Lymnaea Peregra Research Articles

Amino acid dating of a molluscan fauna from Bassingham Fen, Lincolnshire: implications for the chronology of the Trent Terraces

This paper re-examines the age of the Fulbeck Sand and Gravel and the chronology of the Trent Terraces following amino acid dating of a molluscan fauna found within a basin at Bassingham Fen, incised c . 3 m below the Fulbeck deposit. Two species were dated and yielded different ratios, Lymnaea peregra 0.074 ± 0.002 (2), and Valvata piscinalis 0.131 ± 0.002 (2). It is argued that the V. piscinalis are derived and that the L. peregra indicate the true age of the deposit. Currently available amino acid geochronologies suggest that the shells of L. peregra were deposited between sub-stage 5a and 4 of the early Devensian ( c. 85–64 ka BP ). Correlation with sub-stage 5a would suggest correlation with the Odderade Interstadial of continental Europe and with the only other known 5a site in England, at Cassington in the Upper Thames. Incision into the temperate sub-stage 5e deposits may perhaps have been associated with adjustment of base levels to the cold climate conditions of the early Devensian and suggests unimpeded drainage through the area to the North Sea. More problematic, is the interpretation of the V. piscinalis ratios; with assignment to either warm sub-stage 5e ( c. 125–115 ka BP ) or cold sub-stage 5d ( c . 115–105 ka BP ) using a younger amino acid geochronology, or using an older geochronology, to cold stage 6 ( c . 128–195 ka BP ). The implications of the V. piscinalis ratios for the chronology of the Trent terraces is discussed.

Photoperiodic cercarial emergence patterns of the digeneans Echinoparyphium recurvatum and Plagiorchis sp. from a mixed infection in Lymnaea peregra

Photoperiodic emergence patterns of the cercariae of two digeneans, Echinoparyphium recurvatum (Echinostomatidae) and Plagiorchissp. (Plagiorchiidae), were studied experimentally using a photoperiodic regime of 12 h light: 12 h dark at a constant 18°C. In single species infections of the first intermediate host snail Lymnaea peregra, the cercariae of E. recurvatum emerged exclusively during the light phase and those of Plagiorchis sp. emerged exclusively during the dark phase. In double infections, each species retained its own discrete photoperiodic emergence pattern. This result demonstrates the absence of interference between the mechanisms responsible for the photoperiodic emergence pattern of each species. A degree of interference between the two parasites in double infections of L. peregra was evident. The presence of E. recurvatum significantly reduced the number of Plagiorchis sp. cercariae emerging in a 24 h period and also delayed the mean peak emergence time of Plagiorchissp. cercariae by 2 h. This result is consistent with the well documented antagonistic effect that the redial stages of echinostomes are known to have against the sporocysts of other digeneans within the same host mollusc.

Phototactic responses of the cercaria of Echinoparyphium recurvatum during phases of sub-maximal and maximal infectivity

Phototactic responses of cercariae of the echinostome Echinoparyphium recurvatum (Digenea: Echinostomatidae) were investigated during phases of sub-maximal and maximal infectivity using a simple light/dark choice chamber. The phototactic responses of a total of 400 cercariae were examined in 20 replicate experiments during their sub-maxinal infectivity dispersal phase (0.5 h post-emergence from the first intermediate host Lymnaea peregra) and during their maximal infectivity phase (2.5 h post-emergence). Cercariae in the sub-maximal infectivity dispersal phase were found to be positively phototactic, significantly larger numbers of them occurring on the light side of a light/dark choice chamber P < 0.001. Cercariae in the maximal infectivity phase were found to be negatively phototactic, significantly larger numbers of them being found on the dark side of the choice chamber, P < 0.001. This report of ‘phototactic switching’ demonstrated by the cercariae of E. recurvatum from a positively phototactic dispersal phase to a negatively phototactic infection phase constitutes the first record of such behaviour in the cercaria of an echinostome.

Photoperiodic cercarial emergence patterns of the digeneans Echinoparyphium recurvatum and Plagiorchis sp. from a mixed infection in Lymnaea peregra

Photoperiodic emergence patterns of the cercariae of two digeneans, Echinoparyphium recurvatum (Echinostomatidae) and Plagiorchissp. (Plagiorchiidae), were studied experimentally using a photoperiodic regime of 12 h light: 12 h dark at a constant 18°C. In single species infections of the first intermediate host snail Lymnaea peregra, the cercariae of E. recurvatum emerged exclusively during the light phase and those of Plagiorchis sp. emerged exclusively during the dark phase. In double infections, each species retained its own discrete photoperiodic emergence pattern. This result demonstrates the absence of interference between the mechanisms responsible for the photoperiodic emergence pattern of each species. A degree of interference between the two parasites in double infections of L. peregra was evident. The presence of E. recurvatum significantly reduced the number of Plagiorchis sp. cercariae emerging in a 24 h period and also delayed the mean peak emergence time of Plagiorchissp. cercariae by 2 h. This result is consistent with the well documented antagonistic effect that the redial stages of echinostomes are known to have against the sporocysts of other digeneans within the same host mollusc.

INBREEDING AND FITNESS IN THE FRESHWATER SNAIL LYMNAEA PEREGRA: AN EVALUATION OVER TWO GENERATIONS OF SELF-FERTILIZATION.

Inbreeding depression was estimated from an outbreeding population of the freshwater snail Lymnaea peregra, on the basis of two successive generations of enforced selling and outcrossing, and 70 maternal lineages. Outcrossing was analyzed under two treatments, groups of two and five individuals. The fitness parameters measured included fecundity, growth, and survival. In the first generation, we contrasted three treatments (selfers vs. paired outcrossers and group outcrossers). Very similar results were obtained between the two outcrossing treatments. A strong self-fertilization depression (which includes parental fecundity and progeny fitness) was detected in the selling treatment (about 90%). In the second generation, there was again marked evidence for self-fertilization depression, with the highest contributions coming from parental fecundity and progeny hatching rate. Our results suggest that the decreased parental fecundity is a consequence of the mating system in the previous generation, although the role of partial self-incompatibility and the copulation behavior could not be ruled out. Hatching rate and early survival data are suggestive of purging of lethal mutations. Significant variation in fitness among selfing lineages was found for most fitness traits. Our experimental design also allowed to test for interactions among fitness loci. Only one trait of the nine studied behaved as expected under synergistic interactions. However we cannot rule out some purging during the experiment, which could have biased results towards linearity. Inbreeding depression was also inferred from the change of inbreeding level across generations in the same population. We obtained a value similar to the experimental estimate.

Inbreeding and Fitness in the Freshwater Snail Lymnaea peregra: An Evaluation Over Two Generations of Self-Fertilization

Inbreeding and Fitness in the Freshwater Snail Lymnaea peregra: An Evaluation Over Two Generations of Self-Fertilization

Shell form and habitat choice in Lymnaea

The shell form of aquatic gastropods can vary among closely-related species or populations according to the intensity of water movement in a habitat or to other environmental factors. Freshwater snails can also adapt to local conditions by altering life history traits. There is evidence' that these differences are at least partly under genetic control in certain populations of Lymnaea peregra. The shell morphology of Lymnaea peregra is very variable, though no differences amongst forms in internal anatomy have been found. We consider here two forms, referred to as peregra and ovata, which are almost certainly different species. Form ovata has a rather compact shell with a wide opening while peregra has a longish shell with a narrow opening. The shell form of ovata seems advantageous in running waters or on wave-washed shores, as it allows the expansion of a large foot which enables the animal to attach firmly to the substratum and so withstand strong water movement. Form peregra seems better suited to drying conditions, because the snails can more easily retreat into the mud when a water body dries and their water loss is limited by the narrow opening, as in other lymnaeid species which live in temporarily drying waters. Both forms occur in the Seealpsee, an isolated lake in the Swiss Alps (1142m above sea level; longitude 9 24' E, latitude 47 16' N). This is unusual; different forms of Lymnaea are usually found in different, if close, water bodies.There is a flat, muddy and vegetated shore in the south western end of the lake; otherwise the shore is mostly steeper and stony. The shallow area dries out at least once during the summer. Form peregra is only found in the shallow, drying area whereas ovata is common on the stony shores but also occurs in the shallow area. We examined the habitat choices of snails from two differing shore habitat types in a laboratory experiment by exposing them to a variety of water depths and substratum types. Three categories of snail were used; peregra from the shallow area and ovata from both the shallow and deeper areas. Around 30 newly-collected animals, separated by category, were put into the centre of the water surface of a 100 x 50 x 50 cm tank which contained a 'staircase' of containers (Fig. 1). Each 'stair' was of two containers and there were five along the length of the tank; i.e. five different water depths were available to the snails. These ranged from almost no water to 40 cm depth. At each depth one container was filled with stony substratum and the

SSU rDNA Characterization of Lymnaeid Snails Transmitting Human Fascioliasis in South and Central America

The small subunit (18S) rRNA gene sequences of the lymnaeid morphs I and II (Mollusca: Gastropoda: Basommatophora: Lymnaeidae) transmitting human fascioliasis in the high endemic zone of the northern Bolivian Altiplano and of Lymnaea cubensis from Mexico and Guadeloupe island (Caribbean) have been obtained by direct polymerase chain reaction PCR cycle sequencing and silver staining methods and compared to that of the 6 most common European Lymnaeidae species. Results allow us to establish definitively the distinction between the lymnaeids from the northern Bolivian Altiplano and L. cubensis. Lymnaea cubensis is a valid species distributed in North and Central America but absent in the northern Bolivian Altiplano. Lymnaeid morphs I and II from the northern Bolivian Altiplano both present the same 18S rDNA sequence, which is moreover identical to that of the European species Lymnaea truncatula. Significant nucleotide substitutions in helix E10-1 of the variable region V2 of the secondary structure suggest the need for distinguishing L. cubensis in the subgenus Lymnaea (Bakerilymnaea) with L. (B.) cubensis as type species. The subgenus Lymnaea (Fossaria) is retained, with L. (F.) truncatula as type species. The larger number of nucleotides in the 18S rDNA sequence of L. cubensis (1,860 bp) with regard to the other Lymnaea species (1,843-1,852 bp) is tentatively related to the more ancient paleogeographic origin of L. cubensis. The grouping of L. cubensis with L. truncatula and the relationship of Lymnaea auricularia with Lymnaea peregra in the phylogenetic trees obtained show an evolutionary parallelism with the digenean parasite species they transmit, namely Fasciola hepatica and Fasciola gigantica, respectively.

Selfing and biparental inbreeding: a mating system analysis in Lymnaea peregra (Gastropoda: Lymnaeidae)

The mating system was analysed in three populations of Lymnaea peregra, under the mixed-mating model (MMM) and the effective selfing model (ESM), using progeny array data and allozyme markers. The results revealed variation among populations for the outcrossing rate (tm ranged from 0.646 to 0.983). This finding modifies the earlier classification of L. peregra as a predominantly outcrossing species. The occurrence of biparental inbreeding and population substructure in one of three populations was suggested from positive (tm—ts) under the MMM, covariation of the effective selfing rate with parental fixation index, as well as the great difference between the effective selfing rate of inbred and outbred parents under the ESM. The within-population distribution of the outcrossing rates varied extensively and may involve different patterns of selection and levels of inbreeding depression among the populations studied. Consistently, the estimation of correlated matings showed different levels of the correlation of selfing within progenies in the three populations. Levels of multiple paternity for the outcrossed progeny were positively associated with higher population outcrossing rates, which may reflect the propensity of preferential outcrossers to copulate actively.

Parasite-Induced Gigantism in a Snail: A Host Adaptation?

I experimentally tested the hypothesis that parasite-induced gigantism in molluscs may be a host adaptation by increasing a host's probability of outliving an infection. I exposed juvenile pulmonate snails from two populations of the hermaphrodite Lymnaea peregra to infections with the trematode Diplostomum phoxini. Only one of the two snail populations studied is naturally in contact with this parasite (P + ) and the other is not (P - ). Diplostomum phoxini induced gigantism only in P + snails. Furthermore, P + snails showed a lower parasite-induced mortality compared to P - snails but infected P + snails did not lay eggs before they died. These results are in agreement with three out of four predictions derived from the hypothesis of gigantism as a host adaptation. The fourth prediction, that some P + snails should survive long enough to outlive the infection and reproduce, was not met. My results do not directly support the idea that parasite-induced gigantism is of adaptive value for L. peregra but suggest that gigantism is unlikely to be purely a parasite adaptation. This study cannot exclude the alternative hypothesis that gigantism is a non-adaptive side-effect of infection.

The reaction time of leech and triclad species to crushed prey and the significance of this for their coexistence in British lakes

SUMMARY1. Three species of leeches, Erpobdella octoculata, Glossiphonia complanata and Helobdella stagnalis, and four species of triclads, Polycelis nigra, P. tennis, Dugesia polychroa and Dendrocoelum lacteum, commonly coexist on stony shores in productive British lakes. All species are food limited and there is much overlap in their diet. For both leech and triclad communities, coexistence of species is through the occurrence of food refuges. Leeches are more successful than triclads at capturing live prey, whereas both groups feed on damaged prey, comprising incapacitated, live or dead animals that are leaking body fluids. If triclads are better than leeches at exploiting damaged prey, this could be a mechanism for their coexistence.2. Laboratory experiments investigated the comparative speeds at which leeches and triclads responded to crushed prey. Young and adult predators were offered a crushed specimen of the oligochaete Tubifex tubifex, the snail Lymnaea peregra, the crustacean Asellus aquaticus or the chironomid Chironomus sp., and their reaction times recorded. These four prey groups constitute the main diet of the predators in the field. Only D. polychroa and D. lacteum showed a significantly different reaction time between young and adults to crushed prey, and the reason for this is unclear. All predators, except H. stagnalis and D. polychroa, showed a difference in reaction time to the four types of prey, presumably a consequence of differences in both the ‘quality’ and ‘concentration’ of the different prey fluids, and there were some differences between predators in their speed of reaction to the same prey type. The following sequence, from fastest to slowest, in general reaction time to prey was obtained: E. octoculata, D. polychroa, P. tenuis, D. lacteum, P. nigra, H. stagnalis and G. complanata.3. The location of the damaged food by the predators can be explained partly in terms of their foraging behaviour, with E. octoculata, D, polychroa and P. tenuis exhibiting a more seek‐out strategy than other species which have a more sit‐and‐wait behaviour, and partly on the level of sophistication of their chemosensory system used to detect leaked prey fluids. This system is highly developed in triclad species but poorly developed in leeches.4. In a second type of experiment in which prey, L. peregra, A. aquaticus or Chironomus sp., were offered at different time intervals after crushing to H, stagnalis and P. tenuis, few predators fed on food crushed for 24 h or longer, although a few leeches fed on Chironomus crushed for up to 72 h.5. It is concluded that coexistence of leech and triclad species on stony shores in lakes is assisted by partitioning of food on a damaged or live basis.

Factors influencing the stability of nutrient‐enriched freshwater macrophyte communities: the role of sticklebacks Pungitius pungitius and freshwater snails

SUMMARY1. Enclosures, with the following characteristics, were used to investigate the effects of increased nutrient (nitrate and phosphate) loading on freshwater communities: (a) controls with normal densities of invertebrates, and including macrophytes, molluscs and fish (sticklebacks), and communities similar to the controls but with (b) much reduced snail densities and (c) increased fish densities.2. The addition of nutrients resulted in increases in the biomass of phytoplankton and epiphyton, as well as zooplankton counts, in all the experimental enclosures.3. A reduction in the snail population density resulted in a significant decrease in phytoplankton density and an increase, compared to controls, in the density of epiphytic algae growing on the leaves of the submerged macrophytes, Potamogeton crispus and Groenlandia densa. It also resulted in a significant increase (3.5 times that in the controls) in chlorophyll‐a values of epilithic algae growing on glass slides, an increase in senescent tissue in macrophytes and a decline in the mass of Groenlandia densa by the end of the experiment.4. An increase in the fish population resulted in a significant decline, compared to the controls, in the densities of zooplankton (including Chydorus sphaericus and copepods), Lymnaea peregra (from May to mid‐June), Asellus meridianus, Crangonyx pseudagracilis, Culex (P.) molestus and Physa fontinalis. It also resulted in a change in the composition of the phytoplankton, with an increase in the large colonial Eudorina elegans and a decline in smaller planktonic algae, including Ankistrodesmus and Cryptomonas spp.5. The possible mechanisms responsible for the effects are evaluated and their relevance to conservation and management of freshwater macrophyte communities are discussed.

Food preference of freshwater invertebrates: comparing fresh and decomposed angiosperm and a filamentous alga

SUMMARY1. Fresh and decomposed Mougeotia sp. (a filamentous green alga) and Elodea nuttallii (a vascular plant) were offered as food to three species of aquatic macroinvertebrates (Lymnaea peregra, Asellus meridianus and Endochironomus albipennis) to test: (i) if filamentous algae are preferred to aquatic higher plants (hereafter, called ‘macrophytes’) and (ii), as is known for higher plants, if decomposition also results in greater palatability of filamentous algae.2. Compared with the alga, the macrophyte in both states was of higher nutritional value. Conditioning improved the nutritional value of both food types, but especially of the macrophyte.3. Both fresh alga and fresh macrophyte were eaten little by all animals except A. meridianus feeding upon the macrophyte. Consumption was higher for both plants in their decomposed state. However, L. peregra consumed more decomposed macrophyte than the decomposed alga. Both decomposed plants were eaten most by E. albipennis followed by A. meridianus and L. peregra.4. Digestibility of both plants, but especially of the macrophyte, increased significantly after decomposition. The assimilation efficiencies of all animals on the fresh E. nuttallii were higher than on fresh Mougeotia sp. After decomposition, the efficiency increased significantly only on the alga. Consequently, both decomposed plants were assimilated with similar efficiency by all test animals.5. Amongst aquatic macrophytes, the increase of their consumption and digestibility upon decomposition has hitherto been known only for vascular plants but not for filamentous algae.

Life-cycle description and comparison ofDiplostomum spathaceum (Rudolphi, 1819) andD. pseudobaeri (Razmaskin & Andrejak, 1978) from rainbow trout (Oncorhynchus mykiss Walbaum) maintained in identical hosts

Precise identification of members of the genus Diplostomum is difficult in that many larval stages have been described and named separately from their adult forms. In this study, the life cycles of two Diplostomum spp. were established using metacercariae obtained from the eyes of rainbow trout. The parasites were identified as D. spathaceum and D. pseudobaeri using a procedure and key devised by Shigin. Each life cycle was completed in the laboratory using domestic chickens (Gallus gallus domesticus L.) as definitive hosts and Lymnaea peregra (Muller) and rainbow trout as first and second intermediate hosts, respectively. Utilizing identical hosts minimised host-induced morphological variations and allowed comparisons of anatomical features to be made. Light and scanning electron microscopy were used to study each developmental stage and detailed descriptions of the life cycles were compiled. The results obtained may go some way to resolving the confusion that surrounds Diplostomum taxonomy.

Trophic specialization in Arctic charr Salvelinus alpinus (Pisces; Salmonidae): morphological divergence and ontogenetic niche shifts

Abstract Lake Thingvallavatn supports four trophic morphs of Arctic charr, Salvelinus alpinus (L.); two of the morphs are benthic (small and large benthivorous charr) one exploits pelagic waters (planktivorous charr) and the fourth is found in both habitats (piscivorous charr). The morphological variation among these morphs was analysed by use of principal component analysis and canonical discriminant analysis. The benthic morphs have a short lower jaw and long pectoral fins. The benthic fish also have fewer gillrakers than the other morphs. Small and large benthivorous charrs attain sexual maturity from 2 and 6 years of age, and at fork lengths from 7 and 22 cm, respectively. Small benthivorous charr retain their juvenile parr marks as adults, have beige ventral colours, and are frequently melanized under the lower jaw. Planktivorous and piscivorous charr attain sexual maturity from 4 and 6 years of age, from fork lengths of 15 and 23 cm, respectively. This phenotypic polymorphism is associated with habitat utilization and diet of the fish, and has probably arisen within the lake system through diversification and niche specialization. The pelagic morphs apparently stem from a single population, and are possibly diversified through conditional niche shifts which affect ontogeny. Juveniles reaching a body length of 23 cm may change from zooplankton to fish feeding. Asymptotic length increases thereby from 20.5 cm in planktivorous charr to 30.2 cm in piscivorous charr. The benthic morphs appear to represent separate populations, although both feed chiefly on the gastropod Lymnaea peregra . Their co-existence seems to be facilitated by size dependent constraints on habitat use. The small morph (asymptotic length 13.3 cm) exploit the interstitial crevices in the lava block substratum, whereas the large morph (asymptotic length 55.4 cm) live epibenthically.

Trophic specialization in Arctic charr Salvelinus alpinus (Pisces; Salmonidae): morphological divergence and ontogenetic niche shifts

Lake Thingvallavatn supports four trophic morphs of Arctic charr, Salvelinus alpinus (L.); two of the morphs are benthic (small and large benthivorous charr) one exploits pelagic waters (planktivorous charr) and the fourth is found in both habitats (piscivorous charr). The morphological variation among these morphs was analysed by use of principal component analysis and canonical discriminant analysis. The benthic morphs have a short lower jaw and long pectoral fins. The benthic fish also have fewer gillrakers than the other morphs. Small and large benthivorous charrs attain sexual maturity from 2 and 6 years of age, and at fork lengths from 7 and 22 cm, respectively. Small benthivorous charr retain their juvenile parr marks as adults, have beige ventral colours, and are frequently melanized under the lower jaw. Planktivorous and piscivorous charr attain sexual maturity from 4 and 6 years of age, from fork lengths of 15 and 23 cm, respectively. This phenotypic polymorphism is associated with habitat utilization and diet of the fish, and has probably arisen within the lake system through diversification and niche specialization. The pelagic morphs apparently stem from a single population, and are possibly diversified through conditional niche shifts which affect ontogeny. Juveniles reaching a body length of 23 cm may change from zooplankton to fish feeding. Asymptotic length increases thereby from 20.5 cm in planktivorous charr to 30.2 cm in piscivorous charr. The benthic morphs appear to represent separate populations, although both feed chiefly on the gastropod Lymnaea peregra. Their co-existence seems to be facilitated by size dependent constraints on habitat use. The small morph (asymptotic length 13.3 cm) exploit the interstitial crevices in the lava block substratum, whereas the large morph (asymptotic length 55.4 cm) live epibenthically.

Does body size difference in the leeches Glossiphonia complanata (L.) and Helobdella stagnalis (L.) contribute to co-existence?

The effect of predator and prey body size on the feeding success of the British lake-dwelling leeches Glossiphonia complanata and Helobdella stagnalis was examined in the laboratory, and any involvement of size difference between the leeches in allowing coexistence in the field assessed. G. complanata breeds in advance of H. stagnalis and maintains a body size advantage throughout their annual life-cycle. In experiments, conducted at 14 °C and a photoperiod of 16 hrs L: 8 hrs D, three size classes of leeches of each species were each exposed to each of three size classes of each of five prey species, viz. Tubifex sp., Chironomus sp., Asellus aquaticus, Lymnaea peregra and Potamopyrgus jenkinsi. For each prey species, three different types of experiments were performed: one leech exposed to four prey individuals; four leeches of the same species with sixteen prey; and two leeches of each species with sixteen prey. In the first experiment, all sizes of G. complanata were capable of feeding on all sizes of the prey types offered; the same was true for H. stagnalis with exceptions of feeding on large A. aquaticus and large L. peregra. For both species, but especially for G. complanata, there was a trend within each size class of leech for decreasing proportions of fed leeches with increasing prey size, and within each size class of prey for an increasing proportion of fed leeches with increasing leech size; however there were several exceptions to these trends. Both leeches fed extensively on Tubifex sp. but there were significant differences in the proportions feeding on other prey types; G. complanata fed more on A. aquaticus and the two snail species, and less on Chironomus, than H. stagnalis. The effect of increasing the number of leech individuals from one to four individuals, of the same or mixed species, had little effect on the proportion of leeches which had fed. It is concluded that large G. complanata will have access to large individuals of certain prey taxa denied H. stagnalis, which may lessen the intensity of interspecific competition.

Quantitative Structure‐Activity Relationships of 2‐Benzamido‐5‐Nitrothiazole Molluscicidal Derivatives

AbstractThe pharmacologic action of 2‐benzamido‐5‐nitrothiazole and eleven derivatives was studied in Lymnaea peregra ovata Müller, Gammarus pulex pulex L., and Euglena gracilis Klebs. The toxicity parameters (50 % lethal concentration, 50 % inhibitory concentration) were correlated with the physicochemical parameters of the compounds (lipophilicity, stability). The toxicity of the molecule was related to its lipophilicity and stability. By combining these parameters it was possible to establish specific equations for each species studied. Linear equations of the overall parameters of the molecule were obtained with L. p. ovata and E. gracilis, whereas second‐degree relationships were found with G. p. pulex. Using these pharmacokinetics models, it is possible to predict the pharmacologic action of these compounds knowing only the physicochemical parameters of each molecule.

The life-cycle and morphology of Sanguinicola inermis Plehn, 1905 (Digenea: Sanguinicolidae)

The life-cycle and morphology of Sanguinicola inermis in laboratory infected common carp Cyprinus carpio and Lymnaea peregra at 20°C are described. Furcocercous cercariae develop from two generations of sporocysts within four to five weeks and directly infect a cyprinid definitive host. Post-penetration larvae migrate to the blood circulatory system of the fish and mature into adults which release eggs into the blood by 30 days post-exposure. Eggs reaching the gills mature and liberate miracidia within seven days. The miracidia emigrate through host tissue to the water, where they locate and penetrate a snail host. A detailed description of the developmental stages is given and the surface structure of the cercaria, the oral muscular organ of the adult and the vitelline membrane of the egg are described for the first time.

Competitive interactions between the lake-dwelling leeches Glossiphonia complanata and Helobdella stagnalis: an experimental investigation of the significance of a food refuge.

The glossiphoniid leeches Helobdella stagnalis and Glossiphonia complanata co-exist on the stony shores of British lakes. Because both species have a similar life-style and are food limited, the question arises as to how coexistence persists. Laboratory experiments investigated if either leech has a competitive advantage on three foods, viz. oligochaete worms (Tubifex sp.), snails (Lymnaea peregra), and a worm-plus-snail mixture. A variety of densities with a range of ratios of the two leeches, and monospecific controls were used. The level of feeding ensured increasing food shortage with rising leech density. Experiments, conducted at 14° C, lasted 3 months. Regression values for growth against density in the controls indicated the occurrence of intraspecific competition in both leech species. Light mortality occurred at the highest densities. G. complanata gained significantly less weight in mixed leech cultures than in monospecific cultures at some of the highest densities when fed worms, but more weight when fed on snails. The opposite was true for H. stagnalis. When both prey types were provided, no differences at any density were observed for either leech. Differences in bodysize and feeding strategy between the leeches may be implicated in determining their feeding success on the prey types. Because the anti-predatory responses (e.g. shaking) exhibited by L. peregra are less when encountering an inactive than an active leech, the sit-and-wait strategy of G. complanata may aid snail capture. In contrast, the seek-out strategy of H. stagnalis, coupled with its relatively smaller size, may result in fewer successful captures. The active hunting strategy of H. stagnalis may be advantageous for the capture of oligochaetes which are relatively less mobile than snails. Offering both prey types simultaneously would have had an ameliorating influence on interactions between the two leeches. Findings provide support for the hypothesis that long-term co-existence between the two glossiphoniids in British lakes may depend, in part at least, on the occurrence of a snail food refuge for G. complanata.