Can Estivation Preferences Be Used to Develop Novel Management Tools against Invasive Mediterranean Snails?

Terrestrial snails that live in hot and dry climates have developed strategies to cope with high summer temperatures. Several species estivate during the warmest months of the years by resting on vertical supports, typically in groups. Understanding how snails choose their estivation sites and aggregate may lead to the development of new management tools in areas where these snails are invasive. Here, we investigated the preferences of four snail species for vertical supports varying in widths and heights under laboratory and field conditions, and tested whether the presence of conspecifics or snails of other species affected these preferences. The results show that the snails strongly preferred wider supports in laboratory dual-choice tests, and one species (Theba pisana) showed a consistent preference for taller supports as well. These results were confirmed in the field, where more snails were found on wider and taller supports 24 h after being placed in test quadrats. The percentage of snails found in groups on a support was strongly density-dependent. The presence of conspecifics or their mucus did not affect the choices of the snails, nor did the presence of snails of other species or their mucus. Taken together, these results could lead to the development of attractive supports that could be used to mass-capture snails in the field.

Insights into the ecotoxicological perturbations induced by the biocide abamectin in the white snail, Theba pisana

Abamectin (avermectin B1, ABM) has been widely used as a biocide in agriculture, veterinary and medicine worldwide. In the current study, we aimed to evaluate the acute toxicity and sub-lethal biochemical responses of ABM on the non-target land snail, Theba pisana. Mortality of snails increased with the dose increase, resulting 48h- LD50 value of 1.048 µg/snail. Sub-lethal effects were studied on the survivors of 20% and 60% LD50 ABM doses and the biochemical parameters were assessed for up to 7 days of exposure. The results showed a decrease in glycogen content and lipids for two sub-lethal doses after all time intervals, whereas increased the level of total proteins after exposure to 60% LD50 ABM. Overall, the tested sub-lethal doses significantly decreased the total energy reserves. ABM-exposure to snails elevated γ-Glutamyl transferase (γ-GT) and Lactate dehydrogenase (LDH) activities at all-time intervals. A significant increase of Glutathione-S-transferase (GST) activity was also recorded in snails exposed to 20% and 60% LD50 after 7 days and all time intervals, respectively. However, ABM inhibited the activity of Aspartate aminotransferase (AST) and Alanine aminotransferase (ALT) after 7 days of exposure. Our investigation provides new insights into the disturbances of energy reserves and enzyme activities in T. pisana snails that can be used as useful sentinel organism. Indeed, these tested biochemical parameters of the snails are sensitive and may be used as biomarkers for assessing ABM toxicity.

Multiple Introductions of the Pestiferous Land Snail Theba pisana (Müller, 1774) (Gastropoda: Helicidae) in Southern California

The terrestrial land snail Theba pisana is circum-Mediterranean in native range and widely introduced and pestiferous in regions around the world. In California, USA, T. pisana has been recorded intermittently since 1914, but its source population(s) are unknown, and no morphological or molecular analyses within or between California populations have been published. Therefore, we compared molecular data (CO1, 16S, ITS2) and internal morphology (jaw, radula, reproductive system) in T. pisana collected from Los Angeles and San Diego counties in 2019–2020. DNA barcode (CO1 mtDNA) analysis revealed that T. pisana from Los Angeles County was most similar to T. pisana from the Mediterranean island of Malta, and northern San Diego County-collected specimens were most similar to T. pisana from Morocco. Morphology of the jaw and mucous glands also differed between Los Angeles and San Diego populations, but it is unclear if traits are lineage-specific or artifacts of ontogeny. Several pathways of introduction into Southern California are possible for this species, but evidence for intentional vs. accidental introduction of present populations is lacking. Subsequent investigation(s) could use the data generated herein to assess the provenance of T. pisana elsewhere in California and/or worldwide and inform analyses of reproductive biology and systematics in this widespread species.

Continuous versus discrete quantity discrimination in dune snail (Mollusca: Gastropoda) seeking thermal refuges

The ability of invertebrates to discriminate quantities is poorly studied, and it is unknown whether other phyla possess the same richness and sophistication of quantification mechanisms observed in vertebrates. The dune snail, Theba pisana, occupies a harsh habitat characterised by sparse vegetation and diurnal soil temperatures well above the thermal tolerance of this species. To survive, a snail must locate and climb one of the rare tall herbs each dawn and spend the daytime hours in an elevated refuge position. Based on their ecology, we predicted that dune snails would prefer larger to smaller groups of refuges. We simulated shelter choice under controlled laboratory conditions. Snails’ acuity in discriminating quantity of shelters was comparable to that of mammals and birds, reaching the 4 versus 5 item discrimination, suggesting that natural selection could drive the evolution of advanced cognitive abilities even in small-brained animals if these functions have a high survival value. In a subsequent series of experiments, we investigated whether snails used numerical information or based their decisions upon continuous quantities, such as cumulative surface, density or convex hull, which co-varies with number. Though our results tend to underplay the role of these continuous cues, behavioural data alone are insufficient to determine if dune snails were using numerical information, leaving open the question of whether gastropod molluscans possess elementary abilities for numerical processing.

Interactions between the land snails Theba pisana and Cernuella virgata in the laboratory

ABSTRACT Two Mediterranean snails, Theba pisana and Cernuella virgata, are agricultural pests in southern Australia. The two species are rarely found together in large numbers in the field, at small scales (<1 m2). In laboratory experiments, the presence of T. pisana reduced the survival of C. virgata, but only when food (carrot + lettuce) was provided. When C. virgata was exposed to only the mucus trails and faeces of T. pisana, produced while feeding on lettuce, both the survival and activity of C. virgata were reduced. When carrot was substituted for lettuce, there was less effect. In addition, when C. virgata was exposed to T. pisana’s faeces only, derived from access to a mix of lettuce and carrot, there was no effect on C. virgata’s survival. The observed reductions in the survival of C. virgata were stronger in autumn (the breeding season for both snail species) compared with spring. Inhibitory components within the mucus trails of T. pisana may (1) help explain the observed distribution patterns of the two species at small scales in the field and (2) provide a novel method for control of pest populations of C. virgata, in some situations.

Role of ambient wavelength, shell size and pigmentation intensity in the heating of Mediterranean Theba pisana (Eupulmonata: Helicidae)

ABSTRACT Theba pisana is a polymorphic land snail widely used in ecophysiological research on the biological significance of solar irradiation. We investigated whether, taking size into account, differently pigmented morphs of a Mediterranean population of T. pisana influence the snails’ heating under different wavelengths. We used a laboratory set-up comprising light-emitting diodes emitting visible light of defined wavelengths and quantified shell heating using high-resolution thermography. Shell pigmentation was quantified densitometrically. There were significant effects of both shell size and light wavelength, such that small snails heated more than large ones over 15 min, and blue light (470 nm) raised temperature more than green light (525 nm). Pigmentation alone did not show significance, but a trend towards higher temperature with increasing pigmentation was evident. Despite the observed significances and trends, we could only explain at most 4.67% of variation in shell heating by multiple regression modelling. However, the light intensity used in this experiment was low and the pigmentation intensity of the shells varied, at most, by a factor of 2. This suggests the actual temperature of a snail depends not only on the tested variables, but also on the multifactorial action of a number of unknown, perhaps intrinsic physiological parameters.