Differential infectivity of Transversotrema patialense for naive fish

Fish ectoparasite detection, collection and curation

Fish parasitology is a dynamic and internationally important discipline with numerous biological, ecological and practical applications. We reviewed optimal fish and parasite sampling methods for key ectoparasite phyla (i.e. Ciliophora, Platyhelminthes, Annelida and Arthropoda) as well as recent advances in molecular detection of ectoparasites in aquatic environments. Ideally, fish capture and anaesthesia as well as parasite recovery methods should be validated to eliminate potential sampling bias and inaccuracy in determining ectoparasite population parameters. There are considerable advantages to working with fresh samples and live parasites, when combined with appropriate fixation methods, as sampling using dead or decaying materials can lead to rapid decomposition of soft-bodied parasites and subsequent challenges for identification. Sampling methods differ between target phyla, and sometimes genera, with optimum techniques largely associated with identification of parasite microhabitat and the method of attachment. International advances in fish parasitology can be achieved through the accession of whole specimens and/or molecular voucher specimens (i.e. hologenophores) in curated collections for further study. This approach is now critical for data quality because of the increased application of environmental DNA (eDNA) for the detection and surveillance of parasites in aquatic environments where the whole organism may be unavailable. Optimal fish parasite sampling methods are emphasised to aid repeatability and reliability of parasitological studies that require accurate biodiversity and impact assessments, as well as precise surveillance and diagnostics.

Transversotrema hafniensis n. sp. infection in Poecilia reticulata by cercariae released from Melanoides tuberculata in Denmark

BACKGROUND

Exotic and ornamental fish are highly popular companion animals resulting in a significant transcontinental trade of fish, invertebrates and aquatic plants. A major issue is the diseases associated with these organisms, as they have a major impact on health of the fish in both public and private household aquaria. A secondary issue is the trade with these products, which potentially may expand the distribution area and spread a range of diseases to new habitats.

RESULTS

We here describe how Poecilia reticulata (guppy), produced in a private household aquarium, were invaded by cercariae of an exotic trematode released by imported Melanoides tuberculata snails. The fish presented with severe clinical signs (tremor, flashing, scraping of body against objects). A standard parasitological examination and morphometric identification showed scale pocket infections with a digenean trematode species within the genus Transversotrema. Molecular identification by PCR, sequencing and phylogenetic analyses of a 2646 bp sequence encoding ribosomal RNA (partial 18 S, ITS1, 5.8 S, ITS2, partial 28 S) was performed. The 1107 bp sequence of mitochondrial DNA (cox1) showed that the parasite differed from previously described Transversotrema species in M. tuberculata. Morphometrics of adult and larval specimens of this isolate also differed from previously described freshwater species within the genus. The new species was described and is named after Copenhagen, for its geographic origin.

CONCLUSIONS

The genus Transversotrema comprises a range of species, adapted to a microhabitat in scalepockets of teleosts. A combination of morphological and molecular characterization techniques has been shown to provide a good differentiation between species. The fish were not purchased from a pet shop but produced in the home aquarium. This indicated that an infection pressure existed in the aquarium, where the source of infection was found to be exotic intermediate host snails M. tuberculata, which originally were imported and purchased from a pet shop. The potential spread of fish diseases associated with trade of fish and snails to new geographic regions, where climate conditions are favourable, is discussed.

Autochthonous transmission of the Indomalayan parasite, Transversotrema patialense, in the Caribbean: Molecular, morphological, and experimental evidence

The Asian freshwater snail Melanoides tuberculata has been established since the 1960s in the Americas, where it transmits cercariae of a small number of digenetic trematode species from its native range. In 2021-2022, 24 M. tuberculata were discovered shedding transversotrematid cercariae in Puerto Rico, where parasites of this snail have not been previously studied. Adult transversotrematids (in some cases, gravid) were found on field-caught fish and on fish exposed to shedding snails, including on fish species native to Puerto Rico. Adults and cercariae were identified as Transversotrema patialense (Soparkar, 1924), a species native to the Indomalayan region. Morphological identification was supported with 28S rDNA sequences closely matching that from unidentified transversotrematid cercariae in Thailand. The absence of T. patialense in snails collected prior to 2021, increasing prevalence of infection in snails collected thereafter, and lack of variation in parasite DNA sequences (28S, internal transcribed spacer 2, cytochrome c oxidase I) from three isolates are consistent with a recently introduced and possibly expanding parasite population. Transversotrema patialense has been recorded outside its native range before, but most studies (including a prior record in the Americas) reported the parasite from captive hosts from commercial sources such as pet shops. The present results thus provide the first demonstration of natural transmission of T. patialense in the Americas. Phylogenetic analysis of 28S but not of ITS2 show the transversotrematid genus Transversotrema Witenberg, 1944 is paraphyletic, with Crusziella Cribb, Bray and Barker1992 nested within it.

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.

First Record of Transversotrema Witenberg, 1944 (Digenea) from the Americas, with Comments on the Taxonomy of Transversotrema patialense (Soparkar, 1924) Crusz and Sathananthan, 1960, and an Updated List of Its Hosts and Geographic Distribution

Specimens of Transversotrema patialense (sensu lato) ( Soparkar, 1924 ) Crusz and Sathananthan, 1960 (Digenea: Transversotrematidae) infected the skin (epidermal spaces beneath scales near pectoral fins) of 4 of 126 (prevalence 3%; mean intensity 1.8) zebrafish ( Danio rerio (Hamilton, 1822) [Cypriniformes: Cyprinidae]) purchased in 2009 and cultured by a California (USA) fish supplier. These fish were sold as "laboratory-reared" and "specific pathogen free," purportedly raised in a recirculating aquaculture system that included zebrafish only. We herein describe the morphological features of this transversotrematid using light and scanning electron microscopy, provide a comprehensive list of hosts (snails and fishes) and geographic locality records for specimens reported as T. patialense, which is perhaps a species complex, and provide a brief historical synopsis of the taxonomic and life history research that has been conducted on this fluke. No species of Transversotrema previously had been reported from the Americas; however, this discovery is not surprising given that: (1) a suitable intermediate host (red-rimmed melania, Melanoides tuberculata (Müller, 1774) [Cerithioidea: Thiaridae]) has been established in California and elsewhere in North America, (2) the zebrafish is a susceptible definitive host, and (3) T. patialense reportedly matures on a broad ecological and phylogenetic spectrum of freshwater fishes. To our knowledge, this is the northern-most geographic locality record for a species of this genus. We suspect this case study represents an example of a parasite that may now be established in North America by the fortuitous co-occurrence of a susceptible, exotic snail host (the red-rimmed melania) and a susceptible, widely distributed, exotic fish host (the zebrafish).

Selective and universal primers for trematode barcoding in freshwater snails

Trematodes are significant pathogens of high medical, veterinary, and environmental importance. They are hard to isolate from their intermediate hosts, and their early life stages are difficult to identify morphologically. Therefore, primers were developed for trematodes to create a species barcoding system and allow selective PCR amplification in mixed samples. The specific oligonucleotide primer was universal for trematodes that infected several freshwater snail species in Israel. The diagnostic tool is based on the 18S rDNA gene. In contrast to morphological identification, trematode barcoding is rapid as it is based on a sequence of only 800 bp, and it classifies species accurately due to high polymorphism between conserved areas.

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.