Spiders and harvestmen as gastropod predators

1. Reports are reviewed of gastropod feeding (malacophagy) by spiders and harvestmen. Although the standard textbooks on arachnids recognise the importance of gastropods as prey of harvestmen, none apparently refers to malacophagy by spiders. A review of several hundred papers on spider feeding habits revealed that species from several families kill and devour slugs and snails in the laboratory and/or field. 2. Malacophagy has been reported most frequently among mygalomorph spiders, and can make up a substantial proportion of the diets of some species, however gastropods make up an insignificant percentage of the prey of most araneomorph spiders. The spiders that eat gastropods are species with broad diets composed predominantly of arthropod prey. No species of spider appears to feed exclusively on gastropod prey. 3. Harvestmen from several families have broad diets that often include gastropods. Several species of the family Trogulidae and at least one species of the family Ischyropsalididae [Ischyropsalis hellwigi (Panzer)] are specialised gastropod predators. The trogulids are slender animals that attack the snail through the shell aperture (shell intruders). Ischyropsalis hellwigi, on the other hand, can crush snail shells with its powerful chelicerae (shell breakers). 4. The review highlights apparent convergent evolution by harvestmen and Carabidae of two mutually exclusive morphologies found among gastropod predators. It also suggests that there is an urgent need for systematic studies to be conducted to establish the extent and ecological importance of malacophagy in natural and anthropogenically altered habitats.     

Contributions of climate and Brassica oleracea cultivar to gastropod abundance and assemblage in southeastern Brazil

1. Terrestrial gastropods (slugs and snails, Mollusca: Eupulmonata) are destructive pests of brassica crops. These organisms defoliate plants and contaminate the harvest, leading to reduced crop yield and marketability. Losses caused by molluscs have escalated in recent years in Brazil.
2. We aimed to determine the seasonal activity of gastropods in brassica fields and unravel the environmental variables associated with these dynamics. Gastropod abundance, assemblage and within-plant distribution were also compared among Brassica oleracea cultivars (broccoli, cabbage and cauliflower).
3. Mild temperatures and rainy periods were found to coincide with gastropod peaks. Regression analyses confirmed a positive association of rainfall and humidity with gastropod abundance, whereas gastropod abundance increased with decreasing temperature. Deroceras laeve (Müller) (Agriolimacidae) and Bradybaena similaris (Férussac) (Bradybaenidae) were the most constant species. Mollusc abundance and assemblage differed amid B. oleracea cultivar; cabbage had the highest abundance and its species composition differed from broccoli and cauliflower. Slugs and snails were more concentrated on lower plant portions, regardless of the cultivar.
4. Management efforts should be reinforced in mild-temperature and rainy periods to reduce damage by gastropods, and cabbage should be monitored more closely than other cultivars.

     

<Emphasis Type="Italic">Theba pisana</Emphasis> (Müller, 1774) (Gastropoda,Helicidae) and other alien land molluscs species in Argentina

The presence and distribution in Argentina of the invasive snail species Theba pisana is presented on the basis of a survey carried out in 10 beach resorts along the Atlantic coast of Buenos Aires Province. Additionally, complementary information and complete revision of material of other exotic terrestrial gastropods housed in museum collections was carried out, reporting herein seven new species for Argentina (Otala punctata, Hawaiia minuscula, Paralaoma servilis, Opeas goodalli, Vallonia pulchella, Vertigo ovata and Pupisoma dioscoricola), one for Colombia (Subulina octona), and two for Peru (S. octona and P. dioscoricola). At present, 42 introduced species of terrestrial gastropods have been recorded in nine countries of South America (Argentina, Bolivia, Brazil, Chile, Colombia, Paraguay, Peru, Uruguay and Venezuela). The most likely pathways for introduction of terrestrial gastropods, at least in Argentina, are horticultural development and urban and suburban transformation of original natural habitats.     

Impact of fire and the recovery of molluscs in south‐east Australian salt marsh

Fire has long been recognised as a natural force in structuring Northern Hemisphere salt marshes, yet little is known about the impact of fire on molluscs and native vegetation dynamics of Southern Hemisphere coastal salt marshes. Following a fire at Ash Island, Hunter River New South Wales, Australia in the summer 2012, we assessed patterns of recovery through time of gastropod populations and resident salt marsh vegetation including biomass for three keystone native plant species, Native Rush (Juncus kraussii Hochst.), a chenopod (Sarcocornia quinqueflora Bunge ex Ungen‐Sternberg A.J. Scott), Salt Couch (Sporobolus virginicus, L. Kunth) and the invasive Spiny Rush (Juncus acutus). In temperate east‐coast Australian salt marshes, Spiny Rush is displacing native salt marsh vegetation. After twelve months, the biomass of Native Rush recovered to similar pre‐burn levels. While fire affected the abundance, richness and composition of the gastropod assemblage differences were also largely driven by spatial variability. Gastropod assemblages associated with two of the higher elevation native species (Native Rush and Salt Couch) were impacted the most by fire. Greater abundance (between 1 and 5 orders of magnitude difference in abundance) and richness of gastropods were found in unburnt compared with burnt Native Rush and Salt Couch vegetation, while more gastropods were found in Spiny Rush in one site. Species prevalent in burnt vegetation included larger species of gastropods Ophicardelus ornatus (Ferussac, 1821) and Phallomedusa solida (Martens, 1878) with an unexpected spike in number of the smaller gastropod Tatea huonensis (Tenison‐Woods, 1876) in the spiny rush at one site only. In salt marsh habitats, many gastropods have planktonic larval dispersal stages which are dependent on the tidal height for transport and the structural complexity provided by vegetation at settlement. Since fire appears to negatively affect salt marsh gastropod populations within structurally complex Native Rush and Salt Couch, due consideration of the importance of these refuges for gastropods is recommended when fire or other disturbances occur in ecologically endangered salt marsh in the Southern Hemisphere. Managers need to consider spatial heterogeneity of molluscs and their recovery in the event of fire in Southern Hemisphere salt marshes.     

Terrestrial Gastropoda from the Pleistocene of Beni Saf,NW Algeria

A study of the terrestrial gastropods of the Pliocene-Quaternary succession of the Beni Saf sea cliff, NW Algeria, at the Playa Port locality, is presented herein. The sedimentary succession is subdivided into four lithostratigraphic units, on the basis of their lithologic and biogenic contents: the three first units (A, B and C) are of marine origin; the last unit (D) is of continental origin and includes three beds yielding terrestrial gastropod fossils, which form laterally traceable horizons. Herein, we describe the section's lithology, present stratigraphical considerations regarding its age and thoroughly described its terrestrial gastropod fossil fauna. In total, 13 species are reported here from Beni Saf: 2 caenogastropods (family Pomatiidae) and 11 stylommatophorans (family Achatinidae and superfamily Helicoidea). The sedimentological data indicate that the depositional setting at Beni Saf was a dune system flanked by wadi floodplains deposits (snail levels); the ecological preferences of the gastropods largely agrees with this scenario.     

Size-dependent effects of an invasive herbivorous snail (Pomacea canaliculata) on macrophytes and periphyton in Asian wetlands

1. The invasive golden apple snail (Pomacea canaliculata), native to South America, is a serious pest on rice seedlings in south‐east Asia and has also been shown to consume large amounts of macrophytes in natural wetlands, with large effects on ecosystem functioning. Earlier studies suggest that the snail undergoes an ontogenetic diet shift, feeding on algae and detritus as juveniles and shifting to aquatic macrophytes as adults. 2. Here, we study the effects of snail populations with a size‐structure typical of either populations at an invasive front or the size‐structure of established populations. In an enclosure experiment performed in a wetland in Laos, we compared treatments with small snails only (3 mm; invasive treatment) to treatments with small, medium sized (10 mm) and adult (>25 mm) snails (established treatment). The effects of snail grazing on three aquatic macrophyte species and periphytic algae were quantified. 3. We found that snails of all sizes had a strong negative effect on the biomass of all macrophyte species and periphytic algae. There was no evidence of an ontogenetic diet change, i.e. snails in both the invasive and established treatments affected macrophyte biomass. Foraging was size‐dependent in that small snails had higher relative foraging capacity (g plant consumed per g of snail) compared with medium and adult snails. Small snails, therefore, depressed growth of medium snails at increasing densities through exploitative competition for preferred resources, while adult snails did not grow at all in the presence of small snails. 4. Density dependence is common in freshwater invertebrates, including gastropod populations, but differences in size dependent foraging‐ and competitive‐ability have rarely been demonstrated in this group of organisms. Knowledge about intra‐specific differences in ecological performance may, however, both deepen our understanding of the processes that underlie population dynamics in invertebrates such as gastropods, and help develop control strategies for invasive golden apple snails.     

Distribution of gastropods in floodplain compartments and feeding preferences for river corridor plant species: Is there an effect of gastropod herbivory on the distribution of river corridor plants?

Herbivory through gastropods has among others been proposed as a potential factor responsible for the river corridor distribution of plant species, which is a well known but poorly understood ecological pattern. Since floodplains are characterised by seasonally changing abiotic conditions, viz. floods during winter and spring and severe summer drought that are unsuitable for gastropods they may present safe habitats for highly palatable plant species.In the present study we compared species composition of gastropods and vegetation of twelve grassland sites situated within three floodplain compartments along the Upper Rhine. Additionally, we studied the palatability of 7 days and 25 days old seedlings of five typical floodplain plant species and five mesic grassland species to the slug Deroceras reticulatum in laboratory experiments.Our results showed that both vegetation and gastropod community composition but not gastropod diversity and abundance differed between floodplain compartments. Owing to omnivory of most gastropods the similarity structure of sites based on plants and gastropods was not significantly correlated. In general, slug herbivory significantly reduced survival and biomass of 7 days old seedlings, but responses were species-specific. In contrast, with the exception of Arabis nemorensis, Viola pumila and Taraxacum sect. Ruderalia biomass of 25 days old seedlings was not significantly affected by slug herbivory. Although the response of floodplain plant species as a group to slug herbivory did not differ from common grassland species, our results suggest that gastropods may potentially influence the distribution pattern of the highly palatable river corridor species Arabis nemorensis and Viola pumila. However, further research is needed to estimate the damage to river corridor plants through gastropod herbivory and its effect on competitive relationships under natural conditions.