Molecular phylogeography and reproductive biology of the freshwater snail Tarebia granifera in Thailand and Timor (Cerithioidea, Thiaridae): morphological disparity versus genetic diversity

The freshwater thiarid gastropod Tarebia granifera (Lamarck, 1816), including taxa considered either congeneric or conspecific by earlier authors, is widespread and abundant in various lentic and lotic water bodies in mainland and insular Southeast Asia, with its range extending onto islands in the Indo-West-Pacific. This snail is, as one of the most frequent and major first intermediate host, an important vector for digenic trematodes causing several human diseases. As a typical thiarid T. granifera is viviparous and parthenogenetic, with various embryonic stages up to larger shelled juveniles developing within the female’s subhemocoelic (i.e non-uterine) brood pouch. Despite the known conchological disparity in other thiarids as well as this taxon, in Thailand Tarebia has been reported with the occurrence of one species only. In light of the polytypic variations found in shell morphology of freshwater snails in general and this taxon in particular, the lack of a modern taxonomic-systematic revision, using molecular genetics, has hampered more detailed insights to date, for example, into the locally varying trematode infection rates found in populations of Tarebia from across its range in Thailand as well as neighboring countries and areas. Here, we integrate evidence from phylogeographical analyses based on phenotypic variation (shell morphology, using biometry and geometric morphometrics) with highly informative and heterogeneous mtDNA sequence data (from the gene fragments cytochrome c oxidase subunit 1 and 16 S rRNA). We evaluate both the morphological and molecular genetic variation (using several phylogenetic analyses, including haplotype networks and a dated molecular tree), in correlation with differences in the reproductive biology among populations of Tarebia from various water bodies in the north, northwest, central, and south of Thailand, supplementing our respective analyses of parasite infections of this thiarid by cercaria of 15 trematode species, reported in a parallel study. Based on the comparison of topotypical material from the island of Timor, with specimens from 12 locations as reference, we found significant, albeit not congruent variation of both phenotype and genotype in Tarebia granifera, based on 1,154 specimens from 95 Thai samples, representing a geographically wide-ranging, river-based cross-section of this country. Our analyses indicate the existence of two genetically distinct clades and hint at possible species differentiation within what has been traditionally considered as T. granifera. These two lineages started to split about 5 mya, possibly related to marine transgressions forming what became known as biogeographical barrier north of the Isthmus of Kra. Grounded on the site-by-site analysis of individual Tarebia populations, our country-wide chorological approach focussing on the conchologically distinct and genetically diverse lineages of Tarebia allows to discuss questions of this either reflecting subspecific forms versus being distinct species within a narrowly delimited species complex. Our results, therefore, provide the ground for new perspectives on the phylogeography, evolution and parasitology of Thai freshwater gastropods, exemplified here by these highly important thiarids

Trematodes obtained from the thiarid freshwater snail Melanoides tuberculata (Müller, 1774) as vector of human infections in Thailand

Larval stages of trematodes obtained from the freshwater snail Melanoides tuberculata (Cerithioidea, Thiaridae) as intermediate host were studied by using cercarial emergence and crushing snails. Between December 2004 and September 2009 snails from one hundred twenty locations in Thailand were collected every two months for one year at each sampling site. Counts per unit of time method was used in this study, and the samples of snails were collected every 10 minutes per sampling by five collectors. The cercarial stages were examined using shedding and crushing methods. The infection rate was found to be 18.79%, i.e. 6,019 animals infected in a total of 32,026. Nine different types in eighteen species of cercariae were categorized, viz. are (1) Parapleurophocercous cercariae: Haplorchis pumilio, Haplorchis taichui, and Stictodora tridactyla; (2) Pleurophocercous cercariae: Centrocestus formosanus; (3) Xiphidiocercariae: Acanthatrium hitaense, Loxogenoides bicolor, and Haematoloechus similis; (4) Megalurous cercariae: Cloacitrema philippinum and Philophthalmus sp.; (5) Furcocercous cercariae: Cardicola alseae, Alaria mustelae, Transversotrema laruei, Apatemon gracilis, and Mesostephanus appendiculatus; (6) Echinostome cercariae: Echinochasmus pelecani; (7) Amphistome cercariae: Gastrothylax crumenifer; (8) Renicolid cercariae: Cercaria caribbea LXVIII; and, (9) Cotylomicrocercous cercariae: Podocotyle (Podocotyle) lepomis

Trematodes obtained from the thiarid freshwater snail Melanoides tuberculata (Müller, 1774) as vector of human infections in Thailand

Larval stages of trematodes obtained from the freshwater snail Melanoides tuberculata (Cerithioidea, Thiaridae) as intermediate host were studied by using cercarial emergence and crushing snails. Between December 2004 and September 2009 snails from one hundred twenty locations in Thailand were collected every two months for one year at each sampling site. Counts per unit of time method was used in this study, and the samples of snails were collected every 10 minutes per sampling by five collectors. The cercarial stages were examined using shedding and crushing methods. The infection rate was found to be 18.79%, i.e. 6,019 animals infected in a total of 32,026. Nine different types in eighteen species of cercariae were categorized, viz. are (1) Parapleurophocercous cercariae: Haplorchis pumilio, Haplorchis taichui, and Stictodora tridactyla; (2) Pleurophocercous cercariae: Centrocestus formosanus; (3) Xiphidiocercariae: Acanthatrium hitaense, Loxogenoides bicolor, and Haematoloechus similis; (4) Megalurous cercariae: Cloacitrema philippinum and Philophthalmus sp.; (5) Furcocercous cercariae: Cardicola alseae, Alaria mustelae, Transversotrema laruei, Apatemon gracilis, and Mesostephanus appendiculatus; (6) Echinostome cercariae: Echinochasmus pelecani; (7) Amphistome cercariae: Gastrothylax crumenifer; (8) Renicolid cercariae: Cercaria caribbea LXVIII; and, (9) Cotylomicrocercous cercariae: Podocotyle (Podocotyle) lepomis

Exploring the evolutionary potential of parasites: Larval stages of pathogen digenic trematodes in their thiarid snail host Tarebia granifera in Thailand

Minute intestinal flukes from several distinct families of endoparasitic platyhelminths are a medically important group of foodborne trematodes prevalent throughout Southeast Asia and Australasia. Their lifecycle is complex, with freshwater snails as primary intermediate hosts, with infecting multiple species of arthropods and fish as second intermediate hosts, and with birds and mammals including humans as definitive hosts. In Southeast Asian countries, the diversity of snail species of the Thiaridae which are frequently parasitized by trematode species is extremely high. Here, the thiarid Tarebia granifera in Thailand was studied for variation of trematode infections, by collecting the snails every two months for one year from each locality during the years 2004–2009, and during 2014–2016 when snails from the same localities were collected and new localities found. From ninety locations a total of 15,076 T. granifera were collected and examined for trematode infections. With 1,577 infected snails the infection rate was found to be 10.46 %. The cercariae were categorized into fifteen species from eight morphologically distinguishable types representing several distinct families, viz. (i) virgulate xiphidiocercariae (Loxogenoides bicolor, Loxogenes liberum and Acanthatrium histaense), (ii) armatae xiphidiocercariae cercariae (Maritreminoides caridinae and M. obstipus); (iii) parapleurophocercous cercariae (Haplorchis pumilio, H. taichui and Stictodora tridactyla); (iv) pleurophocercous cercariae (Centrocestus formosanus); (v) megarulous cercariae (Philophthalmus gralli); (vi) furcocercous cercariae (Cardicola alseae, Alaria mustelae and Transversotrema laruei); as well as (vii) echinostome-type cercariae, and (viii) gymnocephalous-type cercariae. In addition, a phylogenetic marker (internal transcribed spacers 2, ITS2) was employed in generic and infrageneric level classifications of these trematodes, using sequences obtained from shed cercariae isolated from T. granifera specimens of the second study period collected in various regions in Thailand. We obtained ITS2 sequences of cercariae from nine species (of seven types): Loxogenoides bicolor, Loxogenes liberum, Maritreminoides obstipus, Haplorchis taichui, Stictodora tridactyla, Centrocestus formosanus, Philophthalmus gralli, as well as from one species each of echinostome cercariae and gymnocephalous cercariae. Thus, this analysis combines the parasites’ data on morphology and geographical occurrence with molecular phylogeny, aiming to provide the groundwork for future studies looking into more details of the parasite-snail evolutionary relationships

Survey of Stenomelania Fisher, 1885 (Cerithioidea, Thiaridae): The potential of trematode infections in a newly-recorded snail genus at the coast of Andaman Sea, South Thailand

Stenomelania snails (Fisher 1885) have been reported from the coastal regions of the Indian Ocean and the Pacific Ocean, spanning India to Australia. Here, the species diversity and distribution of these snails in the south of Thailand are recorded. The snails were also examined for trematode infections in 13 locations in three Provinces, viz. Krabi, Trang and Satun, along the coast of the Andaman Sea. A total of 1,551 snails were in five morphs tentatively identified as Stenomelania aspirans, S. crenulata, S. punctata, S. torulosa and the closely-related Neoradina prasongi. With 10 infected snails, the trematode infection rate was 0.64%. The cercariae were categorised into three species from two morphologically-distinguishable types, viz. parapleurolophocercous cercariae (Haplorchis taichui and Procerovum cheni) and xiphidiocercariae (Loxogenoides bicolor), through the morphological characterisation of the larval stage. These trematodes were also analysed using the internal transcribed spacer subunit II region to confirm the species identity at generic and infrageneric levels

Cercarial dermatitis outbreak caused by ruminant parasite with intermediate snail host: schistosome in Chana, South Thailand

A cercarial dermatitis outbreak occurred in Chana district, Songkhla Province, South Thailand, between August and October 2020. A total of 359 cases with cercarial dermatitis were confirmed with three cases of skin biopsy. The species of potential trematodes from infected snails were investigated, and the prevalence of infestation with schistosomes was described. As part of our ongoing studies of trematode diversity in freshwater systems, using morphological traits and sequence data to differentiate species, this study aimed to provide insights into the parasite species that cause cercarial dermatitis in the outbreak area and improve our understanding of parasite species distribution. Snail samples were collected in December 2020 and September and October 2021. Five main areas of outbreaks were investigated, and snails were collected by scooping and examined for infection with cercariae. The cercariae were characterized on the basis of morphological features. We found two species of snails to be infected, viz. Indoplanorbis exustus and Bithynia siamensis siamensis, with infection rates of 2.05% (12/586) and 7.93% (23/290), respectively. Three species of trematodes were found in B. s. siamensis, viz. Gastrothylax crumenifer, Astiotrema monticellii, and Loxogenes liberum. Moreover, three species of trematodes were found in Indoplanorbis exustus, viz. Clinostomum giganticum, Echinostoma spiniferum, and Schistosoma indicum. The latter is a ruminant schistosome, which causes the outbreak of cercarial dermatitis in the area. They were genetically analyzed using the internal transcribed spacer subunit II region to confirm the species identity at generic and infrageneric levels