The state of fish parasite discovery and taxonomy: a critical assessment and a look forward

Biases in parasite biodiversity research: why some helminth species attract more research than others

As the number of known and described parasite species grows every year, one might ask: how much do we actually know about these species beyond the fact they exist? For free-living taxa, research effort is biased toward a small subset of species based on their properties or human-centric factors. Here, using a large data set on over 2500 helminth parasite species described in the past two decades, we test the importance of several predictors on two measures of research effort: the number of times a species description is cited following its publication, and the number of times a species' name is mentioned in the scientific literature. Our analysis highlights some taxonomic biases: for instance, descriptions of acanthocephalans and nematodes tend to receive more citations than those of other helminths, and species of cestodes are less frequently mentioned in the literature than other helminths. We also found that helminths infecting host species of conservation concern receive less research attention, perhaps because of the constraints associated with research on threatened animals, while those infecting host species of human use receive greater research effort. Intriguingly, we found that species originally described by many co-authors subsequently attract more research effort than those described by one or few authors, and that research effort correlates negatively with the human population size of the country where a species was discovered, but not with its economic strength, measured by its gross domestic product. Overall, our findings reveal that we have conducted very little research, or none at all, on the majority of helminth parasite species following their discovery. The biases in study effort we identify have serious implications for future research into parasite biodiversity and conservation.

Review of the metazoan parasites of the economically and ecologically important African sharptooth catfish Clarias gariepinus in Africa: Current status and novel records

One of the most widely distributed African freshwater fish is the African sharptooth catfish Clarias gariepinus (Burchell) that is naturally distributed in 8 of the 10 ichthyofaunal regions of this continent. Clarias gariepinus is a highly valued and cheap staple to local communities and an ideal aquaculture species. Consequently, interest in the parasitic communities of C. gariepinus has increased as parasites may accidentally be ingested by humans when eating uncooked fish or can be introduced into culture systems through fish stocks supplied from local rivers which affect yield, growth, and marketability. This review provides an overview of the ∼107 metazoan parasite species known to parasitise C. gariepinus in Africa and their general life cycles, morphology, paratenic and post-cyclic infections, and the biogeography and validity of records are discussed. A brief overview is included on the application of some of these parasites in environmental studies and their link to human health.

How to use natural history collections to resurrect information on historical parasite abundances

Many of the most contentious questions that concern the ecology of helminths could be resolved with data on helminth abundance over the past few decades or centuries, but unfortunately these data are rare. A new sub-discipline - the historical ecology of parasitism - is resurrecting long-term data on the abundance of parasites, an advancement facilitated by the use of biological natural history collections. Because the world's museums hold billions of suitable specimens collected over more than a century, these potential parasitological datasets are broad in scope and finely resolved in taxonomic, temporal and spatial dimensions. Here, we set out best practices for the extraction of parasitological information from natural history collections, including how to conceive of a project, how to select specimens, how to engage curators and receive permission for proposed projects, standard operating protocols for dissections and how to manage data. Our hope is that other helminthologists will use this paper as a reference to expand their own research programmes along the dimension of time.

Endoparasite fauna of freshwater fish from the upper Juruá River in the Western Amazon, Brazil

The Amazon region may present a high diversity of endoparasites with a high degree of endemism. In this sense, this study describes the endoparasite fauna in freshwater fish from the Upper Juruá, in the Western Amazon. The study was carried out around the municipalities of Cruzeiro do Sul, state of Acre, and Guajará, state of Amazonas, Brazil. Fish were caught between periods of droughts and floods, using passive and active sampling methods. In the laboratory, specimens were biometrically analysed and necropsied. As a result, a total of 23,740 endoparasites were recorded, belonging to 62 species, with 91 new host reports and 91 new occurrences for the Western Amazon. Nematoda and Digenea were the most diverse and abundant groups, and the increase in host fish richness and diversity influenced the diversity and richness of endoparasites in the environments. In this sense, the present study expands the number of new reports, and contributes data on the distribution and richness of endoparasites for South America.

Enenterum kyphosi Yamaguti, 1970 and Enenterum petrae n. sp. (Digenea: Enenteridae) from kyphosid fishes (Centrarchiformes: Kyphosidae) collected in marine waters off eastern Australia

Species of the digenean genus Enenterum Linton, 1910 (Lepocreadioidea: Enenteridae) are characterised primarily by their elaborate oral suckers, which are divided into varying numbers of anteriorly directed lobes, and their host-restriction to herbivorous marine fishes of the family Kyphosidae. We describe Enenterum petrae n. sp. from the brassy chub Kyphosus vaigiensis (Quoy & Gaimard) collected off Lizard Island, Great Barrier Reef, Queensland, Australia. Enenterum petrae n. sp. is readily differentiated from congeners by its unique oral sucker morphology, in having a minute pharynx, and the combination of a genital cap and accessory sucker. We also provide the first record of Enenterum kyphosi Yamaguti, 1970 from Australia based on material obtained from the blue sea chub Kyphosus cinerascens (Forsskål) collected off Lizard Island and North Stradbroke Island, Queensland. Morphologically, our specimens of E. kyphosi agree closely with descriptions of this species from Hawaii and South Africa, and despite lack of molecular data from outside of Australian waters, we consider all three reports to represent a single, widespread species. The first ITS2 and COI mtDNA gene sequences for species of Enenterum are provided and molecular phylogenetic analyses of 28S rDNA gene sequences place these species in a strongly-supported clade with the type-species of the genus, Enenterum aureum Linton, 1910. The oral suckers of both E. kyphosi and E. petrae n. sp. can be interpreted as having varying numbers of lobes depending on the particular specimen and how the division between lobes is defined. Scanning electron microscopical images improves understanding of the morphology of the enenterid oral sucker, and permits speculation regarding the evolutionary history leading to its specialisation in this lineage.

Somewhere I belong: phylogeny and morphological evolution in a species-rich lineage of ectoparasitic flatworms infecting cichlid fishes

A substantial portion of biodiversity has evolved through adaptive radiation. However, the effects of explosive speciation on species interactions remain poorly understood. Metazoan parasites infecting radiating host lineages could improve our knowledge because of their intimate host relationships. Yet limited molecular, phenotypic and ecological data discourage multivariate analyses of evolutionary patterns and encourage the use of discrete characters. Here, we assemble new molecular, morphological and host range data widely inferred from a species-rich lineage of parasites (Cichlidogyrus, Platyhelminthes: Monogenea) infecting cichlid fishes to address data scarcity. We infer a multimarker (28S/18S rDNA, ITS1, COI mtDNA) phylogeny of 58 of 137 species and characterize major lineages through synapomorphies inferred from mapping morphological characters. We predict the phylogenetic position of species without DNA data through shared character states, a morphological phylogenetic analysis, and a classification analysis with support vector machines. Based on these predictions and a cluster analysis, we assess the systematic informativeness of continuous characters, search for continuous equivalents for discrete characters, and suggest new characters for morphological traits not analysed to date. We also model the attachment/reproductive organ and host range evolution using the data for 136 of 137 described species and multivariate phylogenetic comparative methods (PCMs). We show that discrete characters not only can mask phylogenetic signals, but also are key for characterizing species groups. Regarding the attachment organ morphology, a divergent evolutionary regime for at least one lineage was detected and a limited morphological variation indicates host and environmental parameters affecting its evolution. However, moderate success in predicting phylogenetic positions, and a low systematic informativeness and high multicollinearity of morphological characters call for a revaluation of characters included in species characterizations.

A novel Ceratomyxa species (Myxozoa: Cnidaria) infecting an Amazonian catfish

Parasites are important organisms for the health of ecosystems. While the Amazon Basin is home to a great diversity of ichthyofauna, our knowledge of myxozoan diversity in the biome remains relatively limited. The present study describes a new myxozoan species, Ceratomyxa mandii n. sp., parasitizing the gallbladder of the Amazonian catfish Pimelodina flavipinnis (Pimelodidae) from the Solimões River, in the region of Manaus, Brazil. Light and electron microscopy,  small subunit ribosomal DNA (SSU rDNA) sequencing and phylogenetic analysis were performed. The new species exhibited worm-like plasmodia with undulatory motility. The SSU rDNA based phylogenetic analysis revealed it to be a sister taxon of C. gracillima, which also parasitizes an Amazonian pimelodid fish, possibly reflecting a host-parasite co-speciation process. This study contributes to our understanding of this little sampled group of organisms.

Filling the knowledge gap of Middle American freshwater fish parasite biodiversity: metazoan parasite fauna of Nicaragua

The heterogeneous landscape of Nicaragua harbours a large diversity of freshwater fishes. The great Nicaraguan lakes, Managua and Nicaragua, and several adjacent crater lakes harbour numerous endemic fish species. However, information about their parasite fauna is still fragmentary. Here, we surveyed the great Nicaraguan lakes and four crater lakes and provide data for 17 metazoan parasite taxa infecting seven fish host species. We also gathered all the published records from the literature on the parasites reported from Nicaraguan freshwater fishes, as well as those for Costa Rica and Panama to discuss the region of Lower Central America as a whole. With this information we built a parasite-host and a host-parasite checklist. With data from near 50% of the native and endemic freshwater fishes in Nicaragua, the parasite fauna comprises 101 taxa in 51 fish species allocated in 11 families. Cichlids are the most diverse group of fishes in this region and have been the most extensively surveyed for their metazoan parasites. Helminths are the best-represented groups of metazoan parasites, with 42 trematodes, five cestodes, 24 monogeneans, two acanthocephalans, 20 nematodes and one hirudinean. Additionally, freshwater fishes are parasitized by copepods, branchiurans and oribatid mites. Even though the inventory is not yet complete, the patterns of diversity uncovered revealed promising information about the origin, biogeography and evolutionary history of the Nicaraguan freshwater fish parasite fauna. More studies are necessary to complete our knowledge about the diversity, host association and distribution of metazoan parasites in Nicaragua and other Central American countries.

Evidence for enemy release in invasive common dace Leuciscus leuciscus in Ireland: a helminth community survey and systematic review

Invasive species lose parasites in the process of invasion and tend to be less parasitized than conspecifics in the native range and sympatric native species in the invasive range (enemy release). We evaluated enemy release in an invasive freshwater fish in Ireland, common dace Leuciscus leuciscus, using helminth parasite community surveys at the core and front of the invasive range of common dace. Furthermore, we undertook a systematic literature review of helminth infection in common dace across its native range in Great Britain and Europe and invasive range in Ireland. The helminth parasite community survey revealed that invasive common dace were infected with fewer helminth species at the invasion front than at the core. Four helminth taxa - Acanthocephala, Monogenea, Digenea and Nematoda - were present in dace at the invasion core compared to only a single helminth species (Pomphorhynchus tereticollis) at the front. The systematic review revealed that invasive common dace in Ireland hosted fewer species of helminths than common dace in the native range. We report a total of three helminth species in common dace in Ireland compared to 24 in Great Britain and 84 in Continental Europe. Our results support the hypotheses that invasive populations are less parasitized than native populations and that more recently established populations host fewer parasites. However, we demonstrate that invasive species may continue to experience release from parasites long after initial invasion.