Interactions between the leech Glossiphonia complanata and its gastropod prey

Summary Predator-prey interactions between the predatory leech, Glossiphonia complanata, and its gastropod prey were investigated in laboratory experiments, including behavioural observations with the aid of time-lapse video technique. Six gastropod species were investigated, viz. Lymnaea peregra, Planorbis planorbis, Physa fontinalis, Ancylus fluviatilis, Bithynia tentaculata, and Theodoxus fluviatilis. The species studied exhibited anti-predator defences, which had their maximum efficiency at different stages of encounter with G. complanata. The activity of B. tentaculata decreased with increasing leech activity, but was relatively higher when food was present than when not. Handling times were dependent on the time elapsed since the previous meal was captured (intercatch interval), which in turn was related to the size of the previous prey. Handling time was also related to the size of both predator and prey. The capture efficiency was high for small prey and the leeches spent more time in patches with higher yield. They were, however, unable to discriminate between patches of different prey density.     

Grazing interactions between pulmonate snails and epiphytic algae and bacteria

SUMMARY. 1. The effects of grazing by Lymnaca peregra and Planorbis planorbis on Ceratophyllum demersum epiphyton were investigated under simulated natural conditions.
2. Grazing resulted in significant reductions in algal cell densities, number of algal taxa and in diversity. Grazing did not influence equi-tabilily values.
3. The use of Ivlev's Electivity index demonstrated positive and negative selection by feeding snails for large, pedunculate, and small, adnate algae, respectively.
4. Inoculation of artificial surfaces revealed that 61.1% and 62.5% of the algal taxa presenl in the diet of L. peregra and P . planorbis , respectively (mainly the smaller species), were able to survive passage through the gut, resettle and grow.
5. The results are discussed with particular reference to the survival strategies available to the epiphytes, and the possible mutual benefits which algae and snails may derive from coexistence.     

A study on snail location by Fasciola hepatica using radioisotopically labelled miracidia.

A number of common freshwater molluscs were exposed to Fasciola hepatica miracidia labelled in vivo with radioselenium. Radioactivity was rapidly incorporated in all lymnaeid species tested (L. stagnalis, L. tomentosa and L. truncatula) whereas no radioactivity could be demonstrated in non-lymnaeid pulmonate species (Aplexa hypnorum, Physa acuta, Physa fontinalis, Anisus vortex, Gyraulus albus and Planorbis planorbis) or in the prosobranchs Valvata cristata and Valvata piscinalis. However, the prosobranchs Bithynia leachii, Bithynia tentaculata and Potamopyrgus jenkinsi showed a slow but significant uptake of radioactivity which was presumably explained by uptake of labelled miracidia through a 'filter-feeding' mechanism. Sphaerium corneum, a true filter-feeder, showed a similar course of uptake of radioactivity.     

Coprophagy: a supplementary food source for two freshwater gastropods?

1. The freshwater pulmonate snail Radix peregra voluntarily and regularly fed on its faeces, whereas Bithynia tentaculata (Prosobranchia) did not.
2. With high-quality faeces as food (faeces with C : N-values below ten) Radix could grow and survived for 11 weeks. Growth was 24–30% of that achieved with control food (lettuce, Chlamydomonas and Tetramin), and body mass was close to the expected value.
3. As with real food, the percentage organic content of faeces was reduced during gut passage, with the exception of very low-quality faeces.
4. Faeces contained living cells of green algae ( Chlamydomonas reinhardii ) and diatoms ( Achnanthes lanceolata, Eunotia pectinalis ), which survived the gut passage of R. peregra and B. tentaculata . Faecal material was also rich in bacteria, with up to 150 × 10⁶ cells mg^–1 dry mass. Bacteria increased in abundance in faeces which had been evacuated for 7 days or more.
5. A further degradation of partly digested food in the faeces was indicated by the activity of the extracellular digestive enzymes cellobiase and chitobiase. These enzymes hydrolysed material with an activity of 0.2–14.5 pkat mg^–1 faecal dry mass in 1–12-day-old faeces. In faeces of Bithynia fed control food or incubated sycamore leaves, chitobiase activity was correlated with bacterial abundance.
6. Coprophagy is considered to be a suitable strategy for further degradation and utilization of refractory material for which one gut passage is too short and/or too inefficient.     

Specific Developmental Defects in Molluscs after Treatment with Retinoic Acid during Gastrulation

All-trans retinoic acid is known as a teratogen in vertebrate development. To study whether molluscan morphogenesis is sensitive to retinoic acid, the development of Lymnaea stagnalis, Physa fontinalis and Bithynia tentaculata was examined after treatment with retinoic acid. Low concentrations retinoic acid (10^–7M) specifically affected eye formation in each of these species. In Lymnaea , it was shown that 10^–6M retinoic acid resulted in a wider spectrum of deficiencies, including eye defects, arrested development and shell deformations. Pulse treatments revealed that embryos were most sensitive during gastrulation. Soon after gastrulation the embryos had lost their sensitivity to retinoic acid, which indicates that the observed defects do not result from a general non-specific toxic effect on cells. Even though molluscan development differs in numerous respects from vertebrate development, the present results suggest that molluscs share common features with vertebrates in morphogenetic processes that operate in early development.     

Species Specific Enzymes in Some Snail Species (Mollusca, Gastropoda) from Fresh and Brackish Water in Sweden

Horizontal starch gel electrophoresis was employed to reveal possible species specific patterns of esterases and peroxidases in four species of Lymnaea, Myxas glutinosa, Bithynia tentaculata, Theodoxus fluviatilis and the genus Hydrobia. From four of the species specimens from both fresh and brackish water were used. Esterase patterns were species specific in the anodic portion of the zymograms in all species. The remainder of the patterns displayed marked individual variation, the nature of which was not investigated. The peroxidases also revealed clearly species specific patterns. The amount of variation exhibited in fresh and brackish water populations of L. peregra and T. fluviatilis , the most thoroughly investigated species, was found to be roughly of the same magnitude, indicating a similar amount of intra-specific variation in fresh and brackish water. In the four Lymnaea species it appears from the peroxidase patterns that alternative alleles from an ancestral polymorphic locus have become established in different species. The esterase patterns of L. peregra and L. auricularia revealed distinct differences between the two taxa, which is also supported by the appearance of the spermathecal duct, whereas shell morphology and immunological investigations fail to show inter-taxon differences.     

Toxic effects of cadmium and zinc on the transmission of Echinoparyphium recurvatum cercariae

The toxicity of cadmium, zinc and Cd/Zn mixtures to the transmission of Echinoparyphium recurvatum (Digenea: Echinostomatidae) cercariae into the snail second intermediate hosts was investigated at concentrations ranging from 100 microg l-1 to 10 000 microg l-1 in both soft and hard water. A differential response in the infectivity of metal-exposed cercariae into Lymnaea peregra and Physa fontinalis was demonstrated which was dependent on the snail species being infected. Exposure of L. peregra, P. fontinalis, and L. stagnalis to heavy metals caused a differing susceptibility to E. recurvatum cercariae depending on the snail species being exposed. The mechanism and effects of metal toxicity, together with the importance of the parasite/host strain on cercarial transmission are discussed.     

Selective grazing on periphyton: a determinant of freshwater gastropod microdistributions

SUMMARY. 1. In a cuirophic pond in southern England. the snail Lymnaca peregra (Mull.) is associated with submersed macrophyies, mainly Elodea canadensis Michx. In contrast, the snail Planorbis vortex (Linn.) is associated with the emergent macrophyte Glyceria maxima (Hartm.) Holmberg.
2. L. peregra grazed selectively on filamentous green algae found only on E. camtdensis. P. vortex selected diatoms. Detritus, which was 5 times more abundant on G. maxima than on E . canadensis. comprised about 60% of the diet of P. vortex.
3. Results of experiments giving both snail species a choice between periphyton-detritus removed from the two macrophytes were consistent with the field observations. L. peregra c hose pcriphyton-detritus from E. canadensis whereas P. vortex chose that from G. maxima.
4. Although other factors may have influenced the distributions of L. peregru and P mriex in Radley Pond, food choice was probably the most important proximate factor.     

The suitability of several aquatic snails as intermediate hosts for Angiostrongylus cantonensis.

Sixteen species of aquatic snails of four families were tested by quantitative technique under standardized conditions for their suitability as intermediate hosts for Angiostrongylus cantonensis. These species were the planorbid snails Biomphalaria glabrata, Biomphalaria alexandrina, Planorbis planorbis, Planorbis intermixtus, Bulinus truncatus, Bulinus contortus, Bulinus africanus, Bulinus tropicus and Helisoma sp.; the lymnaeid snails Lymnaea natalensis, Lymnaea tomentosa, Lymnaea stagnalis, and Stagnicola elodes; the physid snail Physa acuta (an Egyptian and a German strain) and the ampullariid snails Marisa cornuarietis and Lanistes carinatus. All these snail species proved to be susceptible to infection with A. cantonensis, and first stage larvae reached the infective third stage in all of them. However, the rate and intensity of infection varied with different species. B. glabrata was the most susceptible snail species with a 100% infection rate and an average percentage recovery of third stage larvae of 26.1. This was followed by S. elodes and B. africanus, with a 100% infection rate and an average percentage recovery of third stage larvae of 15.6 and 14.6 respectively. The rest of snail species proved to be less susceptible. For comparative evaluation of the suitability of the various snail species as intermediate hosts of A. cantonensis a "Capacity Index" was determined. This index should provide a useful method for the evaluation of the suitability of various snails as intermediate hosts of nematode parasites under standardized conditions in the laboratory.     

Relationships between structure and function in streams contrasting in temperature

1. We studied 10 first-order Icelandic streams differing in geothermal influence in separate catchments. Summer temperature (August–September) ranged between 6 and 23 °C.
2. Macroinvertebrate evenness and species overlap decreased significantly with temperature whereas taxon richness showed no response. In total, 35 macroinvertebrate species were found with Chironomidae the dominant taxonomic group. Macroinvertebrate density increased significantly with temperature. Dominant species in the warm streams were Lymnaea peregra and Simulium vittatum . Algal biomass, macrophyte cover and richness were unrelated to temperature. Densities of trout ( Salmo trutta ), the only fish species present, reflected habitat conditions and to a lesser degree temperature.
3. Density of filter-feeders increased significantly with temperature whereas scraper density, the other dominant functional feeding group, was unrelated to temperature. Stable isotope analysis revealed a positive relationship between δ¹⁵N and temperature across several trophic levels. No pattern was found with regard to δ¹³C and temperature.
4. Leaf litter decomposition in both fine and coarse mesh leaf bags were significantly correlated to temperature. In coarse mesh leaf packs breakdown rates were almost doubled compared with fine mesh, ranging between 0.5 and 1.3 g DW 28 days⁻¹. Nutrient diffusion substrates showed that the streams were primarily nitrogen limited across the temperature gradient while a significant additional effect of phosphorous was found with increasing temperature.
5. Structural and functional attributes gave complementary information which all indicated a change with temperature similar to what is found in moderately polluted streams. Our results therefore suggest that lotic ecosystems could be degraded by global warming.     

Relationship between freshwater gastropods and plant communities reflecting various trophic levels

Plant communities were examined in ponds in Brittany (north-west France) and then classified into six types reflecting different trophic levels: oligotrophic, oligodystrophic, mesotrophic, meso-dystrophic, meso-eutrophic an eutrophic. In 45 of these ponds, aquatic snails were sampled in order to determine the relationship between the gastropod species richness and the water trophic levels as indicated by plant community types. The second aim of this study was to determine whether some gastropod species were characteristic of a particular trophic level. The number of plant communities in the ponds was also taken into consideration.A trophic gradient was found along the F1 axis on the principal plane of the correspondence analyses. The species richnesses low or zero and especially the lymneid, Lymnaea glabra were close to the oligo-dystrophic and oligotrophic communities. In contrast, the highest numbers of snail species (5 and above) were found in the most eutrophic ponds where Hippeutis complanatus, Planorbis planorbis, Lymnaea stagnalis and Planorbarius corneus were particularly common. The latter species inhabited the ponds including on average the greatest number of macrophyte communities but no significant differences were found between snail species. The ponds which contained the greatest numbers of plant communities included the richest gastropod communities (7 and more) but also the poorest ones (0 or 1 species). Relationships between freshwater snails, macrophytes and trophic levels are discussed.Laboratoire de Zoologie et d'Ecophysiologie     

Variations in the retinal designs of pulmonate snails (Mollusca, Gastropoda): squaring phylogenetic background and ecophysiological needs (I)

Abstract. The eyes of aquatic pulmonates differ from those of terrestrial pulmonates; the latter, in species such as Cepaea nemoralis and Trichia hispida , possess conventional, cup-shaped retinas, but the aquatic species Lymnaea stagnalis, Radix peregra, Physa fontinalis , and Planorbarius corneus have retinas that are partitioned into dorsal and ventral depressions ("pits"). The pits are separated by an internal ridge, called the "crest", and on account of their pigmentation can be seen in vivo . The dominant cellular components of the retinae of terrestrial as well as aquatic snails are pigmented cells and microvillar photoreceptors, the latter occurring in two morphologically distinct types (I and II). Aquatic snails with preferences for shallow water possess eyes with both type I and type II photoreceptive cells, but Pl. corneus , an inhabitant of deeper water, only has type-I receptors, supporting an earlier finding that type I cells represent dim- and type II cells bright-light receptors. On the basis of histological and optical comparisons, we conclude that the eyes of L. stagnalis and R. peregra , species that are known to escape and seek temporary refuge above the water surface, are well adapted to function in water as well as air, but that the eyes of P. fontinalis and Pl. corneus are less modified from those of their terrestrial ancestors.     

Grazing by oligochaetes and snails on epiphytes

SUMMARY. 1. The isotope ³³P was used to assess the effect of grazitig by oligochaetes (mainly Stylaria lacttstris L.) and the snail, Lymnaea peregra (Muller), on epiphytes within an Equisetum fluviatile L. stand.
2. Two 1 m² polystyrene enclosures were set up within the emergent macrophyte zone of the lake. At the start of the experiment ³³P-solution was mixed with the water in both tanks. Algal and animal samples for ³³P- analysis were collected during the peak occurrence of epiphytes in June.
3. Phosphorus release rates from the animals through defaecation and excretion were measured in the laboratory. The grazing rate of oligochaetes was 2.2–4.1 mg P m⁻² (of bottom) d⁻¹ of which about two - thirds was released and recycled. The oligochaete density ranged from 13,400 to 20,900 m⁻². The snails (25 ind. m⁻²) grazed 1.2-1.5 mg P m⁻²d⁻¹of whieh about a quarter was released through defaecation and excretion.
4. Daily consumption by the oligochaetes and snails corresponded to 22–45% of the average phosphorus uptake by the epiphytes.     

Behavioural responses of Biomphalaria glabrata(Say) to chemical factors from aquatic macrophytes including decaying Lemna paucicostata(Hegelm ex Engelm)

SUMMARY. 1. The behavioural responses of the freshwater pulmonate snail Biomphalaria glabrata to homogenates of various aquatic macrophytes were investigated with the aid of diffusion olfactometers.
2. Of the eleven species studied, three lacked any attractants or arrestants, two contained weak arrestants, and three induced strong repellent effects. Only two, Apium nodiflorum and Lemna paucicostata , induced significant attractant and arrestant effects comparable to those obtained with lettuce (Lactuca sativa) controls.
3. Decaying Lemna paucicostata homogenate proved to be a significantly stronger attractant and arrestant than fresh homogenate. Evidence is given that these effects are mainly due to low molecular weight compounds (<1000 mol. wt) of which the major end products of microbial decomposition, short chain carboxylic acids (C₂-C₅), are likely to be the most important. However, as carboxylic acids account for only a fraction of the total response, other low and high molecular weight compounds are also implicated.
4. The ecological relevance of these results is discussed with particular reference to the hypothesis that the relationship between the snails and macrophytes is essentially mutualistic.     

Fixed focal-length optics in pulmonate snails (Mollusca, Gastropoda): squaring phylogenetic background and ecophysiological needs (II)

Abstract. Our results suggest that freshwater pulmonates like Lymnaea stagnalis, Radix peregra, Physa fontinalis , and Planorbarius corneus have inherited from their terrestrial ancestors eyes with a spherical, immobile lens with fixed focal-length optics. Unable to change the dioptric apparatus to form an image under water, modifications to the retina had to occur if sharp vision was required. Computer-assisted calculations and 3-D eye reconstructions demonstrate that the photoreceptors in the deeper, ventral pit are in a position to perceive focused images under water. Vision in air, however, would favour photoreceptive cells located in the shallower, dorsal pit. On the basis of histological, ethological, and optical comparisons, we conclude that the eyes in L. stagnalis and R. peregra , species that are known to escape and seek temporary refuge above the water surface, are well adapted to function in water as well as air, but that the eyes in P. fontinalis and Pl. corneus are less modified from those of their terrestrial ancestors. We also conclude that good resolving power may be of greater importance in the aquatic pulmonates than the terrestrial species, since the former have to locate thin, vertical stems of reeds and sedges to ascend in order to reach the surface to breathe.     

Inter- and intraspecific trait compensation of defence mechanisms in freshwater snails.

Trait compensation occurs when mechanically independent adaptations are negatively correlated. Here, we report the first study to demonstrate trait compensation in predator-defence adaptations across several species. Freshwater pulmonate snails exposed experimentally to predation chemical cues from fishes and crushed conspecifics showed clear interspecific differences in their behavioural avoidance responses, which were negatively correlated with shell crush resistance. The type of avoidance response varied between species: thin-shelled species (Lymnaea stagnalis and Physa fontinalis) moved to the water-line or out of the water, while those with thick shells moved under cover or showed a mixed response. There were also intraspecific size-linked differences, with an ontogenetic increase in shell strength accompanied by a decrease in behavioural avoidance. Such trait compensation in response to predation has important implications for interspecific interactions and food-web dynamics.     

Plant palatability and disturbance level in aquatic habitats: an experimental approach using the snail Lymnaea stagnalis (L.)

1. The palatability of aquatic macrophytes to the snail Lymnaea stagnalis was investigated in the laboratory. Eight species of macrophyte were selected from habitats that differed in either flood disturbance regime or nutrient status.
2. In a non-choice test, single macrophyte species were offered to individual snails. The average amount of plant dry mass consumed per Lymnaea dry mass ranged from 3.6 ± 1.4 (±SE) to 63.6 ± 13.9 mg g^–1 day^–1 across plant species. In a choice test, all eight plant species were presented simultaneously to sets of five snails. The average total consumption was 66.1 ± 3.8 mg g^–1 day^–1 and the maximum average consumption for a single plant was 26.2 ± 3.6 mg g^–1 day^–1.
3. In both tests, the amount consumed by snails differed significantly between the plant species. The species growing in undisturbed habitats were the least consumed. Habitat nutrient status was unrelated to plant palatability.
4. These results suggest that macrophyte species growing in habitats that are rarely disturbed by floods allocate a greater proportion of their resources to resisting herbivory.     

Are the lake-dwelling leeches,Glossiphonia complanata (L.) and Helobdella stagnalis (L.), opportunistic predators on molluscs and do they partition this food resource?

SUMMARY. 1. Predation on the mollusc genera Potamopyrgus, Bithynia, Valvata, Lymnaea, Physa, Planorbis and Pisidium by the leeches Glossiphonia complanata (L.) and Helobdella stagnalis (L.) was examined in five eutrophic lakes at monthly intervals over one year using a serological technique.
2. Feeding intensity varied seasonally, being lowest in winter and highest in summer. Small, medium and large leeches preyed to the same extent on the several mollusc genera; this was true for both leech species. Qualitatively, the diet of both glossiphoniids was similar throughout the year. Snail genera were consumed in proportion to their relative abundance in the field. Quantitatively, Glossiphonia preyed more heavily on snails than did Helobdella.
3. It is concluded that both leech species are opportunistic predators.     

Stable isotope evidence of trophic interactions between introduced brook trout Salvelinus fontinalis and native brown trout Salmo trutta in a mountain stream of south-west France

The potential trophic impact of introduced brook trout Salvelinus fontinalis on native brown trout Salmo trutta in a mountain stream (south-west France) was investigated using stable isotope analysis (SIA). The isotopic signatures (δ¹³C and δ¹⁵N) of S. fontinalis were similar regardless of the absence or presence of S. trutta , and SIA mixing models revealed that S. fontinalis diet consisted mainly of terrestrial invertebrates. Conversely, a significant shift in S. trutta isotopic signatures (depletion of 1·6‰δ¹³C and enrichment of 0·6‰δ¹⁵N) was observed in sympatry with S. fontinalis ; this may be due to a dietary shift towards terrestrial invertebrates. Contrary to an expected dietary divergence in sympatry, an elevated level of dietary overlap was observed between the non-native and native salmonids when in co-occurrence. This dietary convergence is more likely to be due to behavioural interactions than to variations in food availability or fish displacements.