Invasion of ancient Lake Titicaca by the globally invasive Physa acuta (Gastropoda: Pulmonata: Hygrophila)

A vital population of Physa acuta, a globally invasive species of freshwater gastropods was dicovered in ancient Lake Titicaca in April 2007. The population was found near Chucuito (Bahia de Puno) on the Peruvian side in close proximity to a boat landing of the aquaculture facility of the University of Puno. Physids occured in great abundances at this site on macrophytes in depths from 0 to 2 m. Physa acuta was associated with members of the Heleobia andicola species complex as well as with Heleobia otorni, Biomphalaria andecola, and Uncancylus crequi. Species identity was confirmed by morphological and molecular methods. We discuss the mode of introduction and raise a cautionary note on the potential impact of this global invader on the endemic gastropod fauna of ancient Lake Titicaca.     

Spatial distribution of four freshwater gastropod species in a ditch near Orsay,France

The distribution (both specific and individual) of four pulmonate gastropods was studied in a ditch near Orsay, France, from April 1987 to August 1988. Physa fontinalis L. and Anisus albus (Müller) were restricted to the part of the ditch which never dries up and positively associated with the hydrophytes Ceratophyllum submersum L. and Callitriche hamulata Küntz. On the other hand, Anisus rotundatus (Poiret), which is drought resistant, was abundant in the shallowest part of the ditch and was positively associated with helophytes while Lymnaea palustris (Müller) was ubiquitous and present throughout the ditch. Plant morphology appears to be a major factor determining snail-plant relationships. Within their respective habitats, each species presented an aggregated distribution; their variances were stabilized by logarithmic transformation for L. palustris, Downing's transformation for P. fontinalis and Taylor's transformation for the two remaining species.     

Predator–prey interactions of Procambarus clarkii with aquatic macroinvertebrates in single and multiple prey systems

Understanding the interspecific interactions of Procambarus clarkii with other aquatic macroinvertebrates will help to unveil the mechanisms and processes underlying biological invasiveness. The purpose of this study was to investigate predator–prey interactions of two ontogenic phases of P. clarkii with native and exotic species of aquatic macroinvertebrates at a single and multiple prey level. We performed laboratory experiments to determine the consumption and the behavioral responses of Chironomus riparius, Physa acuta and Corbicula fluminea to P. clarkii. The presence of P. clarkii significantly affected the abundance of C. riparius and P. acuta, but not of C. fluminea whether prey species were provided singly or simultaneously. The consumption of C. riparius by P. clarkii was higher than P. acuta for both crayfish sizes and situations (single/multiple prey systems) and C. fluminea was never consumed. Physa acuta was the only species that exhibited an anti-predator behavior to P. clarkii. Our results show that P. clarkii can have strong consumptive and trait effects on aquatic macroinvertebrate prey at a single and multiple prey level, resulting in differential impacts on different prey species. This study clarifies some aspects of the predator–prey interactions between P. clarkii and native as well as other exotic macroinvertebrate species that have invaded freshwater biocenosis worldwide.     

Volatility in the effective size of a freshwater gastropod population

Despite the utility of gastropod models for the study of evolutionary processes of great generality and importance, their effective population size has rarely been estimated in the field. Here, we report allele frequency variance at three allozyme‐encoding loci monitored over 7 years in a population of the invasive freshwater pulmonate snail Physa acuta (Draparnaud 1805), estimating effective population size with both single‐sample and two‐sample approaches. Estimated N_e declined from effectively infinite in 2009 to approximately 40–50 in 2012 and then rose back to infinity in 2015, corresponding to a striking fluctuation in the apparent census size of the population. Such volatility in N_e may reflect cryptic population subdivision.     

Unraveling cryptic invasion of a freshwater snail in Chile based on molecular and morphological data

Cryptic species may cause biological invasions to be overlooked leading to underestimation of the potential impacts of invaders on the new ecosystems. Identification of freshwater snails is challenging because of the scarcity of discriminative morphological characters and the limited taxonomic knowledge of some taxa. Here, molecular and morphological analyses were performed to investigate the identity of viable populations of the genus Physa in aquatic ecosystems of different basins in northern and central Chile, including habitats where the native species Physa chilensis and Physa nodulosa have been traditionally recognized. Molecular analyses based on mitochondrial DNA sequences from the small subunit 16S rDNA and cytochrome c oxidase subunit 1 identified all specimens sequenced as belonging to the globally invasive species Physa acuta. Microscopic examination of the radula and morphological observations of the reproductive system were congruent with these findings. Highly divergent haplotypes found in El Salto suggest multiple introductions of different lineages of the invader in this locality. It is clear that for future management planning a more reliable assessment of the status of P. chilensis and P. nodulosa is required.     

Studies on the Coccidian Parasites (Apicomplexa: Eimeriidae and Pfeifferinellidae) of Pulmonate Gastropods1

Pulmonate and prosobranch gastropods collected in Minnesota and Washington were examined for the presence of coccidian parasites. Typical coccidian oocysts were recovered from the feces of 89 of 543 (16%) pulmonate snails representing the families Lymnaeidae, Physidae, Planorbidae, and Succineidae. No coccidial infections were apparent in 104 specimens representing the prosobranch families Viviparidae and Pleuroceridae. Oocysts identified as Pfeifferinella ellipsoides were recovered from 21 of 101 (21%) Stagnicola elodes, 2 of 94 (2%) Physa gyrina, and 4 of 67 (6%) Aplexa hypnorum although incidence of infection differed greatly (2–40%) dependent upon the locality sample. This coccidian has not been described previously from North America and is reported here from members of the families Lymnaeidae and Physidae for the first time. Twenty percent of Oxyloma retusa from one habitat were shedding oocysts slightly smaller than, but otherwise identical with, those of Alveocystis gugleri. A third as yet unidentified oocyst was recovered from up to 54% of Helisoma trivolvis. As a group, the coccidia of pulmonate molluscs have been poorly studied. The findings of this survey, however, suggest that these parasites may be more common and widely distributed in both the Basommatophora and Stylommatophora than previously believed.     

Experimental evidence of inbreeding avoidance in the hermaphroditic snail Physa acuta

Inbreeding depression should select for inbreeding avoidance behaviours. Here we test this hypothesis in two populations of the simultaneous hermaphroditic freshwater snail Physa acuta. We recorded the copulatory behaviour of 288 pairs of sib-mates, non-kin mates from the same population, or non-kin mates from two different populations. We find that kin discriminatory behaviours exist in this species, exclusively expressed by individuals playing the female role. We discuss the relevance of our finding in the context of the evolution of recognition systems and the consequences of such a behaviour in natural populations.     

Physiologic and environmental factors influencing the calcium-to-tissue ratio in populations of three species of freshwater pulmonate snails

Summary A study of three species of freshwater pulmonate snails, Physa gyrina (11 populations), Physa integra (17 populations), and Helisoma anceps (18 populations) was carried out from 1973 to 1976, primarily in Michigan.The ratio of whole animal calcium content to tissue dry weight (Ca: tissue ratio) changes with age, but these changes are largely restricted to early growth and do not significantly affect individuals over 40 mg total dry weight, regardless of species.Parasitization by larval trematodes was found to have no significant effect on the Ca: tissue ratio in the populations examined regardless of species.Ca: tissue ratio of P. gyrina did not vary greatly between populations and showed a narrow range over a wide range of environmental calcium concentrations. Shell mass in P. gyrina is therefore either physiologically regulated in all populations (at a species specific constant level) or interpopulation variation is minimized by moderately high levels of gene flow between populations.There was no relationship between Ca: tissue ratio and environmental calcium for either P. integra or H. anceps over a wide range of snail and environmental calcium values. In both of the foregoing species, shell calcium content varied greatly between populations. This variation did not conform to any apparent geographic cline.In 7 of the population sites studied, P. integra and H. anceps coexist, and when these sites are listed from highest to lowest Ca: tissue ratios, the rank orders for the two species are nearly identical. This suggests that there is some environmental factor (possibly trophic conditions) at these localities that influences shell mass.In 8 other species of pulmonates briefly examined, differences in Ca: tissue ratio between populations of the same species were often as great as or greater than differences between species.While the patterns of shell mass and environmental calcium for these species are not unlike those previously reported, the proposed causal factors for these patterns are clearly distinct from those suggested for other freshwater pulmonates.     

Effects of environmental calcium deprivation on the egg masses of Physa marmorata Guilding (Gastropoda: Physidae) and Biomphalaria glabrata Say (Gastropoda: Planorbidae)

Eggs of the basommatophoran snails Physa marmorata and Biomphalaria glabrata were cultured in low concentrations of calcium to determine effects on growth and development. In both species there was some development in media with 0.12 mg/l Ca²⁺ but embryos were unable to hatch. 61.04% of embryos of P. marmorata could develop to hatching in 0.22 mg/l Ca²⁺ but those of B. glabrata required a level of 0.42 mg/l Ca²⁺, to attain even a 31.07% hatch. Marked effects on growth rate, embryo size and on time taken to achieve hatching were noted in both species at very low calcium levels. The possibility of cation-controlling mechanisms in the egg membrane is discussed.     

Silent assassins: predation of native New Zealand trichopteran eggs by non-native freshwater gastropods

Aquatic insects that oviposit on rocks may evolve strategies to mitigate loss of eggs to opportunistic predation by common native grazers, but such strategies may be ineffective against non-native grazers. We tested whether the eggs of common New Zealand caddisfly families (Hydrobiosidae and Hydropsychidae) that oviposit on rocks were more susceptible to predation by native or non-native snails. The native snail Potamopyrgus antipodarum (Grey, 1843) and non-native snails Physa acuta Draparnaud, 1805 and Pseudosuccinea columella (Say, 1817) were presented with caddisfly egg masses and the number of individual eggs consumed was recorded after 24 and 48 hours. The larger Pseudosuccinea readily consumed the eggs, especially spumaline-encapsulated Hydrobiosidae eggs. Physa consumed very few Hydrobiosidae eggs and no Hydropsychidae eggs, whereas Potamopyrgus did not consume eggs of either family. The trichopteran egg masses tested did not succumb to predation by the native Potamopyrgus but are highly vulnerable to predation by a larger non-native snail.     

DISENTANGLING PRECOPULATORY AND POSTCOPULATORY SEXUAL SELECTION IN POLYANDROUS SPECIES

Sexual selection operates on a sequence of events, from mating to offspring production. Which stages in this sequence undergo stronger selection, especially the relative importance of pre‐ versus postcopulatory processes, are intensely debated issues. Unequal siring success among mates of polyandrous females is classically taken as evidence for a large contribution of postcopulatory processes to the variance in male reproductive success (var(RS_m)). However, paternity skews also depend on the timing and number of copulations, a source of variation that should be considered precopulatory rather than postcopulatory. We develop a method for decomposing var(RS_m) accounting for copulatory activity and apply it to experimental mating groups of the snail Physa acuta. In our experiment, 40% of var(RS_m) emerges at the precopulatory stage, only half of which depends on variation in mating success (number of partners). Ignoring copulation characteristics can therefore lead to severe underestimation of precopulatory sexual selection. Moreover, although only 36% of var(RS_m) arises at the postcopulatory stage, this is when sexual selection on body weight mostly occurs. Finally, trade‐offs were detected between different components of precopulatory success, whereas pre‐ and postcopulatory success appear independent. Our study opens the way to a detailed quantitative understanding of sexual selection in polyandrous species.     

How did this snail get here? Several dispersal vectors inferred for an aquatic invasive species

1. How species reach and persist in isolated habitats remains an open question in many cases, especially for rapidly spreading invasive species. This is particularly true for temporary freshwater ponds, which can be remote and may dry out annually, but may still harbour high biodiversity. Persistence in such habitats depends on recurrent colonisation or species survival capacity, and ponds therefore provide an ideal system to investigate dispersal and connectivity. 2. Here, we test the hypothesis that the wide distributions and invasive potential of aquatic snails is due to their ability to exploit several dispersal vectors in different landscapes. We explored the population structure of Physa acuta (recent synonyms: Haitia acuta, Physella acuta, Pulmonata: Gastropoda), an invasive aquatic snail originating from North America, but established in temporary ponds in Doñana National Park, southern Spain. In this area, snails face land barriers when attempting to colonise other suitable habitat. 3. Genetic analyses using six microsatellite loci from 271 snails in 21 sites indicated that (i) geographically and hydrologically isolated snail populations in the park were genetically similar to a large snail population in rice fields more than 15 km away; (ii) these isolated ponds showed an isolation‐by‐distance pattern. This pattern broke down, however, for those ponds visited frequently by large mammals such as cattle, deer and wild boar; (iii) snail populations were panmictic in flooded and hydrologically connected rice fields. 4. These results support the notion that aquatic snails disperse readily by direct water connections in the flooded rice fields, can be carried by waterbirds flying between the rice fields and the park and may disperse between ponds within the park by attaching to large mammals. 5. The potential for aquatic snails such as Physa acuta to exploit several dispersal vectors may contribute to their wide distribution on various continents and their success as invasive species. We suggest that the interaction between different dispersal vectors, their relation to specific habitats and consequences at different geographic scales should be considered both when attempting to control invasive freshwater species and when protecting endangered species.     

Costs and limits of phenotypic plasticity: Tests with predator-induced morphology and life history in a freshwater snail

Potential constraints on the evolution of phenotypic plasticity were tested using data from a previous study on predator-induced morphology and life history in the freshwater snail Physa heterostropha. The benefit of plasticity can be reduced if facultative development is associated with energetic costs, developmental instability, or an impaired developmental range. I examined plasticity in two traits for 29 families of P. heterostropha to see if it was associated with growth rate or fecundity, within-family phenotypic variance, or the potential to produce extreme phenotypes. Support was found for only one of the potential constraints. There was a strong negative selection gradient for growth rate associated with plasticity in shell shape (β = ?0.3, P < 0.0001). This result was attributed to a genetic correlation between morphological plasticity and an antipredator behavior that restricts feeding. Thus, reduced growth associated with morphological plasticity may have had unmeasured fitness benefits. The growth reduction, therefore, is equivocal as a cost of plasticity. Using different fitness components (e.g., survival, fecundity, growth) to seek constraints on plasticity will yield different results in selection gradient analyses. Procedural and conceptual issues related to tests for costs and limits of plasticity are discussed, such as whether constraints on plasticity will be evolutionarily ephemeral and difficult to detect in nature.     

Fresh water snails as bioindicator for some heavy metals in the aquatic environment

The aim of this investigation is to study some freshwater snails as bioindicator for heavy metals Cd, Cu and Pb by determining the concentration of these metals in the field water samples and in whole snail tissues. Seven freshwater snails were used in the present study, some of which are considered medically important snails in Egypt, Biomphalaria alexandrina and Bulinus truncatus, the intermediate hosts for schistosomiasis and nontarget snails Bellamya unicolor, Cleopatra bulimoides, Helisoma duryi, Physa acuta and Theodoxus niloticus. Samples of snails were gathered from three Egyptian governorates: Damietta, Giza and Monufia.. The snails were arranged according to their accumulated concentration of the above‐mentioned microelements in descending order as follows: C. bulimoides > H. duryi > B. truncatus > B. alexandrina >P. acuta > B. unicolor > T. niloticus. It is concluded from the analysis of water and the investigated snails that these snails can accumulate Cu, Pb and Cd with high concentrations in their bodies, so they can be used as bioindicators for heavy metals.     

Molluscicidal activity of affinin and other isobutylamides from the asteraceae

Unsaturated aliphatic isobutylamides from Asteraceae, Rutaceae and Piperaceae are potential agents to control schistosomiasis. Affinin (N-isobutyl-2,6,8-decatrienamide) from Heliopsis longipes has strong molluscicidal activity against Physa occidentalis (ld₅₀ca 100 μM) and the cercariae of the fluke. The amide has also been shown to be present in Wedelia parviceps flowers.     

The growth of juvenile snails in water conditioned by snails of a different species

Summary Two species of freshwater snails, Physa acuta and Lymnaea sp. aff. columella, were collected from Asabata marsh, Shizuoka Japan. Individuals of both species inhabit the same plants. Individuals of P. acuta are more abundant than those of L. sp. aff. columella. Experiments were conducted to examine the effect of water conditioned by snails on the growth of 10-day-old juvenile P. acuta snails. Juvenile snails in water conditioned by L. sp. aff. columella grew faster than those in water conditioned by P. acuta or only lettuce. The effects of water conditioned by P. acuta differed among the litters. The results suggest that juvenile P. acuta snails experience accelated growth in the presence of L. sp. aff. columella. The freshwater snails interacted through resource competition as well as through substances disolved in the water.     

Potamopyrgus antipodarum(Mollusca:Hydrobiidae) in continental aquatic gastropod communities: impact of salinity and trematode parasitism

The structure of gastropod communities was examined from January to June 1999 in four sites of the streams of Mont Saint-Michel Bay along a gradient of salinity, and the occurrence of larval trematodes infecting snails was studied. Abundance and species richness of gastropods increased from polyhaline (95 snails, 1 species) to oligohaline waters (6672 snails, 6 species). Whatever the salinity, the most abundant species was Potamopyrgus antipodarum, an invasive non-indigenous species that represented 80% of the gastropods. Only one male was found in P. antipodarum populations suggesting a predominantly parthenogenetic mode of reproduction. Among 7218 gastropods collected, 1.2% were infected by larval trematodes: 5 species in Lymnaea peregra (4.4%), 4 species in Planorbis planorbis (12.0%), one echinostome in Physa acuta (0.2%), and a new species of Sanguinicola in P. antipodarum (0.5%). This is the first record of infected P. antipodarum in Europe. No parasites were found in polyhaline waters. The prevalence per host population varied from 0 to 100% depending on time of collection, salinity and host species. In the lowest-salinity site, abundance of gastropods and prevalence of trematodes were negatively correlated. The dominance of P. antipodarum in the gastropod communities is discussed in relation with euryhalinity, parthenogenesis and weak rate of parasitism.     

Expression of exogenous fluorescent proteins in early freshwater pond snail embryos

We have for the first time succeeded in expressing in vitro-synthesized mRNAs in both the sinistral and the dextral Lymnaea stagnalis early embryos by microinjecting the mRNAs into the eggs before the first polar body stage. Translation of exogenous mRNA in developing embryos was confirmed by expressing various fluorescent proteins; mCherry, DsRed-Express, and enhanced green fluorescent protein. We have found that the protein expression derived from the introduced exogenous mRNA largely depends on the elapsed time after the microinjection and not on the developmental stage of injection, and also on the amount of injected mRNA. Developmental abnormalities were hardly observed. The first notable fluorescent signal was detected within 2–3 h after the injection while the embryos were still in uncleaved stage. Fluorescence gradually increased until 8–9 h and was stable up to 24 h. From these results, it is suggested that there is enough translation machinery necessary for early development and the translation of injected mRNA proceeds immediately and constantly in the early embryos. This is true for both the sinistral and dextral L. stagnalis embryos. Application of the developed method to other freshwater pond snails, dextral Lymnaea peregra, sinistral Physa acuta, and sinistral Indoplanorbis exustus revealed that their early expression mechanisms to be similar to that of L. stagnalis. Thus, in vitro-synthesized mRNA expression is expected to be important for the understanding of evolutional process and the molecular mechanism underlining the handedness determination in these freshwater snail embryos.     

Spatio-Temporal Distribution of Freshwater Snail Species in Relation to Migration and Environmental Factors in an Irrigated Area from Morocco

Nine sites were sampled 19 times over 2 years in an irrigation system in Morocco in order to study species abundance in a snail community in relation to environmental parameters (including human activities) and migration (geographic distance) among sites. Each site was made of a sink and the first meters of the downstream canal. The snail community included four species (Bulinus truncatus, Lymnaea truncatula, Mercuria similis and Physa acuta). Strong spatial variation in species occurrence and abundance was detected which might be partly due to variation in water availability. However abundance in sinks and canals in which water availability differs were correlated. There was, as predicted, limited evidence in favor of isolation by distance which might be due to fast water current. Dispersal might therefore be an important factor structuring this community. On the other hand, the temporal variation was much more limited. This is consistent with the analysis of individual size distributions in B. truncatus, since no clear-cut cohorts were detected. The environmental parameters recorded (e.g. temperature, occurrence of macrophytes or cleaning of sinks) were extremely variable in time and space, except temperature. Analyzing their association with species through multidimensional methods indicated that P. acuta is ubiquitous and B. truncatus positively associated with macrophytes. These two species were associated in sinks. Less clear trends were detected for the two other species. Annual cleaning of sinks affected all species, but population recovery was fast in B. truncatus and P. acuta.     

Animal ecology of a New Mexico springbrook

Summary Lander Springbrook is 77 m long from source to mouth, and the width ranges from 0.8 to 4.0 m. Total residue was 4400 mg per liter (mostly calcium and magnesium sulphates and chlorides). Hydrogen ion concentration readings ranged between 7.0 and 7.3. At the source (Station I) of this rheocrene springbrook the current is swift, and the substrate consists of rubble. Near the mouth (Station III) the current is sluggish, and the substrate consists of sand and organic silt, which supports a growth of Typha and Cyperaceae. At mid-length (Station II) the current is moderate, the substrate consists mostly of rubble and sand, and there is only a sparse growth of rooted aquatics. The annual range of water temperatures for all three stations was 17.5 to 22.0°.Monthly quantitative bottom samples taken throughout the year showed that the three stations had characteristic and distinctly differing metazoan communities involving seven numerically dominant macrometazoans. Station I (rubble bottom and swift current) had large populations of Zaitzevia parvula (Coleoptera: Elmidae), Amnicola neomexicana, Dugesia dorotocephala, and Gammarus fasciatus. The intermediate Station II had small numbers of Amnicola, Dugesia, and Tendipes larvae, and large numbers of Gammarus, Physa integra, and Tubificidae. Station III (silt bottom, vegetation, and sluggish current) had negligible numbers of Amnicola and Dugesia, and large populations of Gammarus, Physa, Tendipes, and Tubificidae.The most abundant metazoan was Gammarus, with a mean annual density of 10,416 organisms per sq. m. at Station I. Amnicola was second most abundant with a mean annual density of 1489 organisms per sq. m. at Station I. Tendipes larvae were least abundant of the seven forms tabulated; it attained a mean annual density. of only 138 organisms per sq. m. at Station III.Organisms generally were most abundant between September and January, while minimal populations were usually found in February. Total density for all bottom organisms ranged from 2690 (February), to 27,260 (November) per sq. m. at Station I, from 2900 (February) to 25,670 (August) at Station II, and from 500 (February) to 6440 (January) at Station III.It is thought that the absence of such typical springbrook macrometazoans as Sphaeriidae, Isopoda, Trichoptera, Simuliidae, Hirudinea, and water cress is caused by the unusually high salinity of Lander Springbrook.

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Zusammenfassung Der Lander Quellbach ist von der Quelle bis zur Mündung 77 m lang; die Breite schwankt zwischen 0.8 and 4.0 m. Der Gesamtrückstand belief rich auf 4400 mg pro Liter (hauptsächlich Kalzium- und Magnesiumsulfats and Chlorids). Die Wasserstoffionenkonzentration wurde als zwischen 7.1 and 7.3 schwankend bestimmt. An der Quelle (Station I) dieses rheokrenen Quellbachs ist die Strömung schnell; die unterliegende Schicht besteht aus Geröll. An der Mündung ist die Strömung langsam; die unterliegende Schicht besteht aus Sand und organischem Schlamm, der das Wachstum von Typha und Cyperaceae unterhdlt. Auf halber Strecke (Station II) ist die Strömung mässig schnell; die unterliegende Schicht besteht aus Geröll und Sand. Hier findet sich nur eine spärliche, aus verwurzehen Wassergewächsen bestehende Vegetation. Die Wassertemperatur schwankte im Laufe des Jahres zwischen 17.5 and 22.0°.Die im Laufe des Jahres monatlich enthobenen quantitativen Grundproben zeigten, dass die drei Stationen sich durch charakteristische und klar unterschiedene metazoische Gerneinschaften, die zahlenmässig sieben vorherherrschende Macrometazoa umfassen, auszeichnen. Station I (Geröllgrund and schnelle Strömung) war stark von Zaitzevia parvula (Coleoptera: Elmidae), Amnicola neomexicana, Dugesia dorotocephala, und Gammarus fasciatus bevölkert. Die Zwischenstation II wies in geringer Zahl Amnicola, Dugesia, und Tendipes Larven und in grosser Zahl Gammarus, Physa integra, und Tubificidae auf. Station III (Schlammgrund, Vegetation, und langsame Strömung) zeigte eine äusserst geringfügige Anzahl von Amnicola und Dugesia und im grossen Umfang Gammarus, Physa, Tendipes, and Tubificidae.Die am hdufigsten vorkommende Metazoa war Gammarus, dessen durchschnittliche Jahresdichte in Station I 10,416 Organismen betrug. An zweiter Stelle stand Amnicola mit einer durchschnittlichen Jahresdichte von 1489 pro Quadratmeter. Unter den sieben tabellarisierten Organismen wiesen die Tendipes Larven das geringste Vorkommen auf; sie erreichten eine durchschnittliche Jahresdichte von nur 138 Organismen pro Quadratmeter.Das Maximalvorkommen der Organismen lag zwischen September und Januar, das Minimalvorkommen gewöhnlich im Februar. Die Gesamtdichte für alle Grundorganismen schwankte zwischen 2690 (Februar) and 27,260 (November) pro Quadratmeter in Station I, zwischen 2900 (Februar) and 25,670 (August) in Station II, und zwischen 500 (Februar) and 6440 (Januar) in Station III.Man darf wohl annehmen, dass die Ursache für das Nichtvorkommen von solchen typischen Quellbachmakrometazoen wie Sphaeriidae, Isopoda, Trichoptera, Simuliidae, Hirudinea, und Wasserkresse in dem ungewöhnlich hohen Salzgehalt des Lander Quellbaches liegt.

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Contribution No. 2o, Limnology Laboratory, Dept. of Biology, University of Colorado.