The life cycle, population dynamics and productivity of Melanoides tuberculata (Muller, 1774) (Gastropoda: Prosobranchia: Thiaridae) in Hong Kong

The physical soldier caste of an invasive, human-infecting flatworm is morphologically extreme and obligately sterile

We show that the globally invasive, human-infectious flatworm, Haplorchis pumilio, possesses the most physically specialized soldier caste yet documented in trematodes. Soldiers occur in colonies infecting the first intermediate host, the freshwater snail Melanoides tuberculata, and are readily distinguishable from immature and mature reproductive worms. Soldiers possess a pharynx five times absolutely larger than those of immature and mature reproductives, lack a germinal mass, and have a different developmental trajectory than reproductives, indicating that H. pumilio soldiers constitute a reproductively sterile physical caste. Neither immature nor mature reproductives showed aggression in in vitro trials, but soldiers readily attacked heterospecific trematodes that coinfect their host. Ecologically, we calculate that H. pumilio caused ~94% of the competitive deaths in the guild of trematodes infecting its host snail in its invasive range in southern California. Despite being a dominant competitor, H. pumilio soldiers did not attack conspecifics from other colonies. All prior reports documenting division of labor and a trematode soldier caste have involved soldiers that may be able to metamorphose to the reproductive stage and have been from nonhuman-infectious marine species; this study provides clear evidence for an obligately sterile trematode soldier, while extending the phenomenon of a trematode soldier caste to freshwater and to an invasive species of global public health concern.

The life cycle of a zoonotic parasite reassessed: Experimental infection of Melanoides tuberculata (Mollusca: Thiaridae) with Centrocestus formosanus (Trematoda: Heterophyidae)

Centrocestus formosanus is a foodborne intestinal trematode that is native to Asia and has been introduced into the Americas and Europe. Although there are several studies of C. formosanus in definitive vertebrate hosts (birds and mammals, including humans), and in intermediate vertebrate hosts (fish and amphibians), there is little published information regarding interaction with its transmitting mollusc. In this study we studied the miracidial development of C. formosanus using a mouse as a source of eggs. Adult parasites were maintained in water in order to develop miracidia in intrauterine eggs. Miracidia appeared at 12 days of incubation, with no hatching observed for up to 40 days. Subsequently, we placed dead C. formosanus containing eggs with miracidia individually in contact with 48 specimens of Melanoides tuberculata, and observed the absence of the parasites after 1h of exposure, suggesting that they were ingested by the snails. Of the 33 experimentally-infected snails that were alive after 84–89 days post-infection (DPI), seven (21%) shed cercariae. We detected young C. formosanus rediae in 21/33 (64%) M. tuberculata at 90 DPI. To our knowledge, this report is the first to show that, in the life cycle of C. formosanus, infection of molluscs occurs passively by ingestion of eggs, followed by a long intramolluscan phase. We compare these data with those described for other Heterophyidae, and discuss on the phylogenetic background of the pattern of miracidial development verified in these parasites.

Cost of resistance to trematodes in freshwater snail populations with low clonal diversity

BACKGROUND

The persistence of high genetic variability in natural populations garners considerable interest among ecologists and evolutionary biologists. One proposed hypothesis for the maintenance of high levels of genetic diversity relies on frequency-dependent selection imposed by parasites on host populations (Red Queen hypothesis). A complementary hypothesis suggests that a trade-off between fitness costs associated with tolerance to stress factors and fitness costs associated with resistance to parasites is responsible for the maintenance of host genetic diversity.

RESULTS

The present study investigated whether host resistance to parasites is traded off with tolerance to environmental stress factors (high/low temperatures, high salinity), by comparing populations of the freshwater snail Melanoides tuberculata with low vs. high clonal diversity. Since polyclonal populations were found to be more parasitized than populations with low clonal diversity, we expected them to be tolerant to environmental stress factors. We found that clonal diversity explained most of the variation in snail survival under high temperature, thereby suggesting that tolerance to high temperatures of clonally diverse populations is higher than that of populations with low clonal diversity.

CONCLUSIONS

Our results suggest that resistance to parasites may come at a cost of reduced tolerance to certain environmental stress factors.

Reprint of "An overview of freshwater snails in Asia with main focus on Vietnam"

Freshwater snails have received much attention for their role as intermediate hosts for trematodes causing disease in people and animals such as schistosomiasis and various food-borne trematodes. While effective medical treatment exists for some of these diseases there is need for preventive measures to reduce transmission, e.g. control of intermediate hosts because transmission patterns are often complicated due to presence of reservoir final hosts. In order to implement control measures against the intermediate host snails with minimal impact on the freshwater ecosystems and their biodiversity, a profound knowledge on transmission patterns of the trematodes is required and this is partly related to distribution, habitat preferences, and seasonal variation in density of the intermediate host species. Identification of snail species can be problematic on the basis of morphological and anatomical characters alone as some species show morphological plasticity and similarly morphological differentiation of cercariae found in snails may be difficult and this could lead to biased perceptions of intermediate host spectra and transmission patterns. In this paper, we give an overview of the snail families and their medical and veterinary importance in Asia but with main focus on Vietnam.

An overview of freshwater snails in Asia with main focus on Vietnam

Freshwater snails have received much attention for their role as intermediate hosts for trematodes causing disease in people and animals such as schistosomiasis and various food-borne trematodes. While effective medical treatment exists for some of these diseases there is need for preventive measures to reduce transmission, e.g. control of intermediate hosts because transmission patterns are often complicated due to presence of reservoir final hosts. In order to implement control measures against the intermediate host snails with minimal impact on the freshwater ecosystems and their biodiversity, a profound knowledge on transmission patterns of the trematodes is required and this is partly related to distribution, habitat preferences, and seasonal variation in density of the intermediate host species. Identification of snail species can be problematic on the basis of morphological and anatomical characters alone as some species show morphological plasticity and similarly morphological differentiation of cercariae found in snails may be difficult and this could lead to biased perceptions of intermediate host spectra and transmission patterns. In this paper, we give an overview of the snail families and their medical and veterinary importance in Asia but with main focus on Vietnam.

Clonal diversity driven by parasitism in a freshwater snail

One explanation for the widespread abundance of sexual reproduction is the advantage that genetically diverse sexual lineages have under strong pressure from virulent coevolving parasites. Such parasites are believed to track common asexual host genotypes, resulting in negative frequency-dependent selection that counterbalances the population growth-rate advantage of asexuals in comparison with sexuals. In the face of genetically diverse asexual lineages, this advantage of sexual reproduction might be eroded, and instead sexual populations would be replaced by diverse assemblages of clonal lineages. We investigated whether parasite-mediated selection promotes clonal diversity in 22 natural populations of the freshwater snail Melanoides tuberculata. We found that infection prevalence explains the observed variation in the clonal diversity of M. tuberculata populations, whereas no such relationship was found between infection prevalence and male frequency. Clonal diversity and male frequency were independent of snail population density. Incorporating ecological factors such as presence/absence of fish, habitat geography and habitat type did not improve the predictive power of regression models. Approximately 11% of the clonal snail genotypes were shared among 2-4 populations, creating a web of 17 interconnected populations. Taken together, our study suggests that parasite-mediated selection coupled with host dispersal ecology promotes clonal diversity. This, in return, may erode the advantage of sexual reproduction in M. tuberculata populations.

Distribution of trematodes in snails in ponds at integrated small-scale aquaculture farms

In integrated small-scale aquaculture farming, animal and human excreta maybe used as fish feed and pond fertilizer, thereby enhancing transmission of fish-borne zoonotic trematodes (FZTs) from final hosts, like humans, pigs and chickens, to snails. Areas within a pond could vary in trematode egg-load due to the immediate bordering land, and this might provide implications for control of these trematodes or sampling in field studies measuring FZT prevalence in snails. We therefore estimated the effect of bordering land use on prevalence and FZT burden in snails in different areas within small-scale aquaculture ponds. Nine sampling areas within a pond were assigned in six ponds. For each sampling area, about 120 Melanoides tuberculata snails were collected. Based on land use bordering a sampling area, these were categorized in 5 risk-categories: low-risk (road, rice planted in pond, agriculture, or middle of pond), human access point to pond, livestock sty (pigs or poultry), both human access point and livestock sty, and water connection to canal. In total, 5392 snails were collected. Percentages of snails with parapleurolophocercous cercariae varied between 6% in areas categorized as low-risk and areas with livestock sty only to 15% in areas with both human access point and livestock sty; only this 15% was significantly different from the prevalence in the low-risk category. Percentages of snails with xiphidio cercariae did not differ between risk-categories and varied between 5% and 10%. Mean snail size was 15.2mm, and was significantly associated with both the probability of infection as well as parasite burden. Very small differences in parasite burden were found at different land use areas; the maximum difference was about 11 cercariae. This study demonstrated only small differences between areas surrounding a pond on risk of snails to be infected with fish-borne trematodes within different pond areas. In field studies on FZTs in M. tuberculata snails in ponds, sampling from ponds can therefore be done without considering areas within ponds.

Associations between two trematode parasites, an ectosymbiotic annelid, and Thiara (Tarebia) granifera (Gastropoda) in Jamaica

This work describes associations of Thiara (Tarebia) granifera, its larval trematode community, and Chaetogaster limnaei limnaei at a freshwater reservoir in Jamaica. Larvae of 2 trematodes were present, i.e., a notocotylid (15.3%) and Philophthalmus sp. (1.3%), in 3,575 T. granifera examined. The prevalence of both infections increased with snail shell length (H  =  56, P < 0.01, H  =  23.1, P < 0.01, respectively). Only 3.0% (n  =  595) of infected snails possessed reproductive stages, compared with 90.3% (n  =  2,980) of uninfected snails (χ(2)  =  2,059.8, df  =  1, P < 0.001); both trematodes negatively impacted snail reproduction. Chaetogaster l. limnaei occurred within the mantle cavity of T. granifera with a prevalence of 2.3% (n  =  3,575); intensity ranged from 1 to 6 annelids. Notocotylid larvae occurred in 32.5% (n  =  83) of snails also harboring C. l. limnaei, compared with 14.9% (n  =  3,492) of snails lacking the annelid (χ(2)  =  18.127; P < 0.001). Chaetogaster l. limnaei appears not to influence the recruitment of egg-transmitted, notocotylid infections to snails. Ingestion of emergent cercariae by the annelid was observed; this may impact transmission of the parasite. The article presents the first report of a notocotylid and C. l. limnaei in T. granifera, and of Philophthalmus sp. in Jamaica.

The introduced snail Melanoides Tuberculatus (Muller, 1774) (Mollusca: Thiaridae) in aquatic ecosystems of the Brazilian Semiarid Northeast (Piranhas-Assu River basin, State of Rio Grande do Norte)

Records of the gastropod Melanoides tuberculatus (Müller, 1774), family Thiaridae, in the Piranhas-Assu River basin in Rio Grande do Norte reveal the dispersal of this native Southeast Asian and East African species into aquatic environments of the Brazilian semiarid region, including artificial environments (reservoirs) and lotic systems. The eutrophic conditions of the local waterbodies appear to favor the present situation, where this invasive species reaches extremely high densities, sometimes over 10,000 ind.m-2 as in Armando Ribeiro Gonçalves Reservoir. These observations indicate the immediate need for new studies on the spatial distribution of the species and its potential impact on the biodiversity and water quality of the waterbodies of the semiarid region of the state. Implantation of regular and systematic monitoring of the aquatic resources of the region is urgently required.

First record of the invasive snail Melanoides tuberculatus (Gastropoda: Prosobranchia: Thiaridae) in the Paranã River Basin, GO, Brazil

The Thiarid snail Melanoides tuberculatus (Müller, 1774), native to Asia and East Africa was recorded for the first time in the Paranã River basin, Goiás State. There is no evidence concerning introduction vectors but aquarium releases is the most probable vector. Specimens were collected at three different water bodies after twenty-seven rivers were investigated. The possible spread of this species to other habitats and potential effects on native thermal water communities are discussed.

Differential infectivity of Transversotrema patialense for naive fish

The discovery of Transversotrema patialense cercariae in an aquarium harboring tropical fish triggered the present study. The purpose was to estimate the potential range of infection of local freshwater and fishpond populations by T. patialense. Experimental infection of T. patialense differed among 3 species employed: 76.5% for Tilapia zillii, 37.8% for Gambusia affinis, and none for 2 varieties of Cyprinus carpio. These patterns of infectivity are different from those published previously for T. patialense infections, and they may be attributed to the origin of the fish (temperate water vs. tropical and subtropical waters) and the size of their scales. In G. affinis, but not in the other species, worm load and fish length were closely related, and the left side harbored heavier worm burdens than the right side. Taken together, these results suggest that imported T. patialense may spread to various indigenous natural and cultured fish populations, with ensuing potential harm to these populations.

Ovoviviparity and the structure of the brood pouch inMelanoides tuberculata (Gastropoda: Prosobranchia: Thiaridae)

The freshwater gastropod Melanoides tuberculata broods its young in a pouch located in the anterodorsal region of the head-foot. The wall of the brood pouch is composed of smooth muscle surrounded by connective tissue. The lumen of the brood pouch is incompletely partitioned by trabeculae, formed by extensions or folds in the chamber wall that are composed of smooth muscle, connective tissue, nonciliated squamous epithelial cells, and some storage cells containing lipid and glycogen. The lumen of the chamber also contains a few cells with storage products. The general absence of secretory cells suggests that embryos derive little nutrition from the mother, and therefore embryonic development is probably ovoviviparous. Embryos in various stages of development were found within brood pouches, with later stage embryos varying in size. There was a negative relationship between embryo size and number of embryos in the brood pouch.

Interaction between the intermediate host of Schistosomiasis in Brazil Biomphalaria glabrata (Planorbidae) and a possible competitor Melanoides tuberculata (Thiaridae): I. Laboratory experiments

The biological control of Biomphalaria glabrata, intermediate host of Schistosoma mansoni, is one the accepted options to fight schistosomiasis. One of the most promising candidates to control B. glabrata is the snail Melanoides tuberculata, a potential competitor. However, the mechanisms of interaction between the two species are not clear. Our objective is to determine if M. tuberculata indeed compete with B. glabrata, using two laboratory experiments. In Experiment 1, we tested the effect of the presence of M. tuberculata on the fecundity and mortality rates of B. glabrata. In Experiment 2, we tested if there was a direct or indirect interaction between the two species. In Experiment 1, M. tuberculata was eliminated after the peak in reproductive activity of B. glabrata. In Experiment 2, B. glabrata produced more egg masses when raised with M. tuberculata. The conditions leading to this unexpected positive effect of M. tuberculata on the fecundity of B. glabrata need further clarification, but emphasize that detailed studies of the interaction between these species in the conditions of the local environment should be considered.

Variation of shell shape in the clonal snail Melanoides tuberculata and its consequences for the interpretation of fossil series

Interpreting paleontological data is difficult because the genetic nature of observed morphological variation is generally unknown. Indeed, it is hardly possible to distinguish among several sources of morphological variation including phenotypic plasticity, sexual dimorphism, within-species genetic variation or differences among species. This can be addressed using fossil organisms with recent representatives. The freshwater snail Melanoides tuberculata ranks in this category. A fossil series of this and other species have been studied in the Turkana Basin (Kenya) and is presented as one of the best examples illustrating the punctuated pattern of evolution by the tenants of this theory. Melanoides tuberculata today occupies most of the tropics. We studied variation of shell shape in natural populations of this parthenogenetic snail using Raup's model of shell coiling. We considered different sources of variation on estimates of three relevant parameters of Raup's model: (1) variation in shell shape was detected among clones, and had both genetic and environmental bases; (2) sexual dimorphism, in those clones in which males occur, appeared as an additional source of shell variation; and (3) ecophenotypic variation was detected by comparing samples from different sites and years within two clones. We then tested the performance of discriminant function analyses, a classical tool in paleontological studies, using several datasets. Although the three sources of variation cited above contributed significantly to the observed morphological variance, they could not be detected without a priori knowledge of the biological entities studied. However, it was possible to distinguish between M. tuberculata and a related thiarid species using these analyses. Overall, this suggests that the tools classically used in paleontological studies are poorly efficient when distinguishing between important sources of within-species variation. Our study also gives some empirical bases to the doubts cast on the interpretation of the molluscan series of the Turkana Basin.

Predation of Biomphalaria glabrata during the development of Belostoma anurum (Hemiptera, Belostomatidae)

Belostoma anurum was reared under laboratory conditions. Specimens were exposed to semi-natural conditions of photo period. The mortality rate was 26.3% during the post embryonic period (38.6 +/- 0.7 days). During this time the average predation of Biomphalaria glabrata was of 99.0 +/- 9.4 snails. The mean increment ratio of length and dry weight per instar was of 1.4 +/- 0.1 and 2.8 +/- 0.5, respectively. The predation by B. anurum adults can be divided into two different periods: phase I (4.8 +/- 1.4 snails/day) and phase II (1.8 +/- 0.5 snails/day). The higher predation in phase I suggested the sexual maturation of the belostomatid.