (1983), Koste et al. (1983) (Western Australian wetlands); Ganf et at. (1983)(parasitism); Shiel and Walker (1984) and Shiel (1985) (Darling River). Most of thesestudies are synecological; little is known of the autecology of Australian Rotifera. Presentinformation in the context of this contribution is summarized below. As yet there isno information on marine, estuarine or psammon rotifers from the continent.

Autecology

In the only long-term local study to include life-history data on a rotifer, Walker(1973) described seasonality, including birth, population growth and mortality ratesfor Brachionusplicatilis in a saline lake. Fig. 1 shows population dynamics over the studyperiod. Summer peaks reached> 7000 individuals per litre, although production waslower than for comparable studies elsewhere, probably as a result of salinity-alkalinitystress. The Werowrap study also provided evidence that bacteria-detritus comprisedan important dietary item for B. plicatilis. Further information on the autecology ofthis species is given by Walker (1981).

There is some information available on seasonal changes in morphology in the endemicKeratella slacki, which occurs in billabongs as a summer 'dwarf', and as a long-spined

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142

Ecology and Biogeography of Australian Rotifers

morph (to 300 f.lm, the largest species in the genus) in spring. A spine-elongation responseto the predatory rotifer Asplanchna is documented for South Australian K. slackiby Gilbertand Stemberger (1984). Fig. 2 shows a specimen of K. slacki ingested by Asplanchnasieboldi, and demonstrates the selective advantage of long spines (cL Gilbert 1967)!

Other species are mentioned in taxonomic reports or general plankton studies. Byfar the most information on rotifer ecology available, as in the Northern Hemisphere(cL Ruttner-Kolisko 1974), is on community ecology.

Synecology

Most of our present information on rotifer community composition in inland watersis based on collections taken in 1976-1984 from waters of the Murray-Darling basin,with limited collecting from the Northern Territory, Tasmania and Western Australia.Although there may be 50 or more sampling dates from a single locality, providinga reasonably comprehensive estimate of the diversity of rotifers, it has not been possibleto analyse adequately causality of seasonal compositional changes in rotifer populations.Correlations between the occurrence of a plankter and ecological variables are difficultto determine, particularly with rotifers, which may be sensitive to very small changesin a particular environmental variable (cL Ruttner-Kolisko 1974). The only clearevidence of single variables affecting community composition is the depression of speciesdiversity in the Murray with increasing salinity, and similarly with pH changes inthe extreme biotopes of billabongs in the Northern Territory.

Data are accumulating on the tolerances of individual species (cf. Shiel et al. 1982),and it is evident that the cosmopolitan species that occur in Australia have similarecological requirements to their counterparts elsewhere. Some endemics have specificrequirements and are restricted in distribution (e.g. Brachionus kostei, B. pinneenaus),while others are pancontinental, eurytopic opportunists (e.g. Keratellaaustralis) K. slack!).

Clear community similarities are apparent in four broad habitat types.

Saline Lakes (Fig. 3a). All of Australia's major lake basins are ephemeral, and mostare saline (cL De Deckker 1981). The most-studied saline waters are those of westernVictoria (cL Walker 1973). The typical halophilic rotifer assemblage is simple: Brachionusplicatilis and Hexarthra fennica or H. jenkinae, which may reach very high populationdensities (Walker 1973; Hammer 1981). A very large endemic subspecies, B. plicatiliscolongulaciensis(to 440 f.lm), occurs in Lake Colongulac. B. plicatilis and H. fennica alsowere recorded as the dominant taxa in south-west Western Australian saline waters

(Brock and Shiel1983), although a greater diversity ofrotifers occurred in low-salinitywaters «3 gl-l). This was attributed to the persistence of eurytopic taxa (e.g.

Fig. 2. Camera lucida drawing ofAsPlanchna sieboldi after ingestion of long-spined Keratella slacki. The integument ofthe predator is punctured by the caudalspines of the prey.

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Limnology in Australia',,-

B. calyciflorus and K. australis) in ephemeral habitats (Koste et al. 1983). B. plicatilisf. typ. occurs also in saline waters in Tasmania (De Deckker and Williams 1982), withH. oxyuris co-occurring (Koste and Shiel 1986).

Billabongs (Fig. 3b). Billabongs and shallow wetlands associated with rivers have

provided the most diverse (i.e. species-rich) rotifer communities, and the highest rotiferpopulation densities yet recorded from the continent. Invariably, billabong 'plankton'assemblages contain a mix of true plankton and incursion species from submergedmacrophytes. For example, of 80 rotifer taxa recorded in a 1981 tow from a MagelaCreek (Northern Territory) billabong, more than half were littoral in habit, including19 species of Lecane. True plankters in Northern Territory samples commonly areBrachionus falcatus, Keratella lenzi, K. tropica, Asplanchna brightwelli and Filinia opoliensis,a tropical association, with localized populations of acidophilic taxa (e. g. B. urceolarissericus) reaching very high densities (> 70 000 1- 1) in seasonally stressed habitats. Thepoor representation of brachionids generally, in contrast to other tropical areas, probablyis due to the predominance of acid waters, in turn reflecting the interaction of geologyand climatic regime.

Temperate billabongs, e.g. in the Murray basin, do not suffer the seasonal climaticextremes of monsoonal Australia: water-level fluctuations tend to be less extreme and

less rapid. The plankton communities also are less diverse: 4-20 rotifer taxa are common,and densities rarely exceed 1000 1- 1. Most common taxa in the plankton of aGoulburn River series 1976-1984 were B. quadridentatusmelheni, K. cochlearis,K. procurva,

(a)

(c) (d)

Fig. 3. Habitat types from which associations of Rotifera have been categorized: (a) salt lakes(Lake Gillies, Eyre Peninsula, S.A.); (b) billabongs (Goulburn View, Alexandra, Vie.);(c) reservoirs (Eildon Reservoir, Goulburn River, Vie.); (d) rivers (River Murray nearMannum, S.A.).

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Ecology and Biogeography of Australian Rotifers

K. slacki, Euchlanis dilatata, Polyarthravulgaris,A. brightwelli, F. longiseta.In a River Murrayseries over the same time, the dominants were B. calyciflorus, B. quadridentatus melheni,K. australis, K. procurva, K. slacki, Synchaeta pectinata, Pompholyx sulcata and Conochilusdossuarius. These taxa are cosmopolitan or pancontinental, widely tolerant ofenvironmental changes in their shallow lentic habitats.

Billabongs clearly are refuges for aquatic fauna in an otherwise harsh climatic regime.More than palf of the 600 species of Rotifera known from the continent are recordedonly from billabongs or shallow wetlands associated with river and lake margins. Mostof the new species of Rotifera described from Australia are from billabongs (cL Koste1981) .

Reservoirs (Fig. 3c). The proliferation of impoundments on Australia's rivers sincethe advent of European man has provided lacustrine conditions where they did notoccur previously. The rotifer limnoplankton of these storages typically is composedof cosmopolitan taxa, with similar ranges and requirements as reported from NorthernHemisphere studies. Seasonal drawdown for irrigation, stock and domestic use limitsdevelopment of marginal hydrophytes, so that the occurrence of littoral taxa in open-water collections from deep storages is rare.

Predominant genera in 1976-1984 in Murray basin reservoirs were Brachionus,Keratella, Synchaeta, Polyarthra, Asplanchna, Conochilus and Hexarthra. Local differences inwater quality lead to characteristic species assemblages in each reservoir, e.g. in autumn1980, the plankton in Lake Eildon was dominated by K. cochlearisand P. vulgaris, whileLake Hume, on the same day, had B. urceolaris,S. pectinata and A. priodonta as the mostabundant rotifers, and Dartmouth Dam, upstream ofHume, had dominants Lacinulariaismaeloviensis, S. oblonga and C. unicornis. These local differences extend to seasonality:in stable, long-retention-time storages, e.g. Eildon, there are synchronous appearancesof the same taxa at approximately the same time each year, although rotifers usuallycomprise> 20% of the limnoplankton. In short-retention-time storages, opportunistspecies are asynchronous, often of only short duration in the plankton, and may comprise> 80 % of the limnetic zoo plankton community. Fig. 4 shows seasonality of Rotifera

in Dartmouth Reservoir during its filling phase, when rotifers comprised> 70% of

1977 1978 1979

Fig. 4. Seasonality of dominant Rotifera in Dartmouth Reservoir, 1976-1980.

145

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Limnology in Australia

the plankton, compared to the downstream Hume Reservoir, which is dominated byCopepoda (Rotifera < 20%) (Shiel 1986).

With few exceptions, rotifer population densities and species diversity are lower inreservoirs than recorded from billabongs. Densities range from < 1 1- 1 in oligotrophicwaters (e.g. Rocky Valley Dam, Kiewa River) to 200-300 1-1 in downstreammesoeutrophic impoundments (e.g. Burrinjuck Dam, Murrumbidgee River). Thereare occasional summer peaks, e.g. 'blooms' of the colonial alga Volvoxmay be parasitizedby the rotifer AscomoTphellavolvocicola(cL Ganf et al. 1983), which may reach high densities(> 2000 1-1) until the bloom collapses. Species diversity generally is highn inoligotrophic than in eutrophic reservoirs, although there is a masking effect of non-planktonic taxa in shallow downstream reservoirs, e.g. Lake Mulwala on the Murray.

Rivers (Fig. 3d). It is evident from the long-term study by Shiel et al. (1982) that acomplex rotifer potamoplankton is maintained in the slow flows of the westward-flowingrivers of the Murray basin. Long travel times, high turbidities of unimpounded rivers,allochthonous nutrient input, and moderate physical and chemical conditions providea habitat combining features of reservoirs and billabongs. In consequence, the riverplankton that persists combines limnoplankton and heleoplankton elements (fromupstream reservoirs, with an increasing contribution from billabongs and floodplainwaters further downstream) and a riverine component. The lower Murray, for example,may have 20-30 rotifer species in spring and autumn: a complex species-rich communityof tropical taxa derived from Darling River flows, mixed with a temperate assemblagefrom the River Murray. Highly turbid, alkaline Darling waters are dominated bybrachionids (three to six species of Brachionus and four to seven of Keratella are common)and warm stenotherms, e.g. Synchaeta, TTichoceTcaand Filinia spp. In contrast, Murraywaters carry essentially a limnoplankton, including cold stenotherms, of different speciescomposition to the true potamoplankton of the Darling. The Murray rotifers usuallyare associated with diatom or cyanobacterial blooms moving downstr~am from reservoirs.

Principal influences on the Murray plankton are temperature and turbidity, withsalinity and flow variations of lesser significance. The perennial riverine Rotifera arewidely tolerant of environmental fluctuations; approximately half are cosmopolitan andhalf pan continental endemics, with a few taxa apparently confined to the Murray-Darling system. Population densities rarely exceed 200 1-1 in the downstream Murray,partly because of the high level of particulate clay suspensoids.

Little is known of the rotifer fauna of rivers outside the Murray basin. Brock andShiel (1983) reported a halophilic assemblage dominated by B. Plicatilis from south-west Western Australia and this species also is common in the saline drainage of westernVictoria, e.g. Richardson River (Shiel, unpublished data). The Rotifera (if any!) ofthe shorter and more rapidly eastward-flowing rivers, or of rivers of northern Australiaand Tasmania, are unknown.

Feeding. Trophic relationships of the Rotifera in any of these habitats are little known.Most planktonic rotifers are 'filter feeders', obtaining their nutrients from water currentscreated by the coronal cilia. The various types of feeding and food types are reviewedby Pourriot (1965) [see also symposia proceedings edited by King (1977), Dumontand Green (1980) and Pejler et al. (1983)]. Some incidental local observations arerecorded in the literature. In the turbid waters of the Darling, for example, high levelsof montmorillonite-kaolinite clays apparently depress photosynthetic algae. Thepredominance of microphagous detritivores in the Darling Rotifera (BrachionusangulaTis,KeTatellaspp., Filinia spp.) suggests that these zooplankters derive nutrients from bacteriaor organic particles associated with fine suspensoids (cL Starkweather and Bogdan 1980).

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Ecology and Biogeography of Australian Rotifers

Diverse rotifer assemblages associated with diatom blooms in the lower Murray maybe a direct trophic effect; those associated with cyanobacterial peaks in summer mostlyare small detritivores.

Direct evidence of feeding by AsPlanchna, a predatory species, was given by Kosteand Shiel (1980a, 1980b), who found entire Volvoxcolonies, B. falcatus, B. novaezealandia,F. opoliensis, F. pejlerigrandis and K. procurva in the guts of A. sieboldi from River Murraybillabongs. Subsequently, we have recorded a diverse range of taxa in the gut contentsof A"planchna spp., including algae, rotifers, copepods, small cladocerans and detritus.

Specific prey interactions are known also from Mount Bold Reservoir, South Australia,where a pulse of Synchaeta spp., predominantly S. pectinata, followed a chlorophyteflagellate, Carteria sp. The rotifer community reached densities> 25000 1- 1 inNovember 1981, and disappeared within a week as the food supply was exhausted.Also from Mount Bold is the report of the decimation of a Volvox population by itsparasite, Asromorphella(Canf et at. 1983). There is little other evidence for rotifer parasitesin Australian waters, although species known to be parasitic from Northern Hemispherestudies are recorded, e.g. CePhalodellaspp., ~ DicranoPhorus spp., Proales spp.

Also from South Australian waters comes the first evidence of epizoic Rotifera inAustralian waters (Shiel and Koste 1985). Fig. 5 sbows B. novaezealandia epizoic onDaPhnia carinata. The species also occurs on Pseudomoznalemnae.Other species of Brachionusknown to be epizoic in habit, e.g. B. rubens, occur in Murray-Darling waters, but havenot been collected on a host.

Apart from these incidental observations, little is known of the communityrelationships of the Australian Rotifera. Research on feeding is long overdue. On thebasis of NortherwHemisphere studies, we assume that similar relationships exist forthe cosmopolitan species that occur here, i.e. there are small bacterial-detritivore feeders,

Fig. 5. Scanning electron micrograph of Brachionusnovaezealandiaepizoic on Daphnia carinata s.l. from a farm dam near Hahndorf,South Australia. Scale bar, 100 p.m.

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Limnology in Australia '-

herbivores, omnivores, carnivores and parasites on algae and other zooplankton. Thereis evidence of extraordinary size development in endemic species of Keratella andBrachionus, possibly in response to predatory Asplanchna, but the reasons for such responsesin the characteristically highly turbid waters are unknown. There also is evidence thathighly complex trophic webs occur in billabongs, where species diversity is high.

BiogeographyAreas of Australia from which rotifers are known represent more than 1 million km2,however this is a relatively small proportion of the Australian continent (one-seventh).It is likely that a third to a half of the Australian species remain to be recorded, andthese comments here must be considered preliminary.

On a global basis, continental Australia appears to have c. 15% endemism in theRotifera. This is low compared to other planktonic groups, e.g. c. 35% in Cladocera,> 60 % in cyclopoid Copepoda, and> 90 % in calanoid Copepoda, and undoubtedly

will increase with further study. Endemics are distributed across all families, withbrachionids best represented on present evidence. A recent review (Dumont 1983) lists23 species of Brachionus from Australia, with six endemic species (B. baylyi, B. dichotomus,B. keikoa, B. kostei, B. lyratus and B. pinneenaus). Additionally, we have recorded B.

Jorficula, B. nilsoni, B. patulus and B. variabilis, bringing the total to 27, second onlyto South America (31 species and eight endemics). The incidence of Keratella also issimilar to that in South America: 15 species, three endemic (K australis, K shieli andK slacki) versus 15 species, six endemic. K ahlstromi and K sancta, described asAustralian taxa (Pejler 1977; Dumont 1983) more correctly are New Zealand records,and have not been documented from Australia. K. sanctasubsequently has been recordedfrom the sub antarctic Kerguelen Archipelago (Lair and Koste 1984).

More than half the recorded taxa from the continent appear to be limited indistribution, compared to 49% of the European species known from single localities(Berzins 1978). Interestingly, most of those recorded from only single collections orsingle habitats occur in the sheltered waters of billabongs, which could be consideredas 'islands' in terms of population genetics.

The Australian rotifer assemblages provide further evidence for non-cosmopolitan ismin the Rotifera, a group previously considered to be cosmopolitan in distribution (cLDe Ridder 1983; Dumont 1983). Northern Australian assemblages have clear affinitieswith the Indo-Asian fauna, with a pantropical component (Koste 1981; Tait et al. 1984).Habitat extremes, such as pH or conductivity, appear to be limiting in northernAustralia, although 'tropical' assemblages are recorded as far south as 37°S. in south-flowing River Murray waters or relatively stable River Murray billabongs.

Recent work in Tasmania (Koste and Shiel1986) indicates disjunct and anomalousdistributions: rotifers previously considered to be cosmotropical (e.g. Filinia opoliensis,Horaella brehmi) are widely distributed in natural lakes of Tasmania's central plateau,and others regarded as cold stenotherms occurring only in winter or in highland waterson the mainland (e. g. Filinia australiensis, K quadrata) also are widely dispersed. Furtherwork on the rotifer plankton of the island should provide an interesting comparisonwith the mainland fauna, perhaps necessitating a fifth category of rotifers not encounteredon the mainland, a true lake plankton dominated by cold stenotherms comparable tothe abundant lacustrine assemblages of the Northern Hemisphere.

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Ecology and Biogeography of Australian Rotifers

Acknowledgments

The support of the University of Adelaide and the Australian Biological ResourcesStudy is gratefully acknowledged. Thanks also to Margaret Brock, then of MurdochUniversity, Peter Hawkins,James Cook University of North Queensland, Russell Tait,then of Pancontinental Mining, Brian Timms, Avondale College, and Peter Tyler,University of Tasmania, for providing collections or otherwise assisting. Our thanksto Prof. A. Ruttner-Kolisko for providing critical comments on a draft manuscript.

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