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.     

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

1. 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.     

Leech predation on juvenile freshwater snails: effects of size,species and substrate

Summary The leechGlossiphonia complanata does not appear to have substantial impact on snail populations, but this may be due to most studies focusing on adult snails rather than juvenile snails. In this study I investigated how predation rates ofG. complanata feeding on newly-hatched and juvenile snails was affected by snail species, snail size, snail density and substrate, in a laboratory experiment. Number of snails eaten increased with increasing density resulting in a type II functional response curve. Predation rates were higher when leeches were feeding onLymnaea emarginata than onPhysa gyrina, whereas there was no significant difference in predation rates when they were feeding onL. emarginata andHelisoma anceps. Sandy substrates and greater snail size resulted in decreased predation rates. Sand reduced movement speed ofG. complanata, which probably reduced encounter rates. Thus, there was a comparatively large effect of leech predation on newly-hatched snails, due to a high probability of encounter and high predation rates, but spatial and temporal refuges probably reduce the importance of leech predation as a structuring force in freshwater snail assemblages.     

Food partitioning by lake-dwelling triclads and glossiphoniid leeches: field and laboratory experiments

The triclads Polycelis tenuis and Dugesia polychroa and the glossiphoniid leeches Glossiphonia complanata and Helobdella stagnalis are abundant on the stony shores of productive British lakes. All species are food limited and there is considerable overlap in the diets of these triclads and leeches. This paper investigates interactions between the two groups using field and laboratory experiments to try to identify the mechanism of their co-existence. Triclad and leech numbers were manipulated inside experimental enclosures, mathced by controls, erected on the stony shore of an eutrophic English lake. Increasing the numbers of P. tenuis and D. polychroa prior to the reproductive season in spring resulted in a significant decrease in the numbers and body size of G. complanata and H. stagnalis compared with control populations in the summer months, and vice versa. However, increases and decreases were temporary with a readjustment of numbers and body size to control levels in the autumn after reproduction had ceased. It is suggested that increasing the numbers of either group elevated the severity of both intra- and interspecific competition for food. The condition of prey may, in part, determine the strength of competition, and this was examined in laboratory experiments in which different densities and ratios of P. tenuis and H. stagnalis were offered either live of recently crushed Asellus aquaticus. In monospecific controls, growth rates of P. tenuis were greater when fed on crushed than live Asellus, but there was no significant difference in the growth of H. stagnalis fed either live or crushed prey. In mixed cultures of predators, P. tenuis and H. stagnalis were the superior competitors when fed on crushed and live Asellus, respectively. However, when competitive pressure was low, at low densities of predators, the presence of H. stagnalis in mixed cultures fed on live prey was beneficial to the growth of P. tenuis. These results are explained in terms of the greater ability of triclads to detect damaged prey, leaking body fluids, due to their sophisticated chemosensory system, and the ability of leeches to capture live prey due to the presence of suckers. It is concluded that co-existence of the two groups in British lakes is assisted by the partitioning of food on a live or damaged basis.     

Fleeing towards death – leech‐induced behavioural defences increase freshwater snail susceptibility to predatory fish

Prey species are often exposed to multiple predators, which presents several difficulties to prey species. This is especially true when the response to one predator influences the prey’s susceptibility to other predators. Predator‐induced defences have evolved in a wide range of prey species, and experiments involving predators with different hunting strategies allow researchers to evaluate how prey respond to multiple threats. Freshwater snails are known to respond to a variety of predators with both morphological and behavioural defences. Here we studied how freshwater snails Radix balthica responded behaviourally to fish and leech predators, both separately and together. Our aim was to explore whether conflicting predator‐induced responses existed and, if so, what effect they had on snail survival when both predatory fish and leeches were present. We found that although R. balthica increased refuge use when exposed to predatory fish, they decreased refuge use when exposed to predatory leeches. When both predators were present, snails showed a stronger response towards leech than fish and responded by leaving the refuge. This response made the snails more susceptible to fish predation, which increased snail mortality when exposed to both fish and leech compared to fish only. We show that predators that have a relatively low predation rate can substantially increase mortality rates by indirect effects. By forcing snails out of refuges such as rock and macrophyte habitats, leeches can indirectly increase predation from molluscivorous fish and may thus affect snail densities.     

Competition and coexistance between Glossiphonia complanata and Helobdella stagnalis (Glossiphoniidae: Hirudinoidea)

Summary The prey utilization of Glossiphonia complanata and Helobdella stagnalis from Bruce Lake, Alberta, Canada was examined quantitatively using serological techniques. While considerable prey range overlap occurred between the species, niche overlap was low during the winter, peaked in March and declined through the summer. Temporal differences in feeding and intraspecific weight class differences in prey utilization were found in both species but more distinctly in G. complanata. It is suggested that co-existance of G. complanata and H. stagnalis is based on adjustments of weight class food resource partitioning on a temporal basis.     

Study on foraging mechanism of leeches with different feeding habits based on chemoreception and foraging behavior

The leeches Whitmania pigra and Hirudo nipponia live in similar environments but have different feeding habits. At present, there are few studies of the foraging mechanism of leeches with different feeding habits. In this study, we first used maze tests to show that these two species of leeches could locate and distinguish their prey through chemosensory activity without mechanical stimulation. However, the two leech species have different foraging behaviors: Individuals of W. pigra move slowly and repeatedly adjust direction through probing and crawling to detect the location of prey (snails), whereas individuals of H. nipponia move quickly, and after determining the location of food (porcine blood), they quickly swim or crawl to the vicinity of their prey. Scanning electron microscopy (SEM) revealed that there are two types of sensory cilia and pore structures related to mucus secretion in the heads of both leeches. There are two differently sized types of chemoreceptors on the dorsal lip in W. pigra, which may have different functions during foraging, whereas in H. nipponia there is only one type of chemoreceptor, which is small. We detected the chemical components in the natural food of these two leech species by UHPLC–MS. There were 934 metabolites in the body fluid of snails and 751 metabolites in porcine serum; five metabolites unique to the body fluid of snails and to porcine serum were screened as candidate feeding attractants. Of these metabolites, betaine and arginine effectively attracted individuals of W. pigra and H. nipponia, respectively. In summary, leeches with different feeding habits use chemoreceptors to sense external chemical signals when foraging, and there are significant differences between species in foraging behavior, chemoreceptors, and attractants.     

A comparative study of the food niches of lake-dwelling triclads and leeches

The food niches of four species of triclads and three species of non-parasitic leeches living in the littoral zone of British lakes of different trophic status were investigated, over one year, using a serological technique. Antisera against ten potential prey groups were employed. The basic data were adjusted to compensate for differences in predator size and seasonal changes in field temperatures. Data on two of the leech species have been presented elsewhere, but comparisons of the diet of all seven predator species are made in this paper. Much overlap in diet between the various predator species occurs, but, with one exception, each of the genera has a major food resource. The Polycelis spp. feed extensively on oligochaetes, Dugesia polychroa on molluscs, Dendrocoelum lacteum on Asellus, Erpobdella octoculata on chironomids and Glossiphonia complanata on molluscs. Helobdella stagnalis is a generalist feeder. Problems of coexistence of the leech species, and of the leech and triclad species are discussed.     

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.     

Interactions between freshwater snails and tadpoles: competition and facilitation

Summary Freshwater snails and anuran tadpoles have been suggested to have their highest population densities in ponds of intermediate size where abiotic disturbance (e.g. desiccation) is low and large predators absent. Both snails and tadpoles feed on periphytic algae and, thus, there should be a large potential for competitive interactions to occur between these two distantly related taxa. In a field experiment we examined the relative strength of competition between two closely related snail species, Lymnaea stagnalis and L. peregra, and between L. stagnalis and tadpoles of the common frog, Rana temporaria. Snail growth and egg production and tadpole size at and time to metamorphosis were determined. Effects on the common food source, periphyton, were monitored with the aid of artificial substrates. Periphyton dry weight was dramatically reduced in the presence of snails and/or tadpoles. There were no competitive effects on growth or egg production of the two snail species when they were coexisting. Mortality of L. peregra was high (95%) after reproduction, but independent of treatment. Growth of L. stagnalis was reduced only at the highest tadpole densities, whereas egg production was reduced both by intraspecific competition and by competition with tadpoles. Differences in egg production were retained after tadpole metamorphosis. Tadpole larval period increased, weight of metamorphosing frogs decreased and growth rate was reduced as a function of increasing tadpole density. However, contrary to expectation, snails had a positive effect on tadpole larval period, weight and growth rate. Further, in experimental containers without snails there was a dense growth of the filamentous green alga Cladophora sp. We suggest that the facilitative effects of snails on tadpoles are due to an indirect mutualistic mechanism, involving competition between food sources of different quality (microalgae and Cladophora sp.) and tadpoles being competitively dominant over snails for the preferred food source (microalgae). In the presence of tadpoles snails will be forced to feed on low-quality Cladophora, increasing nutrient turnover rates, which results in enhanced productivity of microalgae, increasing tadpole food resources. Thus, tadpoles have a negative effect on snails through resource depression, while snails facilitate tadpole growth through an indirect enhancement of food availability.     

Biological control of the exotic invasive snail Pomacea canaliculata with the indigenous medicinal leech Whitmania pigra

The apple snail Pomacea canaliculata has been an invasive species in China for decades and causes enormous losses to agriculture. The predation capability of the leech Whitmania pigra on P. canaliculata was studied for its economic benefit and potential application. In the present study, the leech W. pigra exhibited a strong predatory capacity in controlling P. canaliculata under both laboratory and field conditions, and it showed no bias towards consuming smaller snails during the experiments. More than 80% of the 80 snails (of which half had reached maturity) were preyed upon by 20 developing leeches (2–7?g) in miniature rice fields over a 15-day period, and the number of rice seedlings damaged by P. canaliculata was decreased in the presence of W. pigra. In a separate experiment, 15 developing snails were exposed to caged leeches and conspecific snails for four weeks. The food intake and growth of P. canaliculata were inhibited under the predation risk imposed by W. pigra, but the feeding rate, food conversion efficiency and survival of P. canaliculata were not conspicuously influenced.     

Operculum closing as a defence against predatory leeches in four British freshwater prosobranch snails

The defence reaction of operculum closing in response to the presence of the molluscivorous leech Glossiphonia complanata (L.) and the non-molluscivorous Erpobdella octoculata (L.) was studied in four species of freshwater prosobranch gastropod. Bithynia tentaculata (L.) and Valvata piscinalis (Müller) can distinguish between the leeches, reacting only to G. complanata. V. piscinalis is capable of a greater degree of distance chemoreception of the leech ‘scent’. Valvata cristata Müller and Potamopyrgus jenkinsi (Smith) did not react to either leech. V. cristata may not be a potential prey item for G. complanata, while P. jenkinsi is fed on by the leech, but is a relative newcomer to the freshwater fauna. Animal Ecology Research Group, Department of Zoology, University of Oxford Commonwealth Forestry Institute, Department of Plant Sciences, University of Oxford     

The Feeding Strategies of the Leech Erpobdella octoculata (L.): A Laboratory Study

The feeding behaviour of the freshwater leech Erpobdella octoculata was analysed by exposing potential invertebrate and vertebrate prey organisms (alive, wounded or dead) to leech attack in dishes, in the laboratory. The fact that E. octoculata is a macrophagous feeder that swallows living prey organisms whole (preferentially Chironomus larvae) is documented. However, it was repeatedly observed that adult leeches sucked the body fluids from wounded larvae. Likewise, cut pieces of earthworms, dead crustaceans and crushed water snails were attacked and the soft parts sucked in with the aid of the unarmed pharynx. Adult leeches sucked the body fluids from dead, decaying bodies of vertebrates (fish, newt larvae). Newly hatched young are blood suckers that rapidly attacked the wounded regions of Chironomus larvae and other invertebrates such as crushed water snails. It is concluded that the common leech E. octoculata is not only a predator, but also a fluid sucker and a scavenger. The ecological implications of this finding are discussed.     

Feeding of the leech <Emphasis Type="Italic">Glossiphonia weberi</Emphasis> on the introduced snail <Emphasis Type="Italic">Pomacea bridgesii</Emphasis> in India

The predation potential of the indigenous leech Glossiphonia weberi on the snail Pomacea bridgesii, introduced in India, was evaluated in the laboratory. Snails used belonged to the size-classes ≤‰3.0, 3.1–5.0, 5.1–7.0 and 7.1–9.0 mm in shell height, using them both separately and together (mixed) in combinations. In each experiment lasting 24 h a single leech belonging to the size-classes 2.0–3.9, 4.0–5.9, 6.0–7.9, 8.0–9.9 and 10.0–11.9 mm in length was used. Except the 4.0–5.9 mm size-class, leeches were able to capture and kill P. bridgesii irrespective of latter’s size; the predation, however, was confined to snails ≤3.0 mm. The rate of predation varied with the size of the predator and the prey, and a leech was able to kill a maximum of three snails per day. In India, in nature G. weberi feeds mostly on the pulmonate snail, Lymnaea (Radix) luteola. Experimental studies, however, revealed that G. weberi prefers the snails P. bridgesii and L. (R) luteola at the same rate from amongst the many other either less or not-preferred native operculate and non-operculate snails.     

The life history,diet and migration of a lake-dwelling population of the leech Alboglossiphonia heteroclita (L.)

Alboglossiphonia heteroclita has an annual life cycle in Tabley Mere, England. Egg-carrying leeches occurred from early June to mid-July. Young were released from parents in August, and the old cohort died soon after. The mean weight of recruited young increased in early autumn, declined over winter due to low feeding activity, and increased again from early spring until the leeches reproduced.The disappearance of leeches from the littoral zone in winter was due to their migration into deeper waters.Six species of snails collected from shallow and deep waters did not harbour leeches inside their shells or mantle cavities at any time of the year. Laboratory experiments confirmed that the leech does not use snails as a refuge.A serological technique identified the gut contents of field-collected A. heteroclita. Leeches had fed most extensively on oligochaetes followed by snails and then chironomids, lightly on Asellus and caddisflies, and scarcely on amphiphods and cladocerans. In laboratory experiments, in which four prey taxa were offered simultaneously to leeches, the descending order of utilization was oligochaetes, chironomids, Asellus and snails.     

The leeches of Rutland Water

Seven species of leeches have been recorded in Rutland Water between September 1977 and October 1979 by monthly samples taken by Ekman grab.The preliminary analysis of leeches from the four study sites of the reservoir (46 grab samples) showed that Helobdella stagnalis was the most abundant species with Erpobdella octoculata and Glossiphonia complanata (cf. Bullock et al. 1982). The life cycle of H. stagnalis was elucidated from these samples by dividing the animals into weight classes (2 mg live weight) and revealed that this species has two breeding seasons. The overwintering population reproduces in June. The juveniles grow rapidly and produce the next generation in August and September. This life cycle is similar to that reported in Europe and Canada.The population density ranged from 32 m^–2 to 90 m^–2, being generally higher during Summer and Autumn after breeding, but low in Winter and Spring. The population biomass increased from June until October each year because of high growth and then declined through the Winter. There are marked variations in density and biomass between the two years of the study period, which probably reflects the dramatic change in benthic abundance and distribution found in many new impoundments.In order to calculate the energy budget, the respiration rate of H. stagnalis has been studied by using a Gilson respirometer operated at three temperatures 5 °C, 10 °C and 15 °C, and the respiration rate of individual H. stagnalis increased logarithmically with weight. The relationship between O₂ consumption and dry weight was examined using linear regression (log/log) for the three temperatures. There are significant differences between the three regressions with slopes of 0.54, 0.69, and 0.73 at 5 °C, 10 °C and 15 °C respectively. The repiratory coefficient (Q₁₀) for the range 5 °C to 15 °C was almost constant for all class weights.The food of the leech was determined qualitatively by exposing potential prey organisms to it, using all the common zoobenthos of Rutland Water. H. stagnalis fed extensively on oligochaetes (Tubificidea). Food consumption was estimated at the three different test temperatures for different weight classes. Similar studies have been undertaken on E. octoculata and G. complanata.     

Relative vulnerability of six freshwater gastropods to the leech Nephelopsis obscura (Verrill)

SUMMARY. 1. The vulnerability of six species of freshwater snails to the leech Nephelopsis obscura was determined in laboratory predation experiments.
2. Nephelopsis was unable to prey on prosobranch snails with an operculum, but did consume certain pulmonate species though predation rates were low, ranging from one to two snails per leech per night at 20°C. Apparently, Nephelopsis does not actively select prey, but merely consumes those species most easily handled.
3. Among pulmonates, leeches captured species in the following order: Physa gyrina (Say), Helisoma anceps (Menke). Lymnaea emarginata (Say) and Helisoma trivolvis (Say). Susceptibihty of size classes varied among snail species.
4. Neither environmental structure, such as cobble or macrophytes, nor gastropod escape behaviors, such as clamping to substrates or vertical migration away from bottom-feeding leeches, lowered capture rates by Nephelopsis.
5. In 2 years sampling, only one of seven lakes, however, had appreciable abundances of Nephelopsis. It is therefore suggested that the rarity, low feeding rates, and lack of strong gastropod preference limit the effect of Nephelopsis on field gastropod distributions.     

Observations on the biology of Glossiphonia heteroclita (L.)

Summary A population of Glossiphonia heteroclita (Linnaeus) was studied in a pond situated in the grounds of the University College of North Wales, Bangor, Caernarvonshire, from November 1965 to September 1966.Monthly samples were obtained by placing roofing slates in the pond and collecting the leeches which settled on their undersides. These leeches were weighed, and examined for the presence of food in their gut. This provided the data for the construction of histograms of weight frequency, from which their life-history could be inferred.During the summer, breeding leeches were kept in the laboratory, at an average temperature of 14°C. Experiments were carried out to investigate the relationship of the young leeches with their parent, and the feeding behaviour of G. heteroclita.The evidence suggests that G. heteroclita is an annual. A proportion of the leeches which had over-wintered, began to reach maturity in April and breeding commenced in May. By September most of these had produced offspring and many of the heavier leeches had died.About twenty-five eggs were laid into a thin-walled cocoon, which was held under the posterior end of the body. After three or four days the eggs became arranged in a single layer, and were firmly attached to the parent by means of the embryonic attachment organ. The embryos hatched sixteen days later, at 14°C, and then attached to the parent by their posterior suckers. The young leeches remained attached to the parent for at least ten days, by which time their store of yolk was used up. Young leeches separated from their parent soon after hatching died.Although young leeches which were placed in a separate dish from their parents several days after hatching, survived well, they were not able to take advantage of certain types of food.Young leeches separated from their parents, but kept in the same dish usually re-attached in a few hours. They also readily attached themselves to other adult G. heteroclita, but not to other species in the same family e.g. Glossiphonia complanata and Helobdella stagnalis.In the field, leeches which hatched early in the summer, weighed up to 8 mg in September. Those which hatched later did not grow so rapidly. Many small leeches were found in the population throughout the winter. The data on feeding showed that it occurred infrequently during the winter. There was considerable mortality in April, just before the onset of breeding.The behaviour experiments, supported by immunological evidence, suggested that the main food of G. heteroclita in the College Pond, was Lymnaea stagnalis. The leeches did not accept undamaged snails. They also fed on damaged Enchytraeus albidus.

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Zusammenfassung Von November 1965 bis September 1966 wurde in einem Teich im Gelände des University College of North Wales, Bangor, Caernarvonshire, eine Population von Glossiphonia heteroclita (Linnaeus) wissenschaftlichen Untersuchungen unterzogen.An der Unterseite von Dachschiefern, die in den Teich gestellt wurden, siedelten sich Egel an. Jeden Monat wurden these Egel, die einen Ausschnitt der ganzen Population im Teich darstellten, gesammelt. Diese Egel wurden gewogen und der Darm auf das Vorhandensein von Nahrung geprüft. Die Untersuchungen ergaben Daten zur Aufstellung von Histogrammen der Gewichtsfrequenz, von denen der Lebenszyklus konstruiert werden konnte.Im Sommer wurden Egel während der Vermehrung im Laboratorium bei einer Durchschnittstemperatur von 14°C gehalten. Die Beziehungen von jungen Egeln zum Muttertier und die Art und Weise der Nahrungsaufnahme von G. heteroclita wurden experimentell untersucht.Die Beobachtungen weisen darauf hin, daß G. heteroclita eine einjährige Lebensdauer hat. Ein Teil der Egel, die überwintert hatten, begannen im April ihre Geschlechtsreife zu erreichen und sich im Mai zu vermehren. Bis September hatten die meisten dieser Egel Junge produziert, und viele der schwereren Egel waren eingegangen.Etwa 25 Eier wurden in einen dünnwandigen Kokon gelegt, der sich unter dem hinteren Ende des Körpers befand. Nach 3 bis 4 Tagen batten sich die Eier einschichtig gruppiert und waren mit Hilfe der embryonischen Haftorgane fest an dem Muttertier befestigt. Bei 14°C schlüpften- die Jungen nach 16 Tagen aus und befestigten sich mit ihren hinteren Saugnäpfen an das Muttertier. Nach Verbrauch des Dottervorrats blieben die jungen Egel nach mehrere Tage an dem Muttertier festgesaugt. Die Jungen, die kurz nach dem Ausschlüpfen vom Muttertier getrennt wurden, gingen ein.Obwohl junge Egel, die mehrere Tage nach dem Ausschlüpfen vom Muttertier getrennt und in einem anderen Behalter untergebracht wurden, gut überlebten, waren sie jedoch nicht fähig, gewisse Arten von Nahrung aufzunehmen.Junge Egel, die vom Muttertier getrennt wurden, jedoch in demselben Behälter blieben, saugten sich gewöhnlich nach wenigen Stunden wieder fest. Sie saugten sich ohne weiteres nicht nur bei dem Muttertier sondern auch bei anderen erwachsenen G. heteroclita wieder fest, nicht aber bei anderen Arten der gleichen Familie, z.B. Glossiphonia complanata und Helobdella stagnalis.Freilebende Egel, die am Anfang des Sommers ausschlüpften, wogen im September bis zu 8 mg. Diejenigen, die später ausschlüpften, wuchsen nicht so schnell. Während des ganzen Winters wurden viele kleine Egel in der Population gefunden. Die Daten der Ernährungsfrequenz zeigen, daß während des Winters Nahrung nicht häufig aufgenommen wurde. Kurz vor Beginn der Vermehrung im April war die Sterblichkeit beträchtlich.Die Verhaltensexperimente, gestützt auf immunologische Nachweise, deuten darauf hin, daß die Hauptnahrung von G. heteroclita im College-Teich Lymnaea stagnalis war. Die Egel nahmen unbeschädigte Schnecken nicht an. Sie frassen auch verletzte Enchytaeus albidus.

     

MAINTENANCE OF GENETIC VARIATION IN IMMUNE DEFENSE OF A FRESHWATER SNAIL: ROLE OF ENVIRONMENTAL HETEROGENEITY

Natural populations often show genetic variation in pathogen resistance, which is paradoxal because natural selection is expected to erode genetic variation in fitness‐related traits. Several different factors have been suggested to maintain such variation, but their relative importance is still poorly understood. Here we examined if environmental heterogeneity and genetic trade‐offs could contribute to the maintenance of genetic variation in immune function of a freshwater snail Lymnaea stagnalis. We assessed the immunocompetence of snails originating from different families and maintained in different feeding treatments (ad libitum feeding, no food) by measuring the density of circulating hemocytes, phenoloxidase activity, and antibacterial activity of snail hemolymph. Food limitation reduced snail immune function, and we found significant among‐family variation in hemocyte concentration and PO activity, but not in antibacterial activity. Interestingly, food availability modified the family‐level variation observed in PO activity so that the relative immunocompetence of different snail families changed over environmental conditions (G × E interaction). We found no evidence for genetic trade‐offs between snail growth and immune defense nor among immune traits. Thus, our findings support the idea that environmental heterogeneity may promote maintenance of genetic variation in immune defense, but also suggest that different immune traits might not respond similarly to environmental variation.     

One-trial reward learning in the snail Lymnea stagnalis

We present evidence that the pond snail Lymnaea stagnalis is capable of aquisition and extensive retention of an appetitively reinforced feeding response after only a single training trial. Food-deprived snails presented with a single pairing of a phagostimulant (a mixture of sucrose and casein digest) and a novel, non-food chemostimulus (amyl acetate) subsequently made feeding responses to the amyl acetate and retained the association for at least 19 days. This demonstration of one-trial, non-aversive classical conditioning enhances the utility of Lymnaea stagnalis as a model system for the detailed analysis of neural mechanisms underlying plasticity.