Helicid Snails Research Articles

Dart receipt promotes sperm storage in the garden snail Helix aspersa

During courtship, many helicid snails attempt to pierce the body walls of their mating partners with mucus-coated calcareous darts. The mucus covering the dart induces conformational changes in the female reproductive tract of the recipient, closing off the entrance to the gametolytic bursa copulatrix. We have tested the effect of dart receipt on the number of sperm stored by once-mated snails, Helix aspersa. Snails that were hit by darts stored significantly more sperm than did snails that were missed. Additionally, the effect of the dart was stronger in smaller animals and the number of sperm stored decreased with the shell volume of the recipient. Although larger animals produced larger spermatophores, dart-shooting success was not related to the number of sperm transferred. These data suggest a role for dart-shooting in post-copulatory sexual selection.

Spermatophore Formation in the Simultaneously Hermaphroditic Land Snail Arianta Arbustorum (Pulmonata: Stylommatophora: Helicidae)

In helicid gastropods including Arianta arbustorum (LINNAEUS, 1758) sperm are reciprocally exchanged during copulation through spermatophores. A. arbustorum has been extensively investigated with respect to sexual selection and a considerable amount of data on sperm and spermatophore transfer are available. However, nothing is known about spermatophore formation and its possible influence on sperm exchange. As a contribution to the very limited knowledge on spermatophore formation in stylommatophoran gastropods in general and in order to complement the existing behavioural studies from a functional-anatomical point of view, we investigated spermatophore formation in A. arbustorum. Spermatophores were collected at certain intervals from 2 to 90 min after the onset of copulation either through forceful separation of the pairs (after which spermatophores were expelled by the snails) or through dissection of electrocuted and fixed animals. We found that spermatophore formation was initiated more or less synchronously in mating partners a few minutes after the onset of copulation and continued until shortly before transfer, which occurred after more than 60 min. Growth of the sperm container, which is probably mediated through the incoming ejaculate dilating the viscous, workable material secreted by the epiphallus, followed a saturation curve where the final length was reached slightly earlier than the final volume. However, growth and final size were not adjusted among partners confirming earlier findings that there is no sperm trading in A. arbustorum. Spermatophores collected from separated snails were larger than those from fixed individuals indicating that spermatophore formation is not immediately stopped after separation. This means that the former protocol is less precise for the investigation of spermatophore formation than a treatment involving rapid fixation. In A. arbustorum, spermatophore formation differs largely from that in the only other investigated helicid snail, Helix pomatia Linnaeus 1758, in which the container is assembled and the spermatophore transferred in less than 10 min.

Phenotypic plasticity in reproductive traits: importance in the life history of Helix aspersa (Mollusca: Helicidae) in a recently colonized habitat

Abstract Reproductive traits of the land snail Helix aspersa Muller were investigated under artificial conditions from two samples, one collected from a population exposed to unpredictable human pressures in its natural environment, i.e. a recently created polders area with intensive agriculture, and the other from a snail farm in which animals were reared under constant conditions defined as «optimal» for growth and reproduction. Results were compared with data collected from natural populations of the same region (Brittany) and from habitats spanning the environmental heterogeneity of the range of the species. A large part of the variation among populations could be explained by different phenotypic covariances between shell size, clutch size and egg size, but not by the number of clutches per snail. Thus, the higher egg production of snails from the polders was related to (i) a strong correlation between clutch size and shell size, shell size being in the upper limit of the overall range for the region concerned; (ii) an uncommonly low egg weight in comparison with the «norm» of Helix aspersa , this trait seeming to be involved in a trade-off with clutch size. Second clutches were smaller than the first ones, but their eggs were significantly heavier. This difference may be linked to a size-dependent mortality of juveniles during winter which arises in all populations in which hibernation occurs as an adaptation to low temperatures. In addition to the selective regime usually involved for populations of helicid snails from Western Europe, several unpredictable mortality factors occurred in the polders area: herbicide and pesticide treatments (lethal for young snails), human predation (lethal for adults) and burning (lethal for all snails). Life-history patterns of Helix aspersa are discussed in relation to its ability to successfully colonize a large range of habitats modified by humans, to such an extent that it can become a pest.

Phenotypic plasticity in reproductive traits: importance in the life history of Helix aspersa (Mollusca: Helicidae) in a recently colonized habitat

Reproductive traits of the land snail Helix aspersa Müller were investigated under artificial conditions from two samples, one collected from a population exposed to unpredictable human pressures in its natural environment, i.e. a recently created polders area with intensive agriculture, and the other from a snail farm in which animals were reared under constant conditions defined as «optimal» for growth and reproduction. Results were compared with data collected from natural populations of the same region (Brittany) and from habitats spanning the environmental heterogeneity of the range of the species. A large part of the variation among populations could be explained by different phenotypic covariances between shell size, clutch size and egg size, but not by the number of clutches per snail. Thus, the higher egg production of snails from the polders was related to (i) a strong correlation between clutch size and shell size, shell size being in the upper limit of the overall range for the region concerned; (ii) an uncommonly low egg weight in comparison with the «norm» ofHelix aspersa , this trait seeming to be involved in a trade-off with clutch size. Second clutches were smaller than the first ones, but their eggs were significantly heavier. This difference may be linked to a size-dependent mortality of juveniles during winter which arises in all populations in which hibernation occurs as an adaptation to low temperatures. In addition to the selective regime usually involved for populations of helicid snails from Western Europe, several unpredictable mortality factors occurred in the polders area: herbicide and pesticide treatments (lethal for young snails), human predation (lethal for adults) and burning (lethal for all snails). Life-history patterns of Helix aspersa are discussed in relation to its ability to successfully colonize a large range of habitats modified by humans, to such an extent that it can become a pest.

Laboratory Screening of Nematodes Isolated from South Australia for Potential as Biocontrol Agents of Helicid Snails

The helicid snails Cernuella virgata, Theba pisana, Cochlicella acuta, and Cochlicella barbara are introduced pests in grain crops and pastures in southern Australia. A survey for nematodes with potential for biocontrol of these snails was carried out and six local nematode isolates were tested for their ability to kill C. virgata, T. pisana, and C. acuta using a soil-based laboratory bioassay. The rhabditid isolate R 954 was highly pathogenic to all three snail species with mortality increasing with increasing nematode densities and exposure time. C. acuta was most susceptible to R 954. In addition, high mortalities followed exposure of C. acuta to two cephalobid isolates, C 916 and C 920. Snail mortality caused by the other nematode isolates tested was too low for further consideration as possible biocontrol agents.

Recognising and responding to the influences of agriculture and other land-use practices on soil fauna in Australia

The abundance and diversity of soil fauna are influenced by a wide range of agricultural and other land use practices. This paper provides three Australian examples of the ways in which land use has affected the different components of the soil fauna and discusses strategies that might be applied to respond to these changes. The paper also discusses an emergent issue (genetic engineering of plants) which may prove to have implications for the soil fauna in future. European settlers dramatically altered the earthworm fauna when they established agriculture on the Australian landscape. This change in the fauna, from native megascolecids to introduced lumbricids, happened by chance, not through thoughtful management. Whilst the most common species now present have the potential to enhance soil properties and plant production significantly, they are patchy in distribution, generally rare in numbers and poor in behavioural diversity. Adoption of the management practices which increase earthworm abundance (e.g. reduced tillage and stubble retention), redistribution of beneficial species to regions lacking them and the introduction of more appropriate taxa from overseas may improve the current fauna's performance. Mediterranean helicid snails are introduced pests of pastures, croplands and vineyards in southern Australia. Their pest status has increased due to the adoption of agricultural practices aimed at soil conservation. Molluscicides are prohibitively expensive to use regularly on a broad-acre basis and environmentally undesirable. Strategic use of such chemicals, as strips to interrupt the seasonal movements of snails or in years when the snail numbers are particularly high, may provide a useful alternative. Fragmentation of native forests is common in Australia but the impact of this fragmentation on the abundance and diversity of soil fauna is poorly understood. Preliminary results are presented for the effects of fragmentation of a Eucalyptus forest on a millipede community. Responses vary between species of soil invertebrates, illustrating that studies need to be focused at population rather than community levels. Responses also vary in time, indicating the need for long-term assessments. Genetically-engineered plants are being developed which incorporate toxins to kill leaf feeding insects. The non-harvested parts of such plants serve as food for the soil fauna which drive nutrient recycling processes. The suitability of this plant material for processing by the soil fauna needs to be researched.

First report of the isolation of an adult worm of the genus Brachylaima (Digenea: Brachylaimidae), from the gastrointestinal tract of a human

A 78-year-old woman presented with an 18-month history of intermittent diarrhoea. Examination of her stools revealed brachylaimid eggs, which were present in three separate specimens over a week. After treatment with praziquantel a degenerate adult Brachylaima species was recovered from her faeces. She lived in a rural area of South Australia and ate vegetables grown in her own garden which had been infested with helicid snails. In South Australia these introduced European helicid snails are commonly infected with brachylaimid intermediate larval stages and are considered to be the source of the human infection.

A new technique to assess the number of spermatozoa in spermatophores of stylommatophoran gastropods

The infrastructure of molluscan spermatozoa is of systematic significance and has been studied fairly extensively. In contrast, little is known in molluscs on the number of spermatozoa transferred to the mating partner during copulation (for exceptions see-). Many species of gastropods and cephaJopods are promiscuous and they have different forms of sperm storage providing the potential for sperm competition. Sperm number is an important determinant in achieving fertilization success in sperm competition.' Accurate assessment of the number of sperm transferred to the partner either in form of free sperm (i.e. as sperm suspension in seminal plasma) or sperm encapsulated into spermatophores is an essential prerequisite of any study of sperm competition. In this note we describe a new technique to assess the number of sperm encapsulated in single spermatophores of stylommatophoran gastropods. Spermatophores of the outcrossing, simultaneously hermaphroditic land snail Arianta arbustorum (L.) were used to develop the technique. Arianta arbustorum is common in moist habitats of north-western and central Europe. Adult snails have a globular shell which measures 17-23 mm in shell breadth. Copulation is reciprocal and includes simultaneous intromission and spermatophore transfer. The spermatophore has a distinct form consisting of a head, a body (sperm container) and a 2-3 cm long tail. Snails were collected at two sites: (1) in a subalpine forest near Gurnigelbad, Switzerland (46° 45' N, 7° 28' E; altitude 1250 m a.s.l.) on 27 April 1996, and (2) on the Ebersangeralm, an alpine meadow in the Ennstaler Alpen, Austria (47° 34' N, 14° 39' E; altitude 1500 m a.s.l) on 6 June 1996. The snails were kept isolated in transparent plastic beakers (6-5 cm in diameter, 8 cm deep) lined with moist soil mixed with powdered limestone. Fresh lettuce was provided ad libitum as food. Snails were allowed to mate with a randomly chosen partner. Spermatophores were obtained from snails frozen immediately after copulation. The female reproductive duct of the receiver was dissected to remove the spermatophore. For each spermatophore the length and width of the sperm containing part were measured to the nearest 0.1 mm using a binocular microscope equipped with an ocular micrometer. The volume of the sperm container was calculated assuming a cylindric form. Spermatophores were kept singly in Eppendorf tubes at -40° C until required. The spermatophore of A. arbustorum consists of a hardened secretion which encapsulates the As in other helicid snails, the spermatophore is attacked by digestive enzymes in the female tract soon after copulation. We mechanically disrupted the spermatophore in 200 ml PBS-buffer (138.6 mM NaCl, 2.7 mM KCI, 8.1 mM Na2HPO4*2H2O and 1.5 mM KH2PO4) using a pair of micro scissors. The sperm suspension was homogenized using a set of Gilson pipettes (in decreasing order 1000 JJLI, 200 JJLI and 20 u.1 pipettes) for 5 to 15 minutes. To obtain openings of different widths, pieces of different lengths were cut from the tips of the pipettes. For a quantitative assessment of the number of sperm, the homogenate was stained with an equal volume of a gallocyanin-chromium complex which stains the DNA in the head of the spermatozoa. This complex is a lake-cation binding optimally at pH 1.6. The staining is independent of its duration and not affected by the presence of ethanol. The sample was stained for 1 to 3 hours and checked for sperm clusters using a light microscope (magnification 400 X). If spermatozoa still occurred in clusters, the sample was treated with a sonicator (35 kHz) for 16 hours (in cases of extremely dense clusters up to 80 hours). If the spermatozoa were well separated in the sperm suspension, two subsamples of known volume were diluted 1:3 with PBS-buffer and transferred to a BUrker-Turk counting chamber. A counting chamber consists of 16 cells each with a volume of 25 nl. All sperm heads were counted in randomly chosen cells until the total number of sperm heads exceeded 400. Sperm were counted in two subsamples, and the average of these subsamples was used to calculate the total number of sperm in a spermatophore.

Influence of snail feces and mucus on oviposition and larval behavior ofPherbellia cinerella (Diptera: Sciomyzidae)

Larvae of the sciomyzid flyPherbellia cinerella are voracious predators of terrestrial helicid snails. Eggs are deposited in areas where snails occur and larvae hunt actively for their prey. Snail feces and mucus were tested to determine if they had any kairomone or stimulatory effects onP. cinerella. Adult flies oviposited more frequently on substrates containing fresh snail feces than on substrates containing snail mucus or water (control). However, mucus and feces both stimulated increased search behaviour in first instar larvae. These results are discussed in relation to snail biology, and the potential for augmentation of these flies in areas affected by pest snails.

The potential of several species of terrestrial Sciomyzidae as biological control agents of pest helicid snails in Australia

The seasonality, habitat preference and prey acceptability of the terrestrial sciomyzids, Coremacera marginata Fabricius, Euthycera cribrata Rondani, Trypetoptera punctulata Scopoli, Pherbellia cinerella Fallén, and Dichetophora obliterata Fabricius are described. Four of the species ( C. marginata, D. obliterata, T. punctulata and E. cribrata) are univoltine and were most abundant in either riverine forest or low scrubland (garrigue) while one species ( Pherbellia cinerella) is multivoltine and was very abundant only in open pasture habitats. All of the species had a wide range of prey acceptability (five to nine species of snails) although they varied widely in their predation efficiency. The potential of the univoltine species as biological control agents for pest pasture snails in Australia can be assumed to be poor due to differences between their preferred habitat and the target habitat. However, the multivoltine, pasture-inhabiting species, P. cinerella may have potential use, if it can be shown that it will not overlap with the habitat of endemic Australian snails.

Susceptibility of Helicid Snails to Isolates of the Nematode Phasmarhabditis hermaphrodita from Southern France

Susceptibility of Helicid Snails to Isolates of the Nematode Phasmarhabditis hermaphrodita from Southern France

Seasonality, Longevity, Host Choice, and Infection Efficiency of Salticella fasciata (Diptera: Sciomyzidae), a Candidate for the Biological Control of Pest Helicid Snails

Abstract Host specificity and seasonal incidence of parasitism by the sciomyzid fly Salticella fasciata was studied at a site near Montpellier, France, and in the laboratory. In field collections, only adult snails (> 11-mm diameter) of the pest helicid species Theba pisana and Cernuella virgata were found bearing eggs of S. fasciata in their umbilicus. Flies were present in the field from September to June, which coincides with the active breeding period of the host snails. The percentage of adult snails bearing fly eggs increased with time while the adult snail population decreased through mortality after breeding. In choice experiments, both T. pisana and C. virgata were significantly preferred as hosts when maintained in pairwise combinations with the co-occuring pest helicid species Cochlicella barbara , Cochlicella acuta , and Helix aspersa . Surprisingly, fly larvae were found to be inefficient at infecting and killing host snails though they fed readily on the decaying tissues of manually killed snails. The host-specific oviposition habit of the fly and larval killing inefficiency are discussed in relation to both the behavioral evolution of the Sciomyzidae and the inappropriateness of this species as a biological control agent of terrestrial snails.

Sex and genetic variation in a helicid snail

Variation at enzyme loci has been investigated in the endemic Madeiran land snail Heterostoma paupercula (Gastropoda: Helicidae). There is a high degree of differentiation between populations. Part of the variation (about half of the total as measured by F statistics based on three polymorphic loci) is the result of divergence between populations of different islands. There are morphological differences between populations in shell form and the structure of the aperture, and of the genitalia, which have been used to define separate taxa, but these do not show a clear geographical pattern. In the present results, the variation in the genitalia appears to be a polymorphism restricted to a few populations, in which some individuals are euphallic cross-fertilizing hermaphrodites, while others are hemiphallic individuals exhibiting partial selfing. It is suggested that these may in fact be part selfing, part female. This is a breeding pattern which could result in polymorphism.

Climatic Warming Due to Thermal Radiation from an Urban Area as Possible Cause for the Local Extinction of a Land Snail

A field survey revealed that the land snail Arianta arbustorum had become extinct at 16 (55.2%) out of 29 localities in the surroundings of Basel (Switzerland) between 1908 and 1991. Habitat destruction by urban development was the reason for extinction of eight local snail populations. However, A. arbustorum also became extinct at eight localities, which were covered by vegetation suitable for the species and which still supported abundant populations of other helicid snail species including Cepaea nemoralis (all of them recorded in 1906-8). Localities where A. arbustorum persisted and localities where it went extinct did not differ in exposure, inclination, percentage of woody plant cover, height of ground vegetation, distance to the nearest water body and number of other helicid snail species present. In this paper we provide evidence that local climatic warming due to increased thermal radiation from built-up areas might be a cause for the local extinction of A.arbustorum

The “love-dart” in helicid snails: a gift of calcium or a firm commitment?

The “love-dart” of hermaphroditic helicid snails is an expensive structure of uncertain adaptive value. Two current models of hermaphrodite mating systems, energy investment and “gamete-trading”, make opposite predictions as to its function. Eggtrading predicts that it represents a gift of calcium to induce a partner to act as a female. The gamete-trading model predicts that it should serve to induce the partner to act as male. Review of the evidence suggests that the “love-dart” serves to induce the partner to act as a male, by acting as an honest signal of an individual's willingness to reciprocate in a sperm-trading mating system.

Variation in the genitalia of the land snail Heterostoma paupercula (Lowe, 1831) (Helicidae) in Madeira

The small helicid snail Heterostoma paupercula occurs in the Azores and Madeira. Previously regarded as a single species, it was split into two genera on the basis of variation in shell morphology and genital anatomy: Heterostoma having a toothed shell and being hemiphallic, while Steenbergia had a toothless shell and was euphallic. This division was first questioned when toothed shells {Heterostoma) in the Azores were found not to be hemiphallic. In this study the genitalia of 361 specimens from islands in the Madeira archipelago were examined. Four components of the genitalia were measured (penis/epiphallus, flagellum, bursa stalk and bursa) for 158 individuals. Hemiphally was found to be rare, occurring in only a proportion of the populations at two localities and it did not correspond with the expected shell morphology–the shells were untoothed (=Steenbergia), which is the opposite of that previously described. Variation in the genitalia was geographically based: Madeira and its nearest neighbour islands forming one group and the Porto Santo islands another. There is no evidence to suggest that the hemiphallic animals are more than an intraspecific form. It is suggested that the variation between the two island groups is reflective of allopatric differentiation. Without fur ther evidence of divergence, H. paupercula should be regarded as the sole species throughout the range and the genus Steenbergia should be suppressed.

Production of Eggs and Young Snails by Adult Theba-Pisana (Muller) and Cernuella-Virgata (Da Costa) (Mollusca, Helicidae) in Laboratory Cultures and Field Populations

The fecundities of two terrestrial helicid snails, Theba pisana and Cernuella virgata, were measured in laboratory cultures. The breeding seasons of both species extended from autumn to spring, with most eggs and largest clutches being laid in autumn. Significant positive correlations between shell size and total number of eggs produced, numbers of clutches and clutch sizes were demonstrated for both species, but these correlations were generally weak and not consistently significant for snails collected from different habitats. In field populations in pastures in South Australia, the numbers of juvenile T. pisana and C. virgata (> 6 mm in shell diameter) produced per adult were inversely related to the densities of adults at the start of the breeding season. Possible reasons for this variation in production are discussed. Previous studies with other species of helicid snails have suggested that crowding can reduce the sizes of adults and hence their production of young. Such an explanation is inadequate in this case.

Biology and population development ofAblattaria arenaria [Coleoptera: Silphidae], a snail predator

The biology ofAblattaria arenaria (Kraatz) was studied in the laboratory by using a helicid snail,Xeropicta derbentina (Kyrnicki) as prey. Population development ofA. arenaria in the field was studied in 1985 and 1986.

FEMALE DRY WEIGHT AND FEMALE CHOICE IN CHAULIOGNATHUS PENNSYLVANICUS.

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Neural serotonin receptors in active and hibernating helicid snails (Helix lucorum).

In hibernating snails Helix lucorum, the effect of 5-HT on neurons of an identified group is mainly hyperpolarization associated with a conzation conductance increase, while, inactive snails, the effect is de polarization decrease of the membrane conductance.