1. IntroductionSnails are invertebrate animals of the class Gastropoda and are widely distributed in aquatic ecosystems around the world. Approximately 5000 species have been identified in freshwater habitats such as lakes, rivers, streams, ponds, and dams [1,2]. Among these, some freshwater snails have medical and veterinary health importance, serving as vectors of parasitic diseases. Snail-borne diseases are major parasitic diseases that remain important public health issues worldwide, particularly in impoverished countries [3]. Schistosomiasis is an endemic parasitic disease affecting almost 240 million people worldwide, and an additional 700 million people are at risk of infection [4]. Six species of the blood fluke are reported to infect humans, causing schistosomiasis; among these, Schistosoma haematobium, Schistosoma mansoni, and Schistosoma japonicum are the main pathogenic species. Schistosoma eggs are the main pathogenic factors of schistosomiasis; parasitizing on host tissues, they cause the host to develop immunopathological reactions, which lead to the occurrence of urinary and reproductive system inflammation (Schistosoma haematobium) and obstructive diseases or intestinal diseases, liver and spleen inflammation, and liver fibrosis (Schistosoma mansoni and Schistosoma japonicum) [5]. In China, schistosomiasis japonicum is still prevalent in Hubei, Hunan, Jiangxi, Anhui, Jiangsu, Sichuan, and Yunnan provinces, posing a great threat to social and economic development [6]. Angiostrongyliasis cantonensis is another parasitic disease endemic in many areas, including Southeast Asia, the Pacific Islands, parts of South and Central America, and the Caribbean [7,8]. It is a serious disease with eosinophilic encephalitis and meningoencephalitis as the main clinical manifestations [7,9]. By 2012, more than 3000 cases of Angiostrongyliasis cantonensis had been recorded in nearly 300 countries and regions, of which the main outbreaks occurred in endemic areas, especially in China [10]. For example, 160 cases occurred in 2006 in Beijing, six cases occurred in 2007 in Guangdong, and 35 cases occurred from 2007 to 2008 in Yunnan; these intensive infections have aroused great attention among the public [11,12,13].The distribution of snail-borne diseases largely depends on the spatial distribution of intermediate hosts [14]. It has been proven that snail-borne parasitic disease is endemic in areas where intermediate host snails are identified, while it does not occur in areas without host snails, although imported parasitic disease cases have been detected [15]. Snail distribution and abundance generally depend on various environmental factors, including physical factors such as temperature, precipitation, aquatic macrophyte cover, hydrography, and substrate composition; chemical factors such as pH, electrical conductivity, five-day biochemical oxygen demand (BOD5), chemical oxygen demand, total nitrogen, and total phosphorus; and biological factors such as food, competition, and predator–prey interactions [16,17,18]. However, the relative importance of environmental factors varies considerably in different regions due to the environmental heterogeneity [19], indicating that local surveys are needed to determine the preferred habitats of snail hosts.A better understanding of the environmental factors affecting the distribution and habitat preferences of snail intermediate hosts is crucial for the effective control and elimination of snail-borne diseases. In Shenzhen and adjacent areas, a few studies have been conducted on the biology of several freshwater snails [20,21,22,23,24,25]. However, the sample sites are very scattered, and the surveyed snails are mainly Biomphalaria straminea and Pomacea canaliculata. Little is known about the distribution of the snails and the main factors affecting the snail abundance in the region, which is unfavorable for promoting comprehensive prevention and control measures for snail-borne diseases. Therefore, in this study, we aimed to (i) identify the local distribution and diversity of freshwater snail intermediate hosts of parasites, and (ii) to identify the biotic and abiotic factors that affect the occurrence and abundance of these snails in Southern China. The findings of this study could be helpful for priority habitat identification and to obtain targets for the prevention and control of snail-borne diseases in this area. 4. DiscussionA fundamental understanding of the ecology of snail intermediate hosts is essential to plan and implement effective snail-borne disease control strategies [71]. In our study, we used a decision tree model to identify the most important environmental variables affecting snail distribution in the rivers of Shenzhen and adjacent areas in China. The kappa (K) values show that the models had fair to moderate predictive performance, indicating that certain snail species have clear environmental requirements within the studied habitat gradient. The results show that the occurrence of predators and competitors, canopy cover, chlorophyll-a, pollution, substrates, water depth, and velocity are the main variables by which to determine the abundance and distribution of snail intermediate hosts of parasites. Moreover, the canonical correspondence analysis (CCA) obtained similar results.Our results indicate that biological factors such as predators and competitors should be given priority in terms of snail occurrence and abundance; these may inhibit snail populations through predation and competition [49]. Younes et al. [44] pointed out that the density of snails is related to the predation activities of their predators. Several studies have suggested that benthic invertebrates belonging to the orders Coleoptera, Diptera, Odonata, Hirudata, and Hemiptera play a role in significantly reducing populations and could be considered in snail control strategies [44,46]. During the snail sampling, we noted large numbers of fish around some sample sites; however, their species, quantity, and size have not been surveyed in detail. Many studies [72,73,74,75,76] have suggested that fish predators can dramatically alter the population dynamics and diversity of snail species; this should be given more attention in future research.Aquatic macrophyte cover was another important factor affecting the distribution of freshwater snails [77,78]. It was found that macrophyte coverage had an important influence on snail occurrence and abundance in our study. Abundant macrophytes could provide sufficient food and spawning sites for snails. Many snails were omnivorous species that could feed on large numbers of aquatic plants, and their growth rate is related to the abundance of plants on which they feed [79]. Moreover, macrophytes could provide a refuge for snails to avoid predation by fish and other large animals, as well as the adverse effects of the current and wind [80]. In addition, macrophytes produce dissolved oxygen through photosynthesis and could create better habitat conditions for aquatic macroinvertebrates [81,82].Chlorophyll-a is an important index of phytoplankton biomass, and its content could reflect the nutritional status of the water body, which is a key parameter for water environment research [83]. We observed that the concentration of chlorophyll-a was between 0.52 and 31.43 μg/L, with an average value of 6.16 μg/L. Phytoplankton play an important role in snails’ diets. Our study shows that chlorophyll-a in water has an important impact on snail occurrence and abundance. A high concentration of chlorophyll-a indicated high phytoplankton content in the water column, which could provide sufficient food for the growth and development of snails. For some snail species, such as B. straminea and P. acuta, these snails were present in large numbers at suitable concentrations of chlorophyll-a, even in the absence of macrophyte.As shown in Figure 7, the occurrence of most snail species is strongly correlated with human disturbance factors such as sewage discharge. Shenzhen and its neighboring areas are economically developed and densely populated [84], with a huge amount of daily sewage discharge, and some sewage may be directly discharged into rivers without treatment [85]. In addition, the reclaimed water treated by wastewater treatment plants is still high in nutrient content. Human disturbance, especially the high concentration of organic matter and dissolved ions in sewage discharge, provides abundant nutrients for phytoplankton and algae, which increases the content of chlorophyll-a in the water and provides sufficient food for snails. Moreover, the ions in wastewater discharge, such as calcium ions, are also a key component in snail shell growth and development [86]. These factors contribute to the presence of snail species in wastewater discharge, which has been confirmed by previous reports [18,87,88]. Biomphalaria spp., belonging to Pulmonata, are better adapted to harsher environmental conditions because they can absorb atmospheric air through a vascularized mantle cavity [30]. Since most other freshwater vertebrate and invertebrate fauna cannot cope with low oxygen levels, air-breathing snails are released from competition and predation pressures in hypoxic habitats, which increases their probability of occurrence and abundance [49]. A study by Pinto et al. [89] showed that the discharge of untreated sewage has brought about algal blooms and aquatic macrophyte proliferation in the Pampulha reservoir in Brazil, which contributes to the establishment of high densities of snails in the water body.In our study, it was found that the river substrate type was one of the key factors affecting the species and abundance of snails. In the investigated rivers, the species and number of snails were the largest in mixed pebbles and gravels. This may have been due to the heterogeneity of the riverbed substrate, which generated the diversity of the spatial distribution of water flow and nutrients and contributed to a plentiful habitat environment [90], thus increasing the occurrence of freshwater snails. In addition, our survey also found that there were fewer snails in sandy environments. This was mainly because, under the same water flow conditions, sand was more easily disturbed than gravel, pebbles, and silt, and had poor stability, which causes severe disturbance to the living environments of benthic invertebrates [91]. Our findings are consistent with the observations of Jowett [92], who found that benthic macroinvertebrate assemblages are greatly dependent on the streambed stability at the reach scale.Water depth and velocity are also key variables as determinants of snail occurrence. Beisel et al. [93] pointed out that, in addition to the substrate, the water depth and velocity are relatively more important physical factors affecting the community structure of benthic invertebrates. Our observations indicate that almost all the snails in this area prefer to live in shallow water, and it was difficult to find traces of them if the depth was over 40 cm. The shallow water was generally rich in aquatic plants, phytoplankton, and organic matter, which provide abundant food for freshwater snails. However, Mandahl-Barth [94] demonstrated that Biomphalaria smithi could be found at a depth of 4.3 m in Lake Edward, Uganda, and Biomphalaria choanomphala at 12.2 m in Lake Victoria, Uganda. Freitas [95] observed Biomphalaria glabrata survival at a depth of 4~5 m at the bottom of the Lagoa Santa in Brazil. In our study, it was difficult to determine whether the water depth affected the snail distribution and to what extent, although they appear to prefer shallow waters [18]. Thus, in future studies, further efforts should be devoted to determining the quantitative relationship between the distribution of snails and water depth in Shenzhen and adjacent areas to better understand the snail ecology. Snails often inhabit marshes, puddles, canals, ponds, and rivers with slowly running or stagnant water. Fast-running water appears to hinder their predation and the establishment of breeding colonies of snails [96]. Moreover, an excessive current flow would directly flush away the snails and reduce the abundance of the snail species; these phenomena were commonly observed during the investigation in the wet season.In this study, snails tended to occur at high frequency and were abundant in water bodies with high human activity, such as sewage discharge. The high concentration of organic matter and ions in these polluted waters provided favorable conditions for snail growth and propagation. Our observations also indicated snails with fewer species and smaller numbers in clean water, which were less affected by anthropogenic disturbances. These water bodies host various predators and competitors of invertebrates, such as Coleoptera, Odonata, Hirudinae, and Hemiptera, whose presence significantly inhibits snail density [44]. Several studies have shown that these invertebrate assemblages are responsible for a significant reduction in snail populations that could be considered in integrated snail control measures [46]. Therefore, comprehensive snail control strategies should give priority to reducing the occurrence and abundance of hosts among freshwater snails in order to control the spread of snail-borne diseases at the local scale. This suggests that the proper management of water bodies to reduce water pollution may be one of the most suitable strategies for the comprehensive control of snail-borne diseases in Shenzhen and adjacent areas.