DISTRIBUTION AND PRODUCTION OF THREE HYDROBIA SPECIES (GASTROPODA: HYDROBIIDAE) IN A SHALLOW COASTAL LAGOON IN THE BAY OF CADIZ, SPAIN

The abundance, life span, growth and production of the mud snailsHydrobia minoricensis, H. ulvae and H. ventrosa in a semi-naturallagoon system were studied by taking monthly samples at threesites during 1991 and 1992. The most abundant species, H. minoricensisoccurred at mean densities of 12834 to 26264 snails m^–2(10.7 to 25.8g dry weigh m^–2), depending on the site.The least abundant species, H. ulvae, occurred at mean densitiesof 185 to 353 snails m^–2 (3.2 to 2.2g dry weight m^–2).The numerical abundance and biomass of the three Hydrobia specieswere positively related to the biomass of benthic macroalgae(P<0.01). Although H. ulvae egg capsules were recorded throughoutthe year, newly hatched snailsof this species were not observed,in contrast to the other two species. The early spring and summercohorts of H. minoricensis and H. ventrosa seemed to be themost numerous. The average life spans of these two species wereestimated to be about 18 and 13 months respectively. Annualproduction estimates for the whole lagoon system were 29.0 (6.3),5.5 (0.8) and 5.2 (1.0)g dry weight (ash-free dry weight) m^–2yr^–1 for H. minoricensis, H. ulvae and H. ventrosa respectively.The annual P/B ratio was about 2 for H. minoricensis and H.ventrosa. (Received 5 July 1994; accepted 5 October 1994)     

SURFACING AND WATER LEAVING BEHAVIOUR OF THE FRESHWATER PULMONATE SNAILS LYMNAEA PEREGRA (MULLER), BIOMPHALARIA GLABRATA (SAY) AND BULINUS JOUSSEAUMEI (DAUTZENBERG)

Surfacing and water-leaving behaviour (quitting) of aquaticpulmonate snails was investigated in the laboratory. Regressioncoefficients of surfacing rate per hour for 16 snails in 200cm³ of water were 1.28 for L. peregra, 1.35 for B. glabrata(albino form), 0.47 for B. glabrata (pigmented form) and 0.17for B. jousseaumei. Disturbance, snail size and conditioningof the water were relatively unimportant factors; water volumeadjusted by depth was also unimportant up to 50 mm depth. Importantfactors controlling surfacing appeared to be population density,time of year, temperature, water volume adjusted by perimeterand dissolved oxygen concentration. The response to the latterwas not clear and both B. glabrata and L. peregra orientatedalmost as well to a nitrogen/water interface as to an air/waterinterface. Water-quitting regressed on volume, which had beenadjusted by perimeter variation, revealed a curvilinear relationshipfor B. glabrata, with maximum quitting at 1 snail per 7cm³ (1snail per 14cm of climbable perimeter). L. peregra showed anegative exponential curve, with maximum quitting at 1 snailper 3 cm³ of water (1 snail per 8 cm of climbable perimeter).These data could be relevant to design and management of irrigationchannels where bilharzia might occur. (Received 28 November 1990; accepted 22 October 1991)     

COPULATORY ACTIVITY AND SPERM PRODUCTION IN BULINUS (PHYSOPSIS) GLOBOSUS (GASTROPODA: PLANORBIDAE)

Young Bulinus (Physopsis) globosus performed male copulatoryactivity and cross-fertilized other snails before their femalereproductive tracts were mature. The two most immature snailsshowed preputial eversion when secretion was present only inthe muciparous gland and at the carrefour region of the oviduct.Sixteen snails showed preputial eversion and four snails cross-fertilizedother snails when their oothecal glands and/or major portionof the oviducts contained either no secretion or only scantyamounts. When paired with a partner snail for 12 or 20 consecutive days,adult snails copulated as males on approximately 60% of thedays paired and up to 8 consecutive days. Virgin snails raisedin isolation copulated as male at the same rate as non-virgin,community-raised snails. Ability to copulate as male was notdependent upon previous experience as male or female. Aftera single copulation as male after 7 days isolation, the hermaphroditicducts of maleacting snails contained 87 000 sperm. Sperm productionoccurred at approximately 50 000 sperm.d-¹, until at 10 dayspost-copulation, snails contained 639 000 sperm. (Received 25 May 1982;     

POPULATION DENSITY EFFECTS ON GROWTH IN CULTURE OF THE EDIBLE SNAIL HELIX ASPERSA VAR. MAXIMA

The effects of population density on the growth of H. aspersaMÜller var. maxima under controlled environmental conditionswere examined. Inhibitory effects on snail growth and maturityresulting from increased population density, between 100–800snails m^–2 of floor area, were observed for a range ofcontainer cleaning frequencies. At all population densities,enhanced snail growth was observed when the frequency of containercleaning was increased to a two-day interval. No significantdifferences were recorded, following 19 weeks growth, betweenfinal mean weights of snails from containers cleaned less frequently.The lowest snail mortality was consistently recorded at thelowest population density in the most frequently cleaned containers.At all snail population densities three phases of growth wereobserved: (a) lag (0–5 weeks), (b) rapid (6–15 weeks)and (c) stable (16 weeks and over). During the first three weeksof growth, high population density had a positive effect ondiet consumption, food conversion efficiency and snail growth.Adverse population density effects increased progressively duringphase (b), typically following 9 weeks growth. Juvenile snailstransferred from high to low population densities during phase(b) continued to exhibit slower growth rates associated withhigh population densities. Food conversion efficiency of snailsin all treatments decreased throughout the experimental periodbut with no overall effect of container cleaning frequency apparent.Inherent growth variability of sibling snails was unaffectedby population density or container cleaning frequency. The importanceof the results for intensive snail culture is discussed. (Received 23 June 1994; accepted 1 December 1994)     

WATER EXCHANGE, TEMPERATURE TOLERANCE, OXYGEN CONSUMPTION AND ACTIVITY OF THE NAMIB DESERT SNAIL, TRIGONEPHRUS SP.

Water exchange, temperature tolerance and oxygen consumptionof the snail, Trigonephrus sp., from the southern Namib desertof Namibia were examined and related to activity. At 25°Cand 15% R.H. mean water loss and food and water uptake were5.95 mg. day^–1 and 630 mg.day^–1, respectively. Bodytemperature tracked sand temperature. Snails tolerated sandtemperatures as high as 45°C. Mean ± S.D. oxygenconsumption rates were 32.0 ± 2.94 µlO₂.g totalbody mass^–1.h^–1 at 15°C, when the snails wereactive, and 11.27 µlO₂.g total body mass^–1.h^–1at 25°C, when the snails were inactive. These values are2-6 times lower than those recorded for the similarly sizedmesic snail, Helix aspersa. Activity experiments indicated thatlow ambient temperatures and high humidities were favoured bythe snails. This, together with the burying behaviour of thesesnails during high temperatures, suggests that they limit stressby restricting activity to physiologically-favourable periods,even though more-extreme conditions may be tolerated. (Received 7 June 1990; accepted 20 November 1990)     

THE RELATIONSHIP BETWEEN ENVIRONMENTAL VARIABLES AND THE DENSITY OF THE MUD SNAIL HYDROBIA TOTTENI IN A NOVA SCOTIA SALT MARSH

The relationship between environmental variables and the densityof Hydrobia totteni was examined in a restricted area of LawrencetownMarsh, Nova Scotia. Densities averaged 50/m² in Spartina alterniflora,9800/m² in Zostera marina, and 19,000/m² in sand. Of 8 environmentalfactors considered, depth and sediment grain size were the mostimportant in explaining variations in snail density. Since thesefactors together explained only 42.0% of the total variationno single physical factor controls the density of Hydrobia ina restricted area, though salinity does control densities overa wide area. Salinity and temperature tolerances and the abilityof the snails to select appropriate sediment grain sizes weredetermined experimentally and related to the distribution ofthe snails in the marsh. ^*Present address: Western Australian Museum, Francis Street,Perth, Western Australia, 6000 (Received 20 June 1976;     

FACTORS CONTRIBUTING TO SPATIAL HETEROGENEITY IN THE ABUNDANCE OF THE COMMON PERIWINKLE LITTORINA LITTOREA (L.)

Although Littorina littorea (L.) exhibits a relatively consistentpattern of vertical distribution throughout the North Atlantic,ranging from the mid-intertidal to the shallow subtidal zone,its horizontal distribution and abundance are highly variable.In this study, we first described the snail's horizontal distributionpatterns on Appledore Island, ME, USA and then asked whetherwave exposure, rugosity, predator density (e.g. Carcinus maenasand Cancer borealis), the percentage of the substrate coveredby Ascophyllum nodosum, Chondrus crispus, Polysiphonia spp.,and ephemeral green algae, or the percentage of uncovered substrate(bare rock) were correlated with L. littorea abundance in theintertidal zone (0.6 to 0.0 m relative to Mean Lower LowWater [MLLW]) and the shallow subtidal zone (–1.5 to –3.0 mMLLW) at nine study sites. Intertidal densities of L. littoreawere highly variable across sites, ranging from 0 to >600 m^–2.In this zone, L. littorea density showed a significant positivecorrelation with rugosity and percentage of bare rock. Densitieswere very low in the subtidal zone (range: 0–19 m^–2)and varied little among sites. Exploratory multiple regressionanalysis suggested that L. littorea density was positively correlatedwith the density of C. maenas in the shallow subtidal zone.Additionally, snails in the subtidal zone had thicker shellsthan snails of the same size in the intertidal zone. Our resultssuggest that structural elements of the habitat, such as rugosityand percentage of uncovered substrate, are among the most importantpredictors of L. littorea abundance on moderately protected,rocky intertidal shores. (Received 9 February 2005; accepted 10 August 2005)     

Predatory Potential of Freshwater Animals on an Invasive Agricultural Pest, the Apple Snail Pomacea canaliculata (Gastropoda: Ampullariidae), in Southern Japan

The apple snail Pomacea canaliculata is an invasive species and a serious pest of rice in many Asian countries. We studied predatory activities of various animals living in Japanese freshwater habitats, by keeping each individual of a potential predator species with 36 snails of various sizes for three days in the aquarium. Forty-six species were tested, and 26 in eight classes fed on small snails. A species of leech, crabs, the common carp, turtles, the mallard duck and the Norway rat attacked even adult snails of 20–30 mm in shell height. These findings will be helpful in identifying effective predators for biological control of the pest snail. In addition, most of the animals attacking snails are reported to be common in rivers or ponds, but few live in modernized paddy fields having little connections with natural water systems. This may be a reason why this snail maintains large populations in paddy fields but not in other freshwater habitats.     

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.     

BIOLOGY AND POPULATION DYNAMICS OF A CREVICE-DWELLING LANDSNAIL, CRISTATARIA GENEZARETHANA (CLAUSILIIDAE)

Crevice-dwelling land snails emerge only infrequently to theexposed surface, and very little is known about their naturalhistory. Here we report on the biology, life cycle, populationdynamics and longevity of one such species, Cristataria genezarethana,which we studied over a period of three years. C. genezarethana spends 95–98% of its lifetime withinrock crevices. In winter the crevice serves as an egg layingsite, and as a retreat during periods of inactivity. Throughoutsummer it serves for aestivation adults aestivate near CTeviceopenings, young apparently deeper inside. The surface of the rock serves mainly as a lichen-feeding andcopulating site, to which the snails emerge during brief periodsof activity. Both for the onset and for the continuation ofits activity season, C. genezarethana is absolutely dependentupon rain. However, activity ends before the end of the rainyseason, suggesting that aestivation is not merely a direct responseto dry climate. Though the population as a whole was active on each rainy day,no more than 15%–20% of the population were active simultaneously.An individual snail was active, on average, for only 6–12days per year. Population size of the study-rock reached 2000–2900individuals, and mean density was 150–200 snails m². Thissnail carrying capacity of the rock is broadly similar to thatof densely populated bushy habitats (when snail weight is considered).In these dense populations, low mating frequencies and growthinhibition may regulate population size. In all three seasons, the population consisted of two main large,well defined age groups: adults and young. As compared to theadults, the young were active on different parts of the rockand at different times. Although a third, intermediate groupof sub adults was found, it was always very small. A fourthgroup, of juveniles, appeared during winter; later, most ofthem disappeared. We did not observe any shift from the young to the adult sizegroup. This suggests a broadly stable population, with virtuallyno recruitment, in which final growth to adulthood is inhibitedby the adults. Growth was very slow, suggesting that maturityin nature is reached in about 11 years, and that individualslive sixteen years at least. Mortality occurred in all age groups,and about 5% of the population died each year. There was noevidence for heavy predation. Crevices might be a suitable habitat for land snails in whichthe hatching period, and period of juvenile growth, are verylong. (Received 1 February 1993; accepted 27 March 1993)     

Population size, density, spatial distribution and dispersal in an Austrian population of the land snail Arianta arbustorum styriaca (Gastropoda: Helicidae)

One population of the land snail Arianta arbustorum styriaca(Frauenfeld, 1868), endemic in the Northeastern Alps in Austria,was investigated with regard to population size and density,spatial distribution and movement patterns of the individualsin order to obtain basic information on their population biology.The study was carried out at a site situated about 880 m abovesea level in Styria, in the Gesäuse mountains over twoyears, using the capture–recapture technique. Populationdensity based on the Jolly–Seber model showed variationranging from 0.12–4.03 individuals/m² over the whole studyperiod. The recapture rate after one year was generally high(up to 85% per sampling date). In most cases individuals wereaggregated. The snails showed a high site fidelity; average calculatedminimum areas were 0.115 m²/day. The sum vector of all individual dispersalobservations over the entire study period did not indicate asignificant preference for any direction. Only during winterwas a significant downhill displacement (possibly passive) detected. (Received 20 April 1998; accepted 12 September 1998)     

DIFFERENCES IN THE SHELL SHAPE OF LYMNAEA PEREGRA (MULLER) (GASTROPODA: PULMONATA) FROM LOTIC AND LENTIC HABITATS; ENVIRONMENTAL OR GENETIC VARIANCE?

Lymnaea peregra from lotic habitats (running waters) had largeraperture-length/shell-length ratios than contemporaries fromlentic sites (standing waters). This pattern was observed infield samples from two separate years (1985 and 1986). Laboratoryculture experiments suggested that shell shape could be geneticallydetermined in at least some populations. We hypothesize thatlotic snails possess bigger feet, and hence larger aperturesthan lentic individuals as an adaptation to withstand watercurrent ^*Present address Departments of Biology, King's College (KQC),University of London, Campden Hill Road, London W3 7AH, England (Received 14 September 1987;     

Estimation of sample zooplankton abundance from Folsom splitter sub-samples

Procedures are described for estimating the abundance of eachzooplanklon species in a sample after counting at least twosub-samples from a Folsom splitter. A multinomiai model is assumedfor the splitting process. The method may be summarized as follows:two sub-samples, balanced with respect to the left — rightsplits, are counted. The counts for each species are testedfor homogeneity using a x² test. If the counts are homogeneous,an estimator is given which permits the estimation of the numberof animals in the sample, and the variance of this estimate.If the subsample counts are heterogeneous, it is assumed thatclumping has occurred. A procedure is described in which additionalsub-samples are counted to locate the clump, and estimatorsof abundance (and variance estimators) are derived. If the splitterbias is <5%, bias of the abundance estimator is negligiblecompared to the binomial sampling error. Comparisons of speciesabundance between plankton samples is made more rigorous usingthe estimates of mean abundance, and the variance of this mean,provided by the methods described here. ^*Present address: Dunoon Road, Dorroughby, NSW 2480, Australia.     

Complex interactions among fish, snails and macrophytes: implications for biological control of an invasive snail

The golden apple snail (Pomacea canaliculata), a native of freshwater wetlands of South America, has invaded many Asian countries and grazed heavily in agricultural and wild areas. Common carp (Cyprinus carpio) has been proposed as a biological control agent against this snail, but little is known about its impact on non-target aquatic plants and animals. In a 8-week enclosure experiment, we quantified the impact of common carp on three species of aquatic macrophytes and nine species of snails, including the apple snail, in a shallow pond. The results showed that the apple snail or carp alone significantly reduced the plant biomass, although the apple snail had a stronger overall herbivorous effect than the carp. The carp completely removed juvenile apple snails, but had only a weak predatory effect on larger apple snails and no effect on the adults’ oviposition frequency. Furthermore, the carp significantly reduced the populations of most species of other snails that occurred naturally in the pond. Our results thus indicate that common carp can be an effective biological control agent against the invasive apple snail, but caution should be taken about its potential to reduce wetland floral and faunal diversity.     

The use of species abundance estimates in marine benthic studies

The application of species abundance estimates, commonly used in terrestrial plant ecology, to marine benthic dredge data is advocated. Two possible scales were tested for their ability to measure adequately the structure of an assemblage by computing measures of evenness diversity McNaughton's dominance index, and Morisita's index of dispersion using both real counts obtained from grab samples and these counts transformed into the two respective abundance estimates. It is concluded that species abundance estimators, when applied to grab or dredge data result in significant savings in sample processing time with only a minimal loss of information about assemblage structure.     

The epidemiology of fasciolosis in the inter-Andean valley of Cajamarca, Peru

Fasciolosis is recognised as a major problem in dairy cattle in parts of Peru. A longitudinal study of dairy cattle in Cajamarca, Peru was used to determine the annual pattern of infection with Fasciola hepatica. After a gradual increase from January, peak egg production occurred in August/September and then dropped rapidly. Indirect indicators of infection, eosinophil counts and serum liver-enzyme activities, indicated that the major period of new infection in cattle occurred from December to May each year. Examination of snails demonstrated that, although there was no clear annual cycle in snail abundance, the majority of snails infected with cercariae were found between January and March. Climatological data indicated that there was sufficient moisture for development of the parasite during a limited period each year, coinciding with the period of maximum abundance of cercaria-infected snails, but that irrigation could substantially alter the amount of water available. Infections in tracer-calves decreased from December to September, with little or no infection occurring between June and August, but suggesting that there could be significant infection prior to December. Thus a defined annual cycle of infection was observed, where cattle acquired infection from December to May and this infection matured to produce peak egg counts in August/September which were then available to infect the intermediate host for the next cycle of infection.     

Experimental Test of Preferences for an Invasive Prey by an Endangered Predator: Implications for Conservation

Identifying impacts of exotic species on native populations is central to ecology and conservation. Although the effects of exotic predators on native prey have received much attention, the role of exotic prey on native predators is poorly understood. Determining if native predators actively prefer invasive prey over native prey has implications for interpreting invasion impacts, identifying the presence of evolutionary traps, and predator persistence. One of the world’s most invasive species, Pomacea maculata, has recently established in portions of the endangered Everglade snail kite’s (Rostrhamus sociabilis plumbeus) geographic range. Although these exotic snails could provide additional prey resources, they are typically much larger than the native snail, which can lead to lower foraging success and the potential for diminished energetic benefits in comparison to native snails. Nonetheless, snail kites frequently forage on exotic snails. We used choice experiments to evaluate snail kite foraging preference in relation to exotic species and snail size. We found that snail kites do not show a preference for native or exotic snails. Rather, snail kites generally showed a preference for medium-sized snails, the sizes reflective of large native snails. These results suggest that while snail kites frequently forage on exotic snails in the wild, this behavior is likely driven simply by the abundance of exotic snails rather than snail kites preferring exotics. This lack of preference offers insights to hypotheses regarding effects of exotic species, guidance regarding habitat and invasive species management, and illustrates how native-exotic relationships can be misleading in the absence of experimental tests of such interactions.     

Invasiveness Does Not Predict Impact: Response of Native Land Snail Communities to Plant Invasions in Riparian Habitats

Studies of plant invasions rarely address impacts on molluscs. By comparing pairs of invaded and corresponding uninvaded plots in 96 sites in floodplain forests, we examined effects of four invasive alien plants (Impatiens glandulifera, Fallopia japonica, F. sachalinensis, and F.×bohemica) in the Czech Republic on communities of land snails. The richness and abundance of living land snail species were recorded separately for all species, rare species listed on the national Red List, and small species with shell size below 5 mm. The significant impacts ranged from 16–48% reduction in snail species numbers, and 29–90% reduction in abundance. Small species were especially prone to reduction in species richness by all four invasive plant taxa. Rare snails were also negatively impacted by all plant invaders, both in terms of species richness or abundance. Overall, the impacts on snails were invader-specific, differing among plant taxa. The strong effect of I. glandulifera could be related to the post-invasion decrease in abundance of tall nitrophilous native plant species that are a nutrient-rich food source for snails in riparian habitats. Fallopia sachalinensis had the strongest negative impact of the three knotweeds, which reflects differences in their canopy structure, microhabitat humidity and litter decomposition. The ranking of Fallopia taxa according to the strength of impacts on snail communities differs from ranking by their invasiveness, known from previous studies. This indicates that invasiveness does not simply translate to impacts of invasion and needs to be borne in mind by conservation and management authorities.     

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.