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1. Plants critical for Hawaiian land snail conservation: arboreal snail plant preferences in Puʻu Kukui Watershed, Maui

   https://doi.org/10.1017/S0030605320000873  

   Meyer, Wallace M.; Evans, Lily M.; Kalahiki, Connor J.K.; Slapcinsky, John; Goulding, Tricia C.; Robinson, David G.; Kaniaupo-Crozier, D. Pomaikaʻi; Kim, Jaynee R.; Hayes, Kenneth A.; Yeung, Norine W. (May 2021, Oryx)

   null (Ed.)

   Abstract The Hawaiian archipelago was formerly home to one of the most species-rich land snail faunas (> 752 species), with levels of endemism > 99%. Many native Hawaiian land snail species are now extinct, and the remaining fauna is vulnerable. Unfortunately, lack of information on critical habitat requirements for Hawaiian land snails limits the development of effective conservation strategies. The purpose of this study was to examine the plant host preferences of native arboreal land snails in Puʻu Kukui Watershed, West Maui, Hawaiʻi, and compare these patterns to those from similar studies on the islands of Oʻahu and Hawaiʻi. Concordant with studies on other islands, we found that four species from three diverse families of snails in Puʻu Kukui Watershed had preferences for a few species of understorey plants. These were not the most abundant canopy or mid canopy species, indicating that forests without key understorey plants may not support the few remaining lineages of native snails. Preference for Broussaisia arguta among various island endemic snails across all studies indicates that this species is important for restoration to improve snail habitat. As studies examining host plant preferences are often incongruent with studies examining snail feeding, we suggest that we are in the infancy of defining what constitutes critical habitat for most Hawaiian arboreal snails. However, our results indicate that preserving diverse native plant assemblages, particularly understorey plant species, which facilitate key interactions, is critical to the goal of conserving the remaining threatened snail fauna. 

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2. Rapid range expansion of an invasive predatory snail, Oxychilus alliarius (Miller 1822), and its impact on endemic Hawaiian land snails

   Curry, Patrick A.; Yeung, Norine W.; Hayes, Kenneth A.; Meyer, Wallace M.; Taylor, Andrew D.; Cowie, Robert H. (April 2016, Biological invasions)

   The invasive predatory snail Oxychilus alliarius is established in many locations around the world including the Hawaiian Islands. Anecdotal evidence suggests that it negatively impacts indigenous snail species where it has been introduced, although such impacts have not been quantified. On the Hawaiian island of Oahu, we tested the hypothesis that indigenous snails, especially small ones (<3 mm in maximum dimension), would be less abundant where O. alliarius had established populations. Fifty-six sites at four locations were repeatedly surveyed for snails between July 2010 and April 2011. The composition of the snail fauna differed in relation to O. alliarius abundance, as well as location. Notably, the abundance of the native Succineidae was negatively related with that of O. alliarius. The abundance of the native Tornatellidinae was significantly related to O. alliarius abundance but this relationship differed among locations, negative at one site and positive at the other three; these snails do not appear to be negatively impacted by O. alliarius. We also monitored the rate of expansion of a newly introduced O. alliarius population along a transect through a bog on the summit of Oahu’s highest mountain, Mt. Kaala. The population’s range expanded linearly between 2008 and 2011 by approximately 300 m (mean c. 113 m/year). This is the first attempt to quantify the impacts of O. alliarius on threatened native island snail faunas. While the results are complex, its high abundance, rapid rate of population expansion and probable negative impacts on certain species caution vigilance in preventing its introduction and spread to as yet uninvaded islands and locations. 

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3. Reducing disease and producing food: Effects of 13 agrochemicals on snail biomass and human schistosomes

   https://doi.org/10.1111/1365-2664.14087  

   Haggerty, Christopher J. E.; Halstead, Neal T.; Civitello, David J.; Rohr, Jason R. (December 2021, Journal of Applied Ecology)

   Abstract Agricultural expansion is predicted to increase agrochemical use two to fivefold by 2050 to meet food demand. Experimental evidence suggests that agrochemical pollution could increase snails that transmit schistosomiasis, a disease impacting 250 million people, yet most agrochemicals remain unexamined.Here we experimentally created >100 natural wetland communities to quantify the relative effects of fertilizer, six insecticides (chlorpyrifos, terbufos, malathion, λ‐cyhalothrin, permethrin and esfenvalerate), and six herbicides (acetochlor, alachlor, metolachlor, atrazine, propazine and simazine) on two snail genera responsible for 90% of global schistosomiasis cases.We identified four of six insecticides (terbufos, permethrin, chlorpyrifos and esfenvalerate) as high risk for increasing snail biomass by reducing snail predators. Hence, malathion and λ‐cyhalothrin might be useful for improving food production without increasing schistosomiasis. This top‐down effect of insecticides on predators was so strong that the effects of herbicides on schistosomiasis risk were masked in the presence of predators because there were so few snails. In the absence of snail predators, herbicide effects on snails were generally negative by reducing submerged vegetationHydrilla verticillata. The exception was that atrazine and acetochlor significantly increased the biomass of infected snails and total snails respectively.Like insecticides, fertilizer had strong positive effects on snail populations. Fertilizer increased both snail habitat (submerged vegetation) and snail food (periphyton), but of these two pathways, the increases in snail habitat resulted in greater snail population growth. Total snail biomass was positively associated with both infected snail biomass and parasite production and thus human infection risk.Synthesis and applications. Our findings suggest that fertilizers and insecticides generally have consistently higher chances of increasing human schistosomiasis than herbicides in natural communities. Furthermore, our results highlight the need to identify other low risk insecticides, which might help reduce crop pests without increasing snails and thus risk of schistosomiasis. 

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4. Local and regional geographic variation in inducible defenses

   https://doi.org/10.1002/ecy.4207  

   Corbett, James J.; Trussell, Geoffrey C. (November 2023, Ecology)

   Abstract Invasive predators can cause substantial evolutionary change in native prey populations. Although invasions by predators typically occur over large scales, their distributions are usually characterized by substantial spatiotemporal heterogeneity that can lead to patchiness in the response of native prey species. Our ability to understand how local variation shapes patterns of inducible defense expression has thus far been limited by insufficient replication of populations within regions. Here, we examined local and regional variation in the inducible defenses of 12 native marine snail (Littorina obtusata) populations within two geographic regions in the Gulf of Maine that are characterized by vastly different contact histories with the invasive predatory green crab (Carcinus maenas). When exposed in the field to waterborne risk cues from the green crab for 90 days, snails expressed plastic increases in shell thickness that reduced their vulnerability to this shell‐crushing predator. Despite significant differences in contact history with this invasive predator, snail populations from both regions produced similar levels of shell thickness and shell thickness plasticity in response to risk cues. Such phenotypic similarity emerged even though there were substantial geographic differences in the shell thickness of juvenile snails at the beginning of the experiment, and we suggest that it may reflect the effects of warming ocean temperatures and countergradient variation. Consistent with plasticity theory, a trend in our results suggests that southern snail populations, which have a longer contact history with the green crab, paid less in the form of reduced tissue mass for thicker shells than northern populations. 

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5. Transmission potential of human schistosomes can be driven by resource competition among snail intermediate hosts

   https://doi.org/10.1073/pnas.2116512119  

   Civitello, David J.; Angelo, Teckla; Nguyen, Karena H.; Hartman, Rachel B.; Starkloff, Naima C.; Mahalila, Moses P.; Charles, Jenitha; Manrique, Andres; Delius, Bryan K.; Bradley, L. M.; et al (February 2022, Proceedings of the National Academy of Sciences)

   Predicting and disrupting transmission of human parasites from wildlife hosts or vectors remains challenging because ecological interactions can influence their epidemiological traits. Human schistosomes, parasitic flatworms that cycle between freshwater snails and humans, typify this challenge. Human exposure risk, given water contact, is driven by the production of free-living cercariae by snail populations. Conventional epidemiological models and management focus on the density of infected snails under the assumption that all snails are equally infectious. However, individual-level experiments contradict this assumption, showing increased production of schistosome cercariae with greater access to food resources. We built bioenergetics theory to predict how resource competition among snails drives the temporal dynamics of transmission potential to humans and tested these predictions with experimental epidemics and demonstrated consistency with field observations. This resource-explicit approach predicted an intense pulse of transmission potential when snail populations grow from low densities, i.e., when per capita access to resources is greatest, due to the resource-dependence of cercarial production. The experiment confirmed this prediction, identifying a strong effect of infected host size and the biomass of competitors on per capita cercarial production. A field survey of 109 waterbodies also found that per capita cercarial production decreased as competitor biomass increased. Further quantification of snail densities, sizes, cercarial production, and resources in diverse transmission sites is needed to assess the epidemiological importance of resource competition and support snail-based disruption of schistosome transmission. More broadly, this work illustrates how resource competition can sever the correspondence between infectious host density and transmission potential. 

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