Density-dependent growth and reproduction of the apple snail, Pomacea canaliculata: a density manipulation experiment in a paddy field

To examine density dependence in the survival, growth, and reproduction of Pomacea canaliculata, we conducted an experiment in which snail densities were manipulated in a paddy field. We released paint-marked snails of 15–20 mm shell height into 12 enclosures (pens) of 16 m² at one of five densities – 8, 16, 32, 64, or 128 snails per pen. The survival rate of released snails was 95% and was independent of snail density. The snail density had a significant effect on the growth and egg production of individual snails. This density dependence may have been caused by reduced food availability. The females at high density deposited fewer and smaller egg masses than those at low density, and consequently produced fewer eggs. The females at densities 8 and 16 deposited more than 3000 eggs per female, while the females at density 128 oviposited only 414 eggs. The total egg production per pen was, however, higher at higher snail density. The survival rates of juvenile snails were 21%–37% and were independent of adult density. The juvenile density was positively correlated with the total egg production per pen and hence was higher at higher adult density. However, the density of juveniles larger than 5 mm in shell height, i.e., juveniles that can survive an overwintering period, was not significantly different among density treatments. These results suggest that snail density after the overwintering period is independent of the density in the previous year. Thus, density dependence in growth and reproduction might regulate the population of P. canaliculata in paddies. Received: October 23, 1998 / Accepted: July 16, 1999     

Impact of invasive apple snails in Hong Kong on wetland macrophytes,nutrients, phytoplankton and filamentous algae

1. Grazing by invasive species can affect many aspects of an aquatic system, but most studies have focused on the direct effects on plants. We conducted mesocosm and laboratory experiments to examine the impact of the invasive apple snail Pomacea canaliculata on macrophytes, filamentous algae, nutrients and phytoplankton. 2. In a freshwater pond, we confined 500 g of Myriophyllum aquaticum or Eichhornia crassipes with 0, 2, 4 or 8 apple snails in 1 m × 1 m × 1 m enclosures for approximately 1 month. Apple snails grazed heavily on both species of macrophytes, with higher overall weight losses at higher snail densities. The damage patterns differed between the two macrophytes. In M. aquaticum, both leaves and stems suffered from substantial herbivory, whereas in E. crassipes, only the roots suffered significant weight reduction. 3. In addition to grazing on macrophytes, apple snails appeared to have controlled the growth of filamentous algae, as these did not develop in the snail treatments. The ability of P. canaliculata to control filamentous algae was supported by a laboratory experiment where the consumption was as high as 0.25 g g⁻¹ snail DW d⁻¹. Because of a lack of native herbivorous snails in the pond, the growth of filamentous algae (mainly Spirogyra sp.) reached 80.3 g m⁻², forming a spongy pond scum in the no‐apple snail control. Together with previous reports that apple snails could eat the juveniles and eggs of other freshwater snails, our results indicated that P. canaliculata could have out‐competed native herbivorous snails from the pond by predation on their juveniles or eggs. Alternatively, P. canaliculata might have out‐competed them by monopolisation of food resources. 4. Nitrogen and phosphorous concentrations remained low throughout both experiments and were not correlated with apple snail density. The treatment effects on chlorophyll a (Chl a) and phytoplankton composition varied in the two experiments. In the M. aquaticum experiment, with increasing snail density, Chl a increased, and the phytoplankton community became dominated by Cryptophyceae. In the E. crassipes experiment, Chl a level was independent of snail density, but with increasing snail density, the phytoplankton community became co‐dominated by Cryptophyceae, Chlorophyceae and Bacillariophyceae. 5. Given the multiple effects of P. canaliculata on wetland biodiversity and function, management strategies should be developed to prevent its further spread. In invaded wetlands, strategies should be developed to eradicate the apple snail and re‐introduce native snails which can control the development of filamentous algae.     

Gonadal maturation,fecundity, spawning,and timing of reproduction in the mud snail,Cerithidea cingulata,a pest in milkfish ponds in the Philippines

Summary

Gonadal maturation, spawning, fecundity and timing of reproduction of the snail Cerithidea cingulata in a brackish water pond in Molo, Iloilo, Philippines, are described. Snails 4–41 mm in shell length were sampled monthly from May 1997 to May 1998; 25% were <25 mm, 67% were 20–30 mm, and 8% were >30 mm. The sexes are separate and could first be distinguished at 15 mm. Males are aphallic, have narrower shells than females of the same length, and have bright yellow-orange testes overlying the digestive gland deep inside the shell. Females have more robust shells, an ovipositor at the right side of the foot, and yellow-green ovaries overlying the digestive gland. The sex ratio was one male to two females in the pond population studied. Gonadal maturation was monitored by means of gonadosomatic index (GSI, gonad weight as a percent of visceral weight); maturation stages were based on the gonad appearance (immature, developing, mature) and histology (immature, developing, mature, redeveloping). GSI increased with snail size, and reached 16% in a 33-mm female. The smallest mature males and females were 18–19 mm, and most snails >20 mm were mature, spawning, or redeveloping. Histological sections showed all stages of gametogenesis in mature male snails. The oocyte size-frequency distributions in mature females showed mostly mature oocytes and secondary oocytes, but also oogonia and primary oocytes. GSI and the frequency of snails at different maturation stages varied over the year. Both GSI and the frequency of mature snails were highest during the summer months, April to August. Nevertheless, mature snails occurred throughout the whole year, as did mating and egg-laying. Fecundity (= number of oocytes >70 pμ) increased with size in mature females 2041 mm; an average 25-mm female produced about 1,500 oocytes and larger females produced a maximum of about 2,500 oocytes. Eggs strings laid on the pond bottom were 45–75 mm long; an average 64-mm string contained 2,000 eggs 210+20 pm in diameter. The density of eggs strings was highest (80–120/m²) during March-September. Eggs hatched after 6–7 d into planktonic veligers, which in turn settle on the pond bottom 11–12 d later as juveniles. Juveniles 2–6-mm long were most abundant in the pond during August-October.     

Lethal and non-lethal effects of multiple indigenous predators on the invasive golden apple snail (Pomacea canaliculata)

1. We investigated the individual and combined effects of two predators (the climbing perch, Anabas testudineus, and the wetland crab, Esanthelphusa nimoafi) indigenous to wetlands in Laos, on the behaviour and survival of the invasive South American golden apple snail (Pomacea canaliculata). The snail is considered a pest, consuming large amounts of rice and other aquatic vegetation in the region. 2. Snail avoidance reactions to released predator chemical cues were investigated in aquaria while the effects of predators on a mixed snail population were studied in field enclosures that contained native aquatic plants (Salvinia cucullata, Ludwigia adscendens and Ipomoea aquatica). 3. In the aquaria experiment, neonate (2–3 mm) and medium‐sized snails (8–10 mm) responded to fish chemical cues by going to the surface, whereas adult snails (35–40 mm) went to the bottom. In contrast, no size class of snails reacted to chemical cues released by crabs. 4. In the field experiment, fish reduced the abundance of neonate snails, and crabs reduced the abundance of all size classes. The effect of the combined predators could not be predicted from the mortality rate observed in single predator treatments. The survival of neonate and medium‐sized snails was greater and of adults less than expected. The presence of predators did not affect egg production. Snails consumed significant amounts of plants despite the presence of predators. 5. Our findings suggest that some indigenous Asian predators have lethal and sublethal effects on P. canaliculata that depend on snail size and predator type. When in the presence of several predators the response of snails to one predator may either increase or decrease the vulnerability of snails to the others.     

Size structure and distribution of Ovassiminea brevicula (Gastropoda) in a Thai mangrove swamp

Ovassiminea brevicula (Pfeiffer) (Assimineidae: Gastropoda) was oneof the most abundant and representative benthic invertebrates in the SamutSongkhram mangrove swamp, Thailand. It lived mainly in the mangrove forest with high average densities ranging from 44 to 340 snails·m^-2and abundant in the sites where much litter, dead branches and seedlingswere present. Average shell length in the mature forest ranged from 4.73to 5.74 mm (range: 2.26–8.30 mm) and size structures showeddifferent patterns among the stations. In the seaward mudflat, O. breviculaappeared immediately after the saplings of Avicennia alba Blume1826 were planted. Thereafter, its densities increased with the growth of A. alba trees, probably due to provision of favourable microhabitats underthe canopies. In the mudflat with only small saplings and outside the canopyof the young forest 2 years after planting, mean shell lengths were 3.45 and3.19 mm respectively. Under the canopy of the young forest, however,mean shell length was 4.34 mm with a few larger snails. Three cohortswere separated for snail populations in the mature forest and larval recruitmentseemed to occur from February to May.     

Recruitment,growth, and vertical distribution of the endangered mud snail Cerithidea rhizophorarum A. Adams, 1855: implications for its conservation

Some fundamental natural-history traits of the endangered mud snail Cerithidea rhizophorarum were investigated for three years in an open, bare, sandy tidal flat located on the east of Shikoku Island in southwestern Japan. Our study demonstrated a highly aggregated distribution of younger snails and complicated differences in vertical migration patterns between younger and older snails that might have prevented comprehensive understanding of the precise demography of C. rhizophorarum in previous studies. From late summer, juveniles of the first-year cohorts appeared patchily at 12–16 cm lower tidal heights than snails of the oldest cohorts and reached 5.0–7.5 mm in mean shell length by the first mid-autumn. In the second year, juveniles gradually moved upward and appeared at higher tidal heights. The second-year cohorts reached 15.6–19.1 mm in mean shell length by the end of the second autumn and converged into the oldest cohorts during the third year or later. Our results suggest that C. rhizophorarum matures sexually and starts reproduction from the fourth summer at the earliest. On the other hand, older snails showed a similar seasonal migration pattern every year—downward movement in summer and upward movement in autumn—probably associated with their reproductive activity and/or a seasonal change in the maximum tidal height. For conservation of C. rhizophorarum, it is very important to preserve both the higher area as a main habitat for older snails and the lower tidal flat as the nursery ground.     

Population regulation in the land snail Arianta arbustorum: density effects on adult size,clutch size and incidence of egg cannibalism

Summary The relationships between local population density and adult size, clutch size and spatial distribution of egg batches were investigated in 11 natural populations of the land snail Arianta arbustorum in a forest near Uppsala, Sweden. Shell size of adults decreased with increasing population density as did clutch size. Within populations, clutch size scaled allometrically with shell size indicating size-specific fecundity. It is hypothesized that food unpalatability caused by mucus deposition slows down juvenile growth rate in high density populations, resulting in small adults and thus reducing their fecundity in subsequent years. The influence of the distance between batches on the incidence of egg cannibalism by hatchlings was examined in a laboratory experiment. In this experiment the number of eggs cannibalized increased with decreasing distance to the batch of hatching snails. Thus, in the field, eggs of highly aggregated batches suffer a high risk of cannibalism. In the 3 populations with the highest snail density, 21–39% of all batches were deposited close to each other (nearest neighbour distance 5 cm, i.e. less than hatchlings more within 1 day). These findings indicate that egg cannibalism can act as a population regulating factor.     

Periphytic food and predatory crayfish: relative roles in determining snail distribution

Summary In the laboratory and field, we examined how periphyton (food of snails) and predatory crayfish influenced snail distribution in Trout Lake, a permanent, northern Wisconsin lake. Laboratory experiments (with no crayfish) tested the importance of periphyton biomass in determining snail preference among rocks, and among rock, sand, and macrophyte substrates. Among rocks with four different amounts of periphyton, periphyton biomass and the number of Lymnaea emarginata, Physa spp., and Amnicola spp. were positively related. A similar, but non-significant, trend occurred for Helisoma anceps. A field experiment at a site in Trout Lake where predation risk was low confirmed the preference by snails for periphyton covered rocks; more snails colonized rocks with periphyton than rocks without. When given a choice of rock, sand, and macrophytes in the laboratory, L. emarginata preferred high periphyton biomass and rock. Laboratory and field results contrasted with the distribution of snails in Trout Lake; no snails occurred in areas with abundant periphyton-covered rocks, but snails were abundant nearby on scattered rocks with little periphyton. However, where snails were absent, crayfish were abundant (14.5 crayfish-trap^–1-day^–1), and where snails were abundant, crayfish were rare (3.2 crayfish-trap^–1-day^–1), suggesting that crayfish predation reduced snails. The hypothesis that the negative association between snail and periphyton biomass resulted from snail grazing was supported by the results of a field snail enclosure-exclosure experiment (1 m² cages; n=3). All experiments and observations therefore suggest that: 1) crayfish predation is more important than a preference for high periphyton biomass in determining snail distribution in Trout Lake; 2) periphyton biomass is negtively related to snail grazing; and 3) crayfish had a positive indirect effect on periphyton by preying on grazing snails.     

红树林恢复初期莱氏锥滨螺分布动态

树栖腹足类动物是红树林底栖动物的重要组成部分,其组成和分布受到红树植被的影响。因此,在红树林生态修复中,腹足类动物的恢复依赖于植被的发育。滨螺科(Littorinidae)锥滨螺属(Mainwaringia)动物是亚洲红树林的广布种,但针对其在红树林中的分布及种群特征的研究较少,对红树林植被恢复过程中锥滨螺属动物的恢复动态更是鲜有研究。调查了厦门下潭尾湿地秋茄(Kandelia obovata)红树林恢复初期,新记录种莱氏锥滨螺(Mainwaringia leithii)种群恢复的时间动态,并分析其与植被参数的关系。研究发现,红树林恢复初期,秋茄植被生长迅速,株高、冠幅、基径和覆盖度分别由0.5年生幼林的41.3 cm、25.0 cm、1.04 cm和25%增长为8年生样地的179.2 cm、76.7 cm、5.96 cm和95%。在秋茄胚轴种植6个月左右开始记录到莱氏锥滨螺的个体,其密度和生物量在1.5 a后达到最大(分别为136个/m²和1.86 g/m²),而后逐渐降低,种植3 a后秋茄林内莱氏锥滨螺仅零星分布。除8年生的样地,莱氏锥滨...     

Indirect interactions in a rice ecosystem: density dependence and the interplay between consumptive and non‐consumptive effects of predators

1. Density‐ and trait‐mediated indirect interactions (DMIIs and TMIIs, respectively) in food chains play crucial roles in community structure and processes. However, factors affecting the relative strength of these interactions are poorly understood, including in widespread and important freshwater rice ecosystems. 2. We studied the strength of DMIIs and TMIIs in a food chain involving a predator (the Reeve’s turtle Chinemys reevesii), its herbivorous prey (the apple snail Pomacea canaliculata) and a plant (rice Oryza sativa) in outdoor containers simulating rice fields. We also evaluated consumptive and non‐consumptive effects of the predator on the snail. We removed a fixed proportion of snails every 2 days to simulate prey consumption and introduced a caged turtle that was fed daily with snails to simulate non‐consumptive effects. 3. Direct consumptive effects increased growth of the remaining snails and their per capita feeding rate. Moreover, consumptive and non‐consumptive effects, and their interaction, affected the proportion of snails buried in the soil. This interaction was presumably because increasing food availability per snail induced their self‐burying behaviour. 4. Both DMIIs and TMIIs affected the number of rice plants remaining, whereas their interaction term was not significant. 5. In summary, density dependence and interactions between consumptive and non‐consumptive effects influenced snail growth and behaviour, respectively. However, no cascading effects of these complicated interactions on rice plants were detected.     

The biology of Placostylus ambagiosus (Pulmonata: Bulimulidae) in New Zealand: Part 2. Population changes,growth, mortality and life expectancy

The large land snail Placostylus ambagiosus (Pulmonata: Bulimulidae) was studied in northernmost New Zealand from 1988 to 2004. At Cape Maria van Diemen (CMvD), more juveniles than adults were found, although estimates showed adults as the most abundant. A cohort, hatched after rodent control commenced in 1990, began maturing in 1995, but 89% died by 1997 (partly through competition with garden snails, Cornu aspersum), before numbers partially recovered by 2003. At Surville Cliffs, adults were estimated to be more abundant than juveniles but few juveniles were observed. Numbers under 12 food plants fluctuated between 425 and 657. Many snails dispersed after horses (Equus caballus) partially defoliated their food plants but returned once these recovered. A translocated population increased after rodent control commenced in 1999, but declined when rodent control ceased in 2002. Another translocated population declined when rodents were present and never recovered even after rodent control commenced in 2002. Snails took 2.8–11.7 years to mature from half-grown juveniles depending on location. Estimated adult life expectancies were 5.2 years at CMvD and 10.0 years at Surville Cliffs. Small juvenile snails experienced the highest mortality, and snails that moved between food plants experienced increased mortality. These results are discussed in relation to conservation management.     

Effects of food availability on reproductive output,offspring quality and reproductive efficiency in the apple snail <Emphasis Type="Italic">Pomacea canaliculata</Emphasis>

Phenotypic plasticity in life history traits favors the establishment of invaders and may magnify their ecological impacts. Pomacea canaliculata, the only freshwater snail listed among the 100 worst invaders worldwide, is able to complete its life cycle within a wide range of conditions, a capacity attributed to its life history plasticity. Using snails from their native range in Argentina we investigated the changes in fecundity, egg mass traits, offspring quality, and efficiency of food conversion into eggs in response to different levels of food availability throughout different life stages. Pre-maturity mortality was not affected by chronic reductions of up to 80% in food availability. Females fed ad libitum demonstrated no significant reproductive output differences when mated with males raised at different food availability levels. For females, the number and total weight of eggs and the size of egg masses decreased at high levels of food deprivation. Their efficiency of conversion into eggs of the food ingested during the reproductive period increased with deprivation, as did the survival time of their offspring. In contrast, the egg mass laying rate and the individual egg weight did not differ under different food availability regimes. Reductions in food availability have been suggested as a control method but our results indicate that fecundity would be lessened only at deprivation levels higher than 50% and would be partially compensated by an increase in hatchling survival.     

Effects of irradiation on the reproductive ability of Zonitoides arboreus, a snail pest of orchid roots

The effect of irradiation on the reproductive ability of the orchid snail, Zonitoides arboreus, a serious pest of potted orchids in Hawaii, was investigated. Weekly egg production averaged between 0.8 and 1.9 over a 9–wk period for snails not exposed to irradiation, and egg hatch averaged 61%. In comparison to untreated controls, irradiation of snails at the lowest dose tested (34–37 Gy) reduced egg production and egg hatch by 63% and 94%, respectively over a 9–wk period. None of the snails treated with levels of irradiation ≥ 69 Gy produced viable eggs. This is the first study measuring the effect of ionising irradiation on a terrestrial snail or slug species using sterilising doses. Overall, the results show that the reproductive ability of this snail species is affected by irradiation in a similar manner as for Biomphalaria glabrata, an aquatic snail for which the effects of irradiation have been studied in detail.     

Density-dependent effects of snail grazing on the growth of a submerged macrophyte,Vallisneria spiralis

In order to better understand the role of herbivorous snails in freshwater ecosystems, we conducted experiments investigating food preference of the snail Radix swinhoei on leaves of the submerged plant Vallisneria spiralis with and without periphyton coverage. The effects of snail grazing on the growth of V. spiralis were assessed in a no-snail control and at three snail densities (80, 160, 240 individuals m^?2). Results showed that the snails chose preferentially leaves covered by periphyton. Grazing activity at low snail density (80 individuals m^?2) was found to stimulate V. spiralis growth, but at higher snail density (240 individuals m^?2), plant growth was apparently suppressed. An increase observed in nutrient concentrations in water column with increasing snail density may be attributed to nutrient release by snails. This study suggests that the nature of the relationship between herbivorous snails and macrophytes in freshwater ecosystems depends on the abundance of the snails. At low snail density, the relationship may be a mutualistic one, but at high density snail herbivory may impact negatively on macrophyte biomass in lakes.     

Factors affecting the distribution and abundance of the commensal <Emphasis Type="Italic">Temnocephala iheringi</Emphasis> (Platyhelminthes: Temnocephalidae) among the southernmost populations of the apple snail <Emphasis Type="Italic">Pomacea canaliculata </Emphasis>(Mollusca: Ampullariidae)

Temnocephala iheringi is the most common temnocephalan inhabiting the mantle cavity of the apple snail Pomacea canaliculata, a freshwater neotropical gastropod that has become a serious rice pest in Southeastern Asia. T. iheringi has been recorded from Mato Grosso (Brazil) to water bodies associated with the Río de la Plata river (Argentina). During an extensive survey in the southern limit of the native area of P.␣canaliculata the presence of T. iheringi eggs was recorded in several apple snail populations, extending the known distribution of the commensal more than 400 km southwards. The aim of this study was to understand the factors affecting the distribution and abundance of T. iheringi among populations of P.␣canaliculata. Only 23% of the apple snail populations inhabiting streams harboured temnocephalans while the occurrence among lentic ones was 71%. T. iheringi was found mostly in populations of apple snails living in non-alkaline sites and where snails attaining sizes larger than 4 cm were very common. The prevalence of the temnocephalans in lentic populations was higher than 90%. The number of eggs on the shell (not including the umbilicus) ranged between 0 and 470 and was different among populations of P.␣canaliculata. The prevalence and number of eggs were lower in the lotic populations, except for a stream population immediately downstream of a lake with commensals. There was no difference between males and females of P. canaliculata neither in the prevalence nor in the number of eggs on the shell. The southernmost population of the world of P. canaliculata harbours commensals that tolerate cold winter water temperatures (4–5 °C) as well as its host. On the other hand, T. iheringi was found only in sites with bicarbonate concentrations lower than 6.6 meq l⁻¹, suggesting that the tolerance of the commensal is very much lower than that of the apple snail (up to 9.95 meq l⁻¹). The number of worms inside each snail or the life history variation of P. canaliculata could explain the influence of the size of the snails on the occurrence of T. iheringi. In the big-sized snails, where the number of commensals is higher, the probability of survival of at least one worm is also higher, specially during the hibernation period, when crawling and feeding are null and snails remain buried. On the other hand, P. canaliculata snails from lentic populations are generally bigger and mostly iteroparous, while those inhabiting streams are smaller and semelparous. In these populations the snails have access to mate only with snails of their same cohort, while in iteroparous populations they can copulate with individuals of other cohorts, allowing the inter-generation transmission of worms and the long term persistence of the population of commensals.     

Population structure of Placostylus hongii (Gastropoda: Paryphantidae) on the Poor Knights Islands

Abstract

Populations of the native land snail Placostylus hongii (Lesson, 1830) on the Poor Knights Islands were investigated during visits in 1976, 1977, and 1978. Marking and recapture of individual snails showed that linear growth of the shell ceased when a lip was formed around the outer margin of the aperture. Measurements of snails suggested that increase in outer lip width was a good indication of age, but that the rate of lip thickening varied between populations. Large Placostylus showed a restricted range of movement. Both tagging studies and random sampling revealed that mortality was size specific. Most small snails (<60 mm shell length) located were dead, whereas most larger snails were alive. Population densities of living snails averaged 2.2 individuals per 0.25 m² in the study area. The local distribution was patchy owing to the tendency of snails to be associated with specific features of the vegetation.     

Location of Biomphalaria pfeifferi by Schistosoma mansoni miracidia in stagnant and running water under field conditions

The efficiency of S. mansoni miracidia in locating and infecting Biomphalaria pfeifferi in Gezira canals has been studied under field conditions. When S. mansoni eggs were introduced into clean stagnant water in small field channels, the miracidia hatched to infect 100% of 30 snails in cages at the release point. Fifteen metres upstream and downstream 13% of caged snails were infected but no infections were found in snails 20 m away.When eggs were released into the same canal in flowing water (8.3 cm · s^–1), no infections were detected in any of the caged snails placed 0–100 m downstream. Releasing hatched miracidia instead of eggs resulted in infections in all cages at 5 m intervals from 0-100 m. The release of eggs into flowing water was likened to the method by which S. haematobium eggs are deposited during urination. The 0% infection suggests that eggs will be swept away from the point of contamination by the flow. Thus only urination into stagnant water will lead to heavy snail infection rates.When eggs were released into a small pond-like minor canal tail end snail infection rates were only 3%. This was probably due to the larger water volume, smaller number of caged snails, and the presence of vegetation and other fauna which may be decoys or predators.The results highlight how very high snail infection rates can be produced under ideal conditions but also show how large snail and miracidia numbers are required in natural situations.     

A preliminary investigation into the use of the invasive golden apple snail,Pomacea canaliculata (Lamarck, 1822), as a water purifier and food source in the breeding ponds of the oriental weatherloach Misgurnus anguillicaudatus (Cantor, 1842)

This study investigated the feasibility of using an invasive snail, Pomacea canaliculata, as a food source and water purifier for the commercial breeding of the loach Misgurnus anguillicaudatus. The predatory potential of M. anguillicaudatus (3.5–5.5?g) against hatchling snails was evaluated in aquaria and simulated paddy fields. Some hatchling snails left the water to avoid being preyed upon by the loaches, and approximately 10 hatchlings died per day in the presence of five loaches in aquaria, whereas a weaker snail control effect was observed in the simulated paddy fields. The growth of rice seedlings (Oryza sativa) was not reduced by the presence of hatchling snails alone, but the shoot biomass of seedlings coexisting with snails was promoted after introducing the loaches. Additionally, the presence of P. canaliculata adults improved the aquatic environment in the short term for loach breeding by decreasing the turbidity of the water. Importantly, M. anguillicaudatus (12–18?g) mortality decreased and its weight increased in the presence of adult snails.     

Snail populations in arctic lakes: competition mediated by predation?

Summary For 2 species of snails in arctic Alaskan lakes, I studied the patterns of snail distribution with respect to habitat, distribution of predatory fish, and the potential for interspecific competition. The snails Lymnaea elodes and Valvata lewisi co-exist in these arctic lakes, either in the presence of lake trout, Salvelinus namaycush, or in the absence of predation. Intensive sediment core sampling of Toolik Lake and Lake N-2, with trout and lacking trout, respectively, showed that the smaller snail, Valvata, was abundant in Toolik but ocurred at very low densities in Lake N-2. On the open sediments of lakes containing trout, diver surveys revealed very low densities of adult Lymnaea (0.12±0.12/m²), but similar surveys in lakes without trout revealed much higher densities of adult Lymnaea (7.1±1.8/m²). A survey of 14 lakes indicated that adult Lymnaea grew to a smaller mean size in lakes with trout than in lakes which lacked trout.In laboratory and field experiments, the presence of Lymnaea lowered the fecundity of Valvata. Laboratory experiments also showed that Lymnaea fecundity was enhanced by the presence of Valvata. Enhancement was not due to predation by Lymnaea on Valvata eggs or young. The observed patterns of distribution and abundance in the absence of trout, combined with results from laboratory experiments, are consistent with the hypothesis that competitive and facilitative interactions control the population dynamics of the two snails. The distribution and abundance patterns of snails where trout are present suggest that trout predation rather than competition controls snail population dynamics in lakes containing trout.     

Snail populations,beech litter production,and the role of snails in litter decomposition

Summary The population densities of snails living in beech litter were studied form March 1968 to April 1969. Litter production over one year was measured and the role of snails in litter disappearance assessed.Snails were extracted from litter using a modified Vágvölgyi (1952) flotation method, extraction efficiencies being 84%. The mean annual population density of the twenty-one species of snail recorded on the main sampling site was estimated at 489/m². Carychium tridentatum was the most numerous species, with a mean density of 200/m². Acanthinula aculeata, Punctum pygmaeum and Vitrea contracta also had fairly high mean densities. The mean annual biomass was 699 mg dry wt./m² or 278 mg ash-free dry wt./m². Hygromia striolata and Oxychilus cellarius/alliarius were the most important species in terms of biomass on the main site. Within the limits of accuracy imposed by the sampling regime the population densities of four out of five of the species (C. tridentatum, A. aculeata, V. contracta, Retinella pura) studied remained unchanged throughout the year, whereas P. pygmaeum had a significantly higher autumn population. C. tridentatum populations were highly aggregated at all times of the year, most markedly so in June. Other species were aggregated at certain times of the year only. Samples taken from other sites showed total population densities of snails ranging from 185–1082 snails/m².A total tree litter production of 652 g/m²/annum was recorded of which 584g/m²/annum was of beech material. 72% fell in the October–December period. 58% of the beech litter-fall was leaves, 5.2% bud-scales, 27% fruits and 10% twigs and bark. Summation of appropriate field layer peak standing crops amounted to 23.3 g/m². This was considered as potential litter and was equivalent to 3.4% of the total litter input. The litter standing on the woodland floor in Septermber 1968 was 2,700 g/m², hence, assuming a steady state, litter turnover time was estimated as 4.5 years.It was calculated that the total snail population ingested 0.35–0.43% of the annual litter input, of which 49% was assimilated. The role of the individual species is examined in relation to concepts of key species in ecosystem functioning. The possible role of slugs in decomposition processes is also discussed.