ECOLOGICAL ADAPTIONS OF A FRESHWATER SPONGE ASSOCIATION IN THE RIVER RHINE, GERMANY (POR1FERA: SPONGILLIDAE) JOCHEN GUGEL Gugel, J. 199906 30: Ecological adaptions ofafreshwatersponge association intheRiver Rhine,Germany (Porifera:Spongillidae).MemoirsoftheQueenslandMuseum44:215-224. Brisbane. ISSN 0079-8835. The species composition and autecology of freshwater sponges (Porifera, Spongillidae) were investigated inthe Rhine between Karlsruheand Bonn (Germany) between 1993 and 1995. Ephydatia fluviatilis, E. muelleri, Trochospongilla horrida, Spongilla lacustris, Eimapiusfragilis and E. carteri were found. Ephydatiafluviatilis was classified as an r-strategist due to its high ability to colonise new habitats, whereas other species placed emphasis on successful establishment in more stable habitats and should therefore be classified as K-strategists (among freshwatersponges). Similarly, the production oflarvae wasanintegralpartofthelifecycleonlyinE.fluviatilis,whereasotherspeciesputtheirmain efforts in producing gemmules as distribution-units. Asexual vs. sexual reproductive strategies in freshwater sponges in running-water habitats is discussed in terms oftheir G prevalence, periodicityandinfluenceoflimnological factors. Porifera, Spongillidae, life cycle, adaptions, centralEurope, runningwater, riverRhine. Jochen Gugel (email: [email protected]) Darmstadt University of Technology, , Institute of Zoology, Schnittspahnstrasse 3, D-64287 Darmstadt, Germany. Present address: Tel Aviv University, George S. Wise Faculty ofLife Sciences, Department of Zoology, RamatAviv, TelAviv69978, Israel: 15March 1999. A considerable number ofpublications on life Kinzelbach, 1995) have been taking place, and cycles offreshwater sponges are now available have influencedthe biocoenosis considerably. (e.g. Gilbert et al., 1975, Frost et al., 1982, Courreges & Fell, 1989, Bisbee, 1992). Mostly Results ofFranz(1992) indicatethattheRhine these focus on single life cycle events, such as isahighlysuitablehabitatforsessilefilterfeeders. formation of larvae or gemmulation, and often Not only is the nutritional situation excellent for include only a single species. Only a fewpapers these animals due to its eutrophic waters, butthe deal with associations of several species, their banks are entirely covered by rocks which colonisation strategies and spatial competition provide a suitable substrate. Filter feeding is not (e.g. Williamson & Williamson, 1979; Mukai, restricted to sessile animals - in particular many 1989; Pronzato & Manconi, 1991). The present insectsalsogaintheirnutritionfromfilterfeeding ioncvceusrtiinggatiionntrheepoRrhtisneonbefitvweeesnympKaatrrliscrushpeecaineds i-vaintydoMfanfinlteertfale.ed(e1r9s72i)s setxattreedmtehlayt thhieghprowdiutchti-n Bonn (Germany), their reproductive strategies waters disturbed by anthropogenic influences. and colonisation. Nevertheless, ourknowledge aboutfreshwater At the beginning ofthe 20th century Lauter- born distinguished 83 macrobenthic animals in sponges in theRhine is still fragmentary, despite regular, general studies on the macrobenthic the Rhine (Tittizer et al, 1990). In the 1970's thesenumbersdecreasedto 12species(Conradet fauna. Such studies usually include sponges, al., 1977), due to an extremely high level of althoughtheyareoftengivenonlycursoryconsid- pollution. Since thattimegreateffortshave been erations. undertakentopurifythewater,andthenumberof This seems surprising since repeatedly high macrobenthic animals has again risen steadily (Scholl et al., 1995). This species diversity now abundances ofsingle species have attracted the exceeds that ofLauterborn, whereas the species attention ofresearchersin thepast(Schon, 1957; compositionisnotthesameasinthebeginningof Bartl, 1984). Large rivers are characterised by thecentury(Tittizeretal., 1990), duetothehuge unpredictable changing water levels. This offers changes in the Rhine. In recent years ongoing and destroys new habitats - a situation which invasions of foreign species (Neozoa: seemsdifficulttodealwithforsessileorganisms. 216 MEMOIRS OF THE QUEENSLAND MUSEUM TABLE 1.Collectingsitesanddatesofcollections Key: 1,NearthefacilitiesoftheBASFAG.2, Outflowofthe coolingwatercircuitofthepowerplant,thetemperatureishereupto 10°Chigherthanthesurroundingriver.3, Slightly polluted stagnant water. *, Collection undertaken with a grap dredger on board ofthe research ship Argus'. Locality eOr<n)nN mOnoSsaa- *mO'4s> #a0Xs0 rn mO"5N! c-i *a>* *'0o>>o »aom>s *a>•rssND o -a4Hr\ 1a/s1 ».cn—i aCIHTsOS a>s< Iffezheim X Neuburg X X X X Leimersheim X X X X Sondernheim X X X X Altrip X X X X Ludwigshafen1 X X X X Lampertheim-Rosen-Garten X X X X X X X X Worms X X Biblis,nuclearpowerplant2 X X X X X X Biblis,downstreamn.p.pl. X X Worms-Rheinduerk-Heim X X X X Gross-Rohrheim X X X Gross-Rohrheim,2kmdownstream X X X X X X X Gernsheim X X X X X X X X X Kornsand X X X X X X X X X X Nierstein X X X X OxbowofGinsheim X X X X X X Mainz-Laubenheim X X X X X X X Mainz X X X X X Heidenfahrt X X Bingen X X X X X X Bacharach X X X X Boppard X X X X Urmitz/Kaltenenger X X X X BadBreisig X X X X Remagen X Bonn-BadGodesberg X METHODS ethanol. From each individual sponge, one microscope slide was prepared. COLLECTION. Samples were collected from Individual sponges growing on approximately many sites intheRhine (Germany) (Fig. 1), with 1.3m2 available substrate at each collecting site were counted. dates ofcollection foreach site listedin Table 1. Collections weremostlymade from thebanks of The averagenumberofindividual spongesper the river, by wading in the water and removing m ofavailable substrate were calculated (Figs substrate by hand. Some collections were made 4-9). Onlyactivecolonieswerecounted,nodead with a grab dredger aboard the research ship colonies or gemmules without their living 'Argus' ofthe federalstateHesse(indicatedwith mother-sponge. an asterisk in Table 1). Methods of preparation for microscopy Spongeswereremovedfromthesubstratewith followed Arndt (1928), with slight a knife and preserved immediately in 70% modifications. Sponge identification was based RIVERRHINE SPONGE ECOLOGY 217 As a rule, the larger rocks (immovable by average :JXj Hamburg Bonn-BadGodesbery currents) were more likely to Jc •Amsterdam BadBreistg be colonised with sponges. Urmitz/Kallenenge' Boppard Smaller rocks (often moved Bacharach Bingen by average currents) were Heidenfahrl rarely settled by sponges or other sessile organisms. These preliminary data agree OxbowofGinsheim with those ofRutzler (1965), Komsand who studied colonisation by Gemsheim GroR-Rohrhelm marine sponges in the GroB-RohrtiBim Biblis Mediterranean Sea. Biblisnuclearpowerplan! Lampertheim-Rosongarten DISTRIBUTION IN RELA- TION TO DIFFERENT FLOODING REGIMES. The species-assemblages varied considerably between sites dependentonfloodingevents, FIG. 1. Rhine collecting sites. comparingsitesfloodedmore than six months before collecting, and those on Arndt (1926, 1928) and Penney & Racek flooded only nineweeks priorto collection (Fig. 3). Ephydatia fluviatilis was the only species (1968). occurring regularly at the more recently flooded Slidespreparedduringthis studyaredeposited sites, whereas at sites flooded more than six intheSenckenbergMuseumofFrankfurt(SMF). months before collecting this species had the same absolute abundance in colony-counts, but EVALUATION OF THE PERIOD OF colonies grew much larger. The other species, T. FLOODINGBEFORE COLLECTION. At each horrida, S. lacustris, E.fragilis and E. muelleru collection site the actual water depth was alsoappearedatboththesecategoriesofsites,but recorded for all sponge samples. Daily inform- only in very small numbers and small size at ationonwaterlevelsatthestationsofWormsand recently flooded sites (with most having a Mainz were recorded (Fig. 2), thus for each site diameter ofless than 1cm). theperiod offloodingbefore collectioncould be Itwasalsoapparentthatdifferencesinflooding calculated. events between the sites is a major factor responsible for different depth preferences of RESULTS sponge species. Places recently flooded were very shallow, often dry, whereas places flooded FAUNISTICS. Six species were found in the over6 months agowere deeper, belowthe levels presentstudy,listedaccordingtotheirprevalence affected duringriver-level fluctuations. from abundant to rare: Trochospongilla horrida Other factors show no depth dependent Weltner, 1893; Ephydatiafluviatilis (L., 1758); variations in the river environment. Nutrient Spongilla lacustris (L., 1758); Ephydatia levels should be evenly distributed within the muelleri (Lieberkiihn, 1855); Eunapiusfragilis waterbody, due to turbulent currents, and light (Leidy, 1851); Eunapius carteri (Bowerbank, canonlypenetrateabout0.75mthroughthewater 1863) column due to high turbitidy. In Figure 3 and subsequent figures only SUBSTRATE. Sponges were found settling on colony-countsaregiven,wherenodistinctionhas all kind ofsolidsubstrates. WithintheRhinethis been made according to the size or local mainly consists of rocks placed to support the abundance ofcolonies. river banks; wood is rarely found. Aquatic macrophytes are almost non existent in the SEASONAL DEVELOPMENT. The general investigated area, only once was a small E. outline of the development of sponge species A fluviatilis found epizootic on Fontinalis sp, associations is given in Figure 4. generalised (Bryophyta, Fontinalaceae). modelofalife-cycleofafreshwaterspongeinthe 218 MEMOIRS OF THE QUEENSLAND MUSEUM clearly visible, not the preceding states of larval development, but it was not the goalofthisstudytodescribethe life history of these sponges, onlytoreportonthepresenceor absence of mature larvae, as important indicators on the ecology ofspecies. Spongilia lacustris. This species formed thick crusts (l-3cm thick); the outline was irregular with rounded edges. Colonies could reach a ^&<P*xp^x^p xP^#^xp<Pxf^&^x^?.of^- a<*?.of^* ,o<f"P.of^" a^*.of^" ,t^> j|f^* x.p^xPVxP^x^pxp^x^pxp^x&pxp<Fd^p *pijrxp considerable size (up to lnr). Only very few colonies showed tendencies towards branching FIG. 2. The niveau of daily water levels at Worms from July 1993- growth forms. Colonies of S. December 1995;thevertical lines indicatetheturnoftheyears(thescale lacustrisalwaysdisintegratedin isrelative, withoutadefined zero point). late autumn (October- November). In winter Rhine is as follows (Fig. 4). In March-April (December-February) only gemmules survived. youngspongeshatched from theiroverwintering Thefirstyoungspongeshatchedfromgemmules gemmules. Sponges grew until midsummer in spring (beginning of April), the number of (July-August), sexually produced larvae may occurfromMaytoJuly. Asexualgemmuleswere TcohleonhiiegshtnhuemnbreorsseostfecaodliolnyiuenstirlepOocrttoebdedru(rFiingg.t5h)e. producedyear-round,butmoreregularlytowards periodfromOctober-Decemberweremainlydue the autumn. In September-October the colonies tothesecoloniesbeingpresentatthebeginningof declined and desintegrated, thus producing October, whereas by the end of October their overwintering units (gemmules). numbers had declined rapidly. Furthermore, the SPECIESAUTECOLOGY. Statementsaboutthe larger colonies observed in October fragmented presence or absence of sexual reproduction intoseveralsmallercoloniesbeforedying,sothat usuallyrequirehistologicalanalysisofspecimens, counts of number of colonies rose before they whichwasnotconducted inthisstudy. Underlow dropped, andeventually disappearedcompletely magnification only fully developed larvae were in December-March. more than 6 months flooded less than 9 weeks flooded T.honida E.muelleri S.lacustris 16.9 colonies/ m2 5.7 colonies/ m2 FIG.3.Species-assemblagesatplaceswithdifferenttimesoffloodingbeforecollection(numbersarecalculated for lnr): 26 collections were made at sites more than 6 months prior to flooding ofcollection sites; 28 collections took placeatsites less than 9 weeks priorto flooding. RIVERRHINE SPONGE ECOLOGY 219 TABLE2. Life cycle dataandecological strategiesofthe five sympatric sponge species. Event E.fluviatilis S. lacustris E. muelleri E. fragilis T. horrida dTainmceeofgreatestabun- October(autumn) October(autumn) June(earlysummer) May-sJuumnmeer{e)arly Aug(luastte-sSuempmteerm)ber Overwinteringunits Wholecolony Weakgleymmfuilxeedsfree Fixedfreegemmules Attachcerdusgtesmmule Attachcerdusgtesmmule Distributionunits Larvae Driftinggemmules Driftinggemmules Larvae 7 Cesotlaobnliissahteidonhaobfitnaetswly sTwhirmomuignhgacltairvvaee Notobserved Notobserved Notobserved Notobserved Ecologicalstrategy r-strategy K-strategy K-strategy K-strategy K-strateg; Gemmules generally appeared from August, gemmules, due to an infestation with unicellular buttheirappearanceseemedtobelessdependent symbiotic algae and a gemmule-polymorphism, on seasonality and more dependent on colony as described by Gilbert & Simpson (1976) and size. This was also true for other species (see Brondsted & Brondsted (1953), were not Rasmont, 1962, 1963; Simpson, 1980). Colonies observed inthis study. Larvaewerenotobserved ofS. lacustrislargerthan3cmindiameteralways in this species from the Rhine. This was very contained at least some gemmules; smaller intriguinggiventhatin otherhabitats colonies of coloniesweremostly free ofgemmules, atwhat- S. lacustris containing larvae were regularly ever time of the year they were encountered. found (e.g. within the outflow of the Gemmules were built singlywithin the tissue of 'Steinbriicker Teich\ a eutrophic pond near themother-sponge, always withinthebasalparts Darmstadt,Germany,nearly 50% ofthecolonies of the colonies. The gemmulation process was in July 1994 contained larvae). more regular towards the end ofthe life span of In early autumn about 30% of colonies were intact colonies. There were often dense, bright green due to the presence of symbiotic single-layeredcarpetsofgemmules,restingwhere algae (Fig. 5). Only during this part ofthe year they formed. The whole sponges disintegrated were water levels low enough to provide the afterdeath,butshelteredpartsoftheskeletonstill preferred habitats forS. lacustris (i.e. in slightly remainedintactsothatgemmulesrestinginthese deeper, permanently flooded water, Fig. 3), with patches ofskeletal refugia were bound together sufficient light forthe successful photosynthesis and weakly fixed to the substrate. Green ofsymbionts. Eunapius fragilis. This species formed low crusts (l-2cm thick), with an irregular outline and rounded edges. Rarely it exceeded a diameterof5cm.ColoniesofE.fragilisusually disintegrated in summer (July-September). In winter (December-February) intact colonies were rarely found (Fig. 6). The first sponges hatched from gemmules in spring (April). Immediatelyafterhatchingthehighestnumbers of colonies appeared (Fig. 6). The species completed its gemmulation process up until summer (July), after which colonies began to JaMnauracrhy- April-June SepJtuelym-ber DOeccleobmebre-r disintegrate. Gemmules were formed in situ producing a pavement-like gemmule crust, Gemmules ;j£ Larvae Gemmulation 9 tairgehtvliyrtfuiaxleldytiomtmhoevsaubblsteraaten.dTihteissedgifefmicmuulltetso perceice how they could contribute to the dispersal within the habitat, whereas in FIG.4.SeasonalappearanceofSpongillidaeingeneral in May-June 1994freemovablelarvaewerefound theRhinestudysites.Thewhitebarindicateswhenonly in about 10% ofcolonies. Colonies containing gemmulesarepresent; theblackbarrepresentstimesof symbiotic algae were not found within the theyearwhenactivecoloniesarepresent;thetime-scale Rhine, probably because their preferred ofthechartcorresponds with thatofthe bar. distribution was in permanently flooded, . ; MEMOIRSOI ENSLANDMUSEUM ill! i,'i c 2.5 «""Evmiicn o i place(June).InJulythe -i urvaehadsettled 10 andbuiltnewcolonics.Thegcmmulation process c1 « i ..v.i r- i?vnn:I: 1 wcoanssiidrerreagbullaerntuhmrobuegrhooutf tchoelownhieoslewayesaralawnadysa g t6 devoid of gcmmules. When gemmules were present their numbers were reduced: in colonies JiQ5 ' H: of 5cm diameter not more than 10 gemmules were round. During fragmentation Of sponges few gemmules were freed from the januarv-Marcn April-June July*Septembsf Odotoer- mother-spongeandthese 'hatched' inspring.The DeGember highest number of colonies was encountered during October-December. As in the case n\ S FIG.5, Seasonalappearanceand numberofsymbiotic lacustrisn the large colonies present in autumn colonics in Spongillalacusfris. fragmented into several smaller colonies, nianv of which died towards winter (December- deeper habitats, where light regimes may be February), the overwintering colonics were also insufficient forphotosynthesis (Fig. J). small. L'j/iapius carteri. This species record from the In May-June about 25% ofcolonies produced Rhine is the first time it has been encountered in larvae. This seemed to be the most important central Europe lsee Gugel, 1995). ft was found event in the life cycle ofE.jlttvialilts, as in July November 1993 within the cooling-water many very small colonies were seen in close outflow ofthe nuclear power plant in Biblis A proximity to each other, a phenomen quoted as detailed description and discussion about its- 'Spruhinfektion' (spray infection) by SleuslolY dispersal are given in Gugel (1995). [1938). As already indicated, E. fiuviatilh was Ephydaiin fluvialUis. This species forms theonlyspecieswInchoccurredinhighernumbers lure-or-less thin encrustations (I-2cm thick). at sites flooded only a few weeks pnoi to Smallercolonics (lessthan 5cm diameter), had a collection (Fig. 3). This was probably due to die circular outline, whereas larger ones (mote than more active dispersal of larvae. Symbiotic "on diameter), were more irregularly shaped. colonies were never found. Colonics of this species grew up to 20cm Ephydaiiamuelleri. Coloniesofthisspecieswere diameter Ephydatiaflitvlat'diswasregularlySeen mostly thickly encrusting (2-4cm thick), with alive in winter(December-February), in contrast irregular outline and rounded edges. The lotheotherspecies. Overwinteringcolonieswere diameter was rarely more than 10cm. The first small crusts, only i.5em diameter, in which no colonics ofE. muelicn appeared in spring (atthe canal systems were visible. These probably beginning of April), and soon after hatching survive inareduced state, as suggested by Arndl colonies were found in large numbers, peaking ( i 928) and Weissenfels (1989) In early spring during summer (July/August). Alter completing (April) their abundance was only slightly gcmmulation colonies died, usually from ihc increased in comparison with winter (Fig. 7), beginning ofAugust to October (Fig. S). Active probably due to hatching of gcmmules (see- colonies were not observed during winter k-low), whereas the number of colonies (November-March), only dead colonies with dramatically increased duringJune-July (Fig. 7), gemmules. Ephydaiia muelkri often used its al which lime a large-scale production oflarvae entire tissue for gemmule-production, whereas its skeleton remained intact for considerable period of time after ihc death of the maternal sponge. Large numbers ofgcmmules were fixed bythe skeletonto theplaceofproduction. In this way a successful recolomsation at the same site was ensured in the following year. In addition, when single gemmules became free and weic i longer fixed to ihc substrate, they could be Jamiwy-Mercn distributed by the current within the habitat, providing an effective mechanism for dispersal PIG.6 Hal appearance ofEunapiusfragiiis. arid recolonisation ofadjacent habitats. Sexually produced latvac were not observed in this . RIVER RHINE SPONGE ECOLOGY 221 mainly distributed in permanently flooded habitats (Fig. 3). Ecological strategies for each species are summarised in Table 2. DISCUSSION Freshwater sponges often display a considerable plasticity in their ecological January-March April-June July-September October-December strategies (Pronzato & Manconi, 1994a), and their life cycles are often adapted to the special FIG. 7. Seasonal appearance ofEphydatiafluviatllis requirementsoftheirhabitats(e.g.E.fluviatilisin temperate regions is usually active during summer and inactive during winter). In hot, arid species, and only a single symbiotic colony was regions the pattern of activity/inactivity is found(fromSondernheim, inAugust 1994),close reversed (Harsha et al., 1983; Corriero et al., to symbiotic colonies ofS. lacustris. Ephydatia 1994). This shows that the life cycle is very muelleri is mainly distributed in deeper waters, adaptabletospecificclimaticconditions(Pronzato below the levels affected during river-level & Manconi, 1994b). According to Pronzato etal. fluctuations (Fig. 3). (1993) the life cycle ofE.fluviatilis seems to be Trochospongilla horrida. Colonies of this controlled by exogenous factors in regions with species formedthin encrustations (less than 1cm strongly oscillatingenvironmentalconditions. In thick), with a very irregular outline. Large morestablehabitatsendogenouscontrolseemsto colonies may cover an area of 0.5m . dominate. Trochospongilla horrida began hatching from For several species the data presented in the gemmules in spring (early April). The highest literature differ from those presented here. For wabausndraenacceh,edinbinothsunmummbeerrs(Aaungdussitz;eoFfigc.ol9o)n.iesI,n eacxtaimvpelec,olBoinsibeesey(e1a9r9-2r)ournedpoirnteSd.tlhaecupsrtersiesncferoomf autumn (September-October) colonies always NorthCarolina.Heobservedgemmulationinlate disintegrated and leftthe gemmule crusts tightly spring-early summer, sexual reproduction in adhered to the substrate. As in E. fragilis, April, and some sponges disappeared during gemmules remain fixed to their place of summer. AccordingtoCheathum &Harris(1953) production and itis difficultto imaginethatthey both E. fragilis and T. horrida were active year- mightbe dispersedwithintheriver. Gemmulation round in Texas. Pronzato & Manconi (1995) commenced in early summer (June-July), and at counted up to 324 gemmules cm" oftissue inE, this time especially T. horrida was a successful fluviatilis from Sardinia. space-competitor against the otherwise dominating neozoan crustacean Corophium Ecological strategiesofthese speciesaregiven curvispinum (Amphipoda). Whengrowing, small in Table 2. Ephydatia fluviatilis is considered to coloniestendedtofusewithothercoloniesofthe beanr-strategist,mainlyforitsabilitytocolonise same species, thus forming larger 'super- new habitats. In contrast, the remaining species are characterised as k-strategists because they colonies'.Neitherlarvaenorcoloniescontaining symbiotic algae were observed. The species was lack this ability. Thisgeneraltendency iscongruentwithresults ofPronzato&Manconi(1991),whocomparedE. fluviatilis and S. lacustris showing the formerto be more successful in colonising new habitats, whereas the latter was more successful as a competitor. Detailsinthe life cycle ofE.fluviatilis seemto contradictthisclassificationasanr-strategist:the dominance of sexual vs. asexual reproduction January-March April-June July-September October- and its year-round presence; these are usually December quotedastypicalfork-strategists(Pianka, 1970). The successful colonisation ofnew habitats is FIG. 8. Seasonal appearance ofEphydatiamuelleri. here considered to be due mainly to active MEMOIRS OK THE QUEENSLAND MUSEUM or advantage in having a capacity for active dispersal. VandeVyver& WiIlenz( 1975)reportedthai in E. fhtviarih's from Belgium sexual reproduction anfined to overwintering colonies. In thai species the developmentofoocytes commenced in autumn ofthe year prior to larval production, which occurred the fallowing June. Jn Belgium, parts ofcolonies of£ fhtviotilis survived winter as living, but reduced colonies, similar to the asona! appearance of TrochaspongitUi Rhine populations. These results arc also fda. continued by Wcissenfels (1989). Williamson& Williamson (1979) discussed whether or not distribution of iree larvae. Hie production of sexual reproduction was triggered by a phero- larvae in Spongillidac is mostly controlled by moneinSpongillidac. Accordingtotheseauthors endogenous faetors (Gilbert et aL IV7S) The sexual reproduction occurs rarely in running seasonal appearanceoflarvaeisconfined toafew water because the postulated pheromone would weeks in theyear, and thistimingissynchronous be diluted and ineffective (in contrast to breeptowreteendvtahartiopursodluoccatliiotineso.fLIeavneaaeuixn(E1.9f4ht1v,i1a0ti4l2i)s swiotuulatdioenxsplmainsttahgenaonctcuwraetnecre),ofThmiasnyhylpaortvhaeesiisn -Utral Europe is confinedto May-June, which colonies SfJS. lacustris within the outflow ofthe corresponds to the results presented here. Since \Steinbriicker Teich' (see above), where the timing in the production of larvae is severely water flows quickly but is only 5-10cm deep. 'ricted to certain weeks in the year and Here, the population ofS. lacustris is so dense c;nnot be altered in the short term, producing that apheromonewould not be dilutedtoomuch. larvae does notappearto be an effectivestrategy In all colonies of this species at least some act tounpredictableenvironmentalchanges. gemmulesoccurredinaddition to larvae.Thefew reportsoflarvae inE. niuelienand T. hoi) Ufaalso The so-called k-strategists (T, horrida, S, originate from populations in stagnant water. ur/r&j E muelteriwA E fragliiz) have the potential to produce a large number ofoffspring The reportoflarvae in E.fragilis occurring in f'.eiaatutrheei.r Tgheemsmeulsepse.citersadidtoionparlloyduacne ra-stlroattecg>yf •rienplryodoucnteioyneoacicur[sISm^MsJomseuygegaerss,tsevethnatinsspeexcuiaels vmmules, but these usually stay at the pla without regular sexual reproductive strategic their production (see Table 2). Many ofthe differentstrategies and life-cycles According to Manconi & Proruato 1)901) £ omrenteinohnaendcaebovtehehenlpcsopmecpieetsiatviovidecoambpieltiittiieso.n (dcustris follows the r-strategy in ihe short-term. Competitive interactions among sponges, or mPuatniysesSsepnotnigalillylaidk-asei both stntrtahteelgoinegsteoncn.cuIrn between sponges and other organisms, are simultaneously (Pronzato&Manconl 1995) regularly observed m the field. As shown abi as diihsitrtihbeutsiaonm-eunsittrsucutnudreesr:atnher-gsetrmamtueglye.sansiedrvaes stpiemceiseosfhtahveeyetaheri(rFhiiggsh5e-s9t,aTbaubnleda2n1c.eMaatndyifdfeetraeinlst Oflife historiescan be interpreted as median; todies under a k-strategy I to enhance competitive ahilj- Jt wduacstiosnurwparsisclienagrly!•.dominant oVe u I ianbculnuddainngcetheoffac&t lhal duri;nvgtpheeripordospoorfthiiognhesotf oduction. Larvae were only observed in E. colonies with symbiotic algai also I \atili& and io a lesser degree in ceonnhsaindceerabtlhee(Fgirgowth fohfestehesiyrmbhioosntts[FsrtOrSoSngl&y n the special limnological environment of liamsou, 1980).In EAdmtfcTsmallercoUi running water it must be questioned whorl,, arly fuse to form larger ones. Neuberl & ii'i gemmules are more suitable distribute, Eppler (1901 ) discussed wether the competitive units than larvae, aside from their role as resting ability was reduced, and therefore T. honicht was bodies lie more robust than larvae relatively rare compared to other freshwater dispersal is passive via currents. Thereis noneed species, However my data show that it was the RIVER RHINE SPONGE ECOLOGY 223 most abundant sponge in the Rhine, and is also J.-C.(eds) Sponges in time and space, (Balkema: very competitive among sponges and in Rotterdam). competitionwithotherorganisms.Itisconcluded COURREGES, V.C. & FELL, P.E. 1989. Sexual and that competition for space is the species' main asexual reproduction by the freshwater sponge Anheteromeyenia ryden\ with emphasis on challenge. spermatogenic activity. Transactions of the ACKNOWLEDGEMENTS American Microscopical Society 108(2): 127-138. I thank the Hessische Landesanstalt fur FRANZ, H.W. 1992. DerRhein und seine Besiedlung Umwellformakingitpossibletousetheresearch im Wandel: Schwebstoffzehrende Organismen ship 'Argus', the crew of the ship is (Hydrozoa, Kamptozoa und Bryozoa) als acknowledged for their good humour and Indikatoren fur den okologischen Zustand eines Gewassers. Pollichia-Buch 25: 1-167. helpfulness. MrJ. Wittmannkindly provided me FROST, H.W. & WILLIAMSON, C. 1980. In situ with Spongillidae of his collections from determination ofthe effect ofsymbiotic algae on 11.05.1994, 08.06.1994 and 30.06.1994. Dr H. the growth of the freshwater sponge Spongilla Pohl helped me a lot with some figures. J am lacustris. 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