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The documented range of the invasive and potentiallypathogenicAsianfish tapeworm,Bothriocephalus acheilognathi Yamaguti, 1934 in the United States and Canada is updated based on examination of museum depo- sitions and original field collections. Gravid specimens of B. acheilognathi were collected from the fathead minnow Pimephales promelas Rafinesque in Peter Lake, at the University of Notre Dame Environmental Research Center (UNDERC) Land o' Lakes, Wisconsin. A single immature specimen of the parasite was collected from a white bass, Morone chrysops (Rafinesque) in Lake Winnipeg, Manitoba, Canada. This is the first record of B. acheilognathi in Canada and extends its northern range in the interior of the continent by more than 600 miles over the last documented record. The previous record of B. acheilognathi in Canada, from the northern pikeminnow, Ptychocheilus oregonensis in British Columbia, is a misidentification of Eubothrium tulipai. Examination of selected records of intestinal cestodes from native cyprinids, in the Harold W. Manter Laboratory of Parasitology (HWML, n ¼ 9) collection and in the United States National Parasite Collection (USNPC, n¼8), provided evidenceof theparasite in Nebraska and possiblyin the upper Colorado River basin. Introductions into Wisconsin- Michigan were due to the stocking of goldenshiners,whereas the source of the introduction in Manitoba remains unknown.

Aim To uncover and describe patterns of biogeography of helminth parasites in freshwater fishes of Mexico, and to understand processes that determine them. Three predictions about host-specificity, faunal exchange in transitional areas, and the biogeographical ‘core’ fauna, are evaluated, all of which follow from a fundamental hypothesis: that parasites show characteristic associations with particular host clades. The parasite fauna of the southern Mexican cichlids and of the fishes of the Mesa Central are examined as case studies that reflect Neotropical and Nearctic historical influences. Location The region covered in this study includes most of Mexico, with emphasis on six biogeographical areas: the Yucatan Peninsula (area 1), the Grijalva-Usumacinta drainage (area 2), the Papaloapan and Panuco drainages (area 3), the Balsas drainage (area 4), the Lerma-Santiago drainage (area 5), and the Bravo drainage (area 6). Methods A parasite data base containing all the records of helminth parasites of freshwater fishes of Mexico was filtered to extract records of adult helminth parasites in freshwater fishes from the six biogeographical areas designated in this study. Jaccard's similarity coefficients and cluster analyses (using upgma) were used to analyse the extent of faunal similarity between the designated biogeographical areas and between host (fish) families. Taxonomic composition of parasite assemblages in different host groups was also qualitatively compared from summary data. These data were used to test the three main predictions. Results To date, 184 species of helminths (120 as adults) have been recorded from 127 freshwater fishes in Mexico (almost 33% of the total fish diversity of Mexico). Of these parasite species, 69 are digenetic flukes, 51 are nematodes, 33 are monogeneans, 25 are tapeworms, and only six are acanthocephalans. The data and analyses from the six biogeographical areas corroborate the predictions that: (1) the adult parasite fauna is largely circumscribed by higher levels of monophyletic host taxa (families, orders, etc.), and that this pattern is independent of areas; (2) areas within a certain biogeographical region, and consequently with similar fish composition (e.g. areas 1, 2 and 3) have more similar parasite faunas compared to areas with less similar fish faunal composition; and (3) ‘core’ parasite faunas persist to some extent in transitional areas with limited host-sharing. Main conclusions Helminth biodiversity in Mexican freshwater fishes is determined by the historical and contemporary biogeography of their hosts. Host lineage specificity, mainly at the level of the host family, appears to be an important factor in the distribution of the parasites. Most fish families (Characidae, Cichlidae, Pimelodidae, Ictaluridae, Catsotomidae, Goodeidae, Atherinidae) possess their own characteristic ‘core’ helminth fauna, with limited host-sharing in transitional areas (e.g. areas 3 and 4). A re-evaluation of the helminth fauna of Mexican cichlids questions the hypothesis that cichlids lost parasites during the colonization of Mexico from South America. The evidence supports the idea that they acquired new parasites by host switching, possibly from marine or brackish-water percomorphs. In contrast, the parasite fauna of the Mesa Central remains enigmatic and reflects the region's history of endemicity with historical marine and Nearctic connections.

Partial sequences of the 28S rDNA (ribosomal gene) were obtained from a total of 11 populations of 5 species (in 3 genera) of North American corallobothriine proteocephalideans. These included Corallobothrium fimbriatum (3 populations), Corallobothrium parafimbriatum (1 population), Corallotaenia minutia (1 population), Megathylacoides giganteum (2 populations), and Megathylacoides lamothei (4 populations). These sequences were used in a phylogenetic analysis to test the monophyly of Corallobothriinae and to investigate the interrelationships of the North American species. The results indicate that Corallobothriinae, as conventionally understood, is not monophyletic and that only the North American corallobothriines, parasites of ictalurid catfishes, form a monophyletic group. Corallobothrium parafimbriatum is sister taxon to a clade that includes Corallotaenia intermedia and C. minutia and not to its congener C. fimbriatum. Also, M. giganteum from Mexico appears to be more closely related to M. lamothei than to its conspecific in Canada. This and the amount of sequence divergence indicate possible cryptic speciation in its endemic host, the Lerma catfish, Ictalurus dugesi.

Spinitectus macrospinosus n. sp., a parasite of the channel catfish Ictalurus punctatus, from the Red and Assiniboine rivers in southern Manitoba, Canada, is characterized by spines reaching a maximum length of 21.8-26.8 microm (means for males and females, respectively) in the anterior esophageal region, arranged in 4 sectors in the anterior region of the body. Spines in this region of the body increase in size and decrease in number (from a maximum of 6 to 3 per sector). The combination of spine length, number, and arrangement and the position of the excretory pore (between spine rows 6 and 7) readily distinguishes it from all its North American congeners, including the species it is most similar to, S. carolini. It is different from S. gracilis and S. acipenseri in having a posteriorly directed vagina, an excretory pore between spine rows 6 and 7, a longer stoma, a right spicule with a terminal ventral barb, and a heart-shaped caudal mucron. Spinitectus macrospinosus is similar to other North American species of Spinitectus, e.g., S. carolini, S. micracanthus, S. mexicanus, S. osorioi, and S. humbertoi, in having the anterior rows of spines arranged in 4 sectors. It is also similar to S. micracanthus and S. carolini in possessing a relatively long stoma, a posteriorly directed vagina, a right spicule with a terminal ventral barb, and a heart-shaped caudal mucron. Furthermore, it is similar to S. carolini in possessing an "area rugosa" with 2 rows of precloacal cuticular cleats. In southern Manitoba, S. macrospinosus appears to mature only in the channel catfish. Reexamination of museum specimens revealed that the nematode also is found in I. furcatus in Kentucky Lake (Kentucky-Tennessee) and in I. lacustris in Lake Texoma (Oklahoma-Texas).

Aim The historical biogeography of host-parasite associations is examined in a geologically old group of fishes, the sturgeons (Osteichthyes: Acipenseridae), and their parasite assemblage. Evidence from phylogenetics and biogeographical studies of several parasite groups is used to formulate and evaluate hypotheses about the coevolutionary history of this host-parasite association and, ultimately, the origin of sturgeons. Location The geographical areas discussed in this study are the temperate regions of North America and Eurasia, including the North Atlantic, North Pacific, Mediterranean and Ponto-Caspian basins and their drainages. Methods Relationships of parasites in several clades (Truttaedacnitis, Diclybothriidae, Deropristiidae, Spinitectus and Acrobothriidae) are analysed or re-analysed using cladistics. These results are combined with hypotheses of relationships of sturgeons and their biogeography. We largely follow an ‘empirical synthetic pattern approach' (Grande & Bemis, 1998) where the search is primarily for broadly congruent patterns, keeping interpretations close to the data and to known or postulated Earth history (palaeogeography). The evidence from the cladistic analyses is first interpreted within the context of vicariance events, but dispersalist scenarios are also discussed where warranted. We also use the concept of Croizat's ‘biogeographical tracks' to characterize broadly congruent distributional patterns. Results Phylogenetic hypotheses and biogeography of parasites indicate three major biogeographical tracks, of which a Holarctic track involving northern and north-eastern Asian regions and North America is predominant (in Crepidostomum auriculatum, Polypodium hydriforme, Truttaedacnitis, Capillospirura, Diclybothriidae, Amphilina), with evidence for a recurring basal trans-Pacific track (in Truttaedacnitis, Diclybothriidae). Trans-Atlantic tracks appear to be either independent of (in Deropristidae) or secondarily derived from (in Bothrimonus and Truttaedacnitis sphaerocephala) a Holarctic track. Evidence from parasites supports the hypothesis of sister relationships between Acipenser medirostris and A. mikadoi, and between A. fulvescens and A. brevirostrum. Main conclusions This host–parasite association was historically structured by colonization and subsequent coevolution. Dispersal and vicariance of acipenserids resulted in small but widespread, highly distinct, monophyletic parasite lineages. Species of such parasite clades remain associated with major and often discrete continental and oceanic formations, and occasionally with related sturgeon species. Biogeographical tracks implicate the Cretaceous and early Tertiary Asian–American peninsula (Beringian region) as a historically important region in the origin and early geodispersion of the parasite fauna. This is consistent with hypotheses of the northern north-eastern Asian region, or the more inclusive Asian–American peninsula, as a key area in the early diversification of sturgeons and the acquisition of their parasite fauna. The parasite fauna supports the hypothesis of a former continuous distribution of Nearctic and eastern Palaearctic (Siberian) sturgeons that was subsequently fragmented by geological and climatic events, resulting in major displacement of ancestral sturgeon populations in North America. The parasites also reflect a historical connection with marine and brackish water environments that pre-dates the diversification of sturgeons, indicating that ancestral acipenserids were diadromous.

Wallinia chavarriae n. sp. is described from the small-bodied characids Astyanax aeneus and Bryconamericus scleroparius in the Area de Conservación Guanacaste, northwestern Costa Rica. The species differs from W. valenciae in possessing an acetabulum that is smaller than the oral sucker and vitelline follicles that are ovoid or rounded rather than elongate and tubular. Detailed comparison between these 2 species is handicapped by the less than satisfactory condition of the type and only museum specimen of W. valenciae. Wallinia chavarriae and W. valenciae belong to a subfamily of trematodes, Walliniinae, that arguably includes Creptotrematina spp., Magnivitellum simplex, and possibly Margotrema. The morphology of walliniines suggests that they are macroderoidids, but a clearer understanding of their classification could be gained from their larval morphology or from molecular systematic studies. The host associations of a monophyletic Walliniinae would indicate diversification within 2 groups of freshwater fishes: the neotropical characids for species of Wallinia, Creptotrematina, and Magnivitellum and the endemic central Mexican goodeids for those of Margotrema. The biogeography and host associations of these parasites provide a system for studies of potential host switching and vicariance, involving the middle-American and neotropical regions.

The helminth fauna of Mexican ictalurids was investigated through original surveys and published literature in order to evaluate the biogeography of host-parasite associations of a primarily nearctic host group in a major faunal transition area. In total, 84 specimens of 4 species of ictalurids (Ictalurus balsanus, Ictalurus dugesi, Ictalurus furcatus, and Ictalurus punctatus) were sampled from 7 localities in 5 hydrological drainages. Ten species of adult endohelminths were recovered from this survey, including 4 species of digeneans (Phyllodistomum lacustri, Genarchella tropica, Alloglossidium corti, and Campechetrema sp.), 3 cestodes (Corallobothrium fimbriatum, Megathylacoides giganteum, and Choanoscolex lamothei), and 3 nematodes (Dichelyne mexicanus, Goezia sp., and Rhabdochona sp.). The adult helminth fauna of ictalurids reported in this study is composed of taxa that are typical of ictalurids in other parts of North America, north of Mexico. This core fauna (in a historical biogeographic sense) includes the corallobothriines C. fimbriatum and M. giganteum and the digeneans P. lacustri and A. corti. Two other helminths, C. lamothei and D. mexicanus, may be added to this group in the future. Other helminths occurred sporadically and provide evidence for host-sharing (ecological host extensions), but we were unable to identify any valid cases of host-switching from more distantly related hosts. The helminth fauna provides evidence that the ictalurids developed their characteristic helminth fauna without neotropical influences and that the phylogenetic affinities of the ictalurids do not appear to be with any neotropical siluriform.

Two genera of caryophyllidean cestodes, Monobothrium Diesing, 1863 and Promonobothrium Mackiewicz, 1968 , from cypriniform fishes (cyprinids and suckers) in the Holarctic Region, are revised using morphological and molecular evidence. Monobothrium, which includes morphologically distinct species that occur in European cyprinids (Cyprinidae) and North American suckers (Catostomidae), is separated into 3 genera. Monobothrium becomes monotypic and is represented by its type species, Monobothrium wageneri Nybelin, 1922 , which occurs in European tench, Tinca tinca (Cyprinidae). Monobothrium auriculatum Kulakovskaya, 1961 from Leuciscus danilevskii (Cyprinidae) in the Ukraine, is tentatively transferred to Caryophyllaeus Gmelin, 1790 as Caryophyllaeus auriculatus ( Kulakovskaya, 1961 ) n. comb. because it possesses morphological characteristics considered to be typical of Caryophyllaeus but which are absent in Monobothrium (shape of the scolex, presence of a seminal receptacle, short neck, and absence of a large, muscular papilla surrounding the large gonopores). The 5 remaining Monobothrium species from suckers in North America are transferred to Promonobothrium based on shared morphological features and molecular data. Species of Promonobothrium differ from M. wageneri by having an external seminal vesicle (absent in M. wageneri), the absence of postovarian vitelline follicles in North American species (present in M. wageneri), and a scolex that is digitiform papillate, loculopapillate, or loculotruncate, i.e., equipped with weak loculi and a terminal introvert (vs. claviform, bluntly ended, with 6 weak, shallow, longitudinal grooves in M. wageneri). Phylogenetic analysis of nuclear ribosomal RNA genes ssr- and lsrDNA placed Promonobothrium minytremi within a clade of 3 of the 5 North American Monobothrium species. The position of M. wageneri, although distinct from the remaining Monobothrium species, is unresolved within the Caryophyllidea. This study further supports the pattern that the biogeography of caryophyllidean taxa is generally constrained by host and continental association.

In Southeast Asia, swamp eels (Synbranchidae: Monopterus spp.) are a common source of human gnathostomiasis, a foodborne zoonosis caused by advanced third-stage larvae (AL3) of Gnathostoma spp. nematodes. Live Asian swamp eels are imported to US ethnic food markets, and wild populations exist in several states. To determine whether these eels are infected, we examined 47 eels from markets and 67 wild-caught specimens. Nematodes were identified by morphologic features and ribosomal intergenic transcribed spacer–2 gene sequencing. Thirteen (27.7%) M. cuchia eels from markets were infected with 36 live G. spinigerum AL3: 21 (58.3%) in liver; 7 (19.4%) in muscle; 5 (13.8%) in gastrointestinal tract, and 3 (8.3%) in kidneys. Three (4.5%) wild-caught M. albus eels were infected with 5 G. turgidum AL3 in muscle, and 1 G. lamothei AL3 was found in a kidney (both North American spp.). Imported live eels are a potential source of human gnathostomiasis in the United States.

Spinitectus osorioi (Nematoda: Cystidicolidae) is described from the freshwater atherinids Chirostoma estor and Chirostoma attenuatum from Lake Pátzcuaro in the Mesa Central, Michoacán State, México. This nematode is characterized by a conspicuous protuberance on the ventral surface of the distal end of the long spicule that distinguishes it from its congeners in North America and in the neotropics. In addition, the species can be readily distinguished from 4 of the 5 nominal species of North American freshwater Spinitectus by the absence of either a terminal barb or heel on the short spicule and from Spinitectus mexicanus by the spination. Previous records of Spinitectus carolini from Chirostoma spp. in México (Lakes Pátzcuaro and Zirahuén) refer to S. osorioi, and the species appears to be specific to Chirostoma spp. The geological history of the Mesa Central drainages and the historical biogeography of freshwater atherinids in this region suggest that the origin of S. osorioi may be associated with either the marine history of their hosts or with host-switching from more distantly related freshwater hosts after colonization of freshwater environments by atherinids.

Crepidostomum opeongoensis Caira, 1985 (Trematoda: Allocreadiidae) is redescribed from fish hosts Hiodon alosoides and Hiodon tergisus in southern Manitoba. The redescription adds details regarding the surface morphology and reproductive structures of the parasite not described previously. These include the characteristic tegumental papillae around the oral opening, paired papillae on the ventral and dorsal surfaces of the body, and tegumental bosses on the dorsal surface of the forebody. Crepidostomum opeongoensis co-occurred with its hypothesized sister species. Crepidostomum illinoiense, in all host individuals harboring the former, and both species are characteristic of hiodontids. The association of C. opeongoensis with hiodontids and the absence of hiodontids in Lake Opeongo where C. opeongoensis was originally reported indicate that the Algonquin Park region may have had hiodontids in the past and that the life cycle may be completed at present without the original fish host.

In Lake Patzcuaro in the Mesa Central of Mexico, a total of 19 species of helminths was found in 598 fishes and comprised five digeneans, two monogeneans, four cestodes, one acanthocephalan and seven nematodes, of which ten species were represented by larval or immature states. The richest and most diverse helminth communities were found in the native carnivorous goodeid Alloophorus robustus. In general, the helminth communities in the different fish species were not particularly species rich and the parasite assemblages were numerically dominated by larvae of the bird trematode, Posthodiplostomum minimum. Patterns of helminth community richness and diversity were similar to those previously observed in north-temperate freshwater fishes. Most enteric helminths occurred with low abundance and only a small proportion of the gut helminth communities was numerically dominated by any one species. Helminths dominating their enteric communities showed some level of host specificity. Helminth communities in carnivorous fish species were generally richer than those in herbivores and detritivores, with the exception of the predominantly herbivorous Goodea atripinnis. The helminth fauna of introduced fishes, Cyprinus carpio, Micropterus salmoides and Oreochromis niloticus, consisted of either few or no host-specific adult helminth(s) translocated from their original geographical areas and by larval stages of helminths of piscivorous birds. Based on the geological history of the area and the biogeography of the endemic fish fauna, it is hypothesized that host-switching and relationships with the nearctic fauna have been fundamental in determining the helminth fauna of the endemic fish hosts.

Aim Published information on the richness and diversity of helminth parasite communities in tropical freshwater fishes is reviewed in response to expectations of species-rich parasite communities in tropical regions. Location Areas covered include the tropics and some subtropical areas. In addition, the north temperate area of the nearctic zone is included for comparison. Methods Data from 159 communities in 118 species of tropical freshwater fish, summarized from 46 published studies, were used for this review. Parasite community descriptors used in the analyses included component community richness and calculated mean species richness. Data from 130 communities in 47 species of nearctic north temperate freshwater fish were summarized from 31 studies and used for comparison. Results The component helminth communities of many tropical freshwater fish are species-poor, and considerable proportions of fish from certain parts of the tropics, e.g. West African drainages, are uninfected or lightly infected. Mean helminth species richness was low and equaled or exceeded 2.0 in only 22 of 114 communities. No single group of helminths was identified as a dominant component of the fauna and species composition was variable among and within broader geographical areas. The richest enteric helminth assemblages were found in mochokid and clariid catfish with a mixed carnivorous diet, whereas algal feeders, herbivores and detritivores generally had species-poor gut helminth communities. Comparisons indicated that certain areas in the north temperate region had higher helminth species richness in fishes than areas in the tropics. Main conclusions Expectations of high species richness in helminth communities of tropical freshwater fishes are not fulfilled by the data. Direct comparisons of infracommunities and component communities in host species across widely separated phylogenetic and geographical lines are inappropriate. Examination of latitudinal differences in richness of monophyletic parasite groups or of compound communities may uncover patterns different from those found in this study. Richness of helminth communities may be ultimately determined not by the number of host species present but by the degree of divergence of host lineages and by their diversification modes. A phylogenetic framework for hosts and parasites will reveal if increased host species richness within host clades, when host speciation is accompanied by habitat or diet specialization, or both, leads to lower helminth diversity in host species by fragmentation of a core helminth fauna characteristic or specific of the larger host clade. This pattern may be analysed in the context of cospeciation and acquisition from other unrelated hosts (host-sharing or host-switching).

We examined the role that predation of infected conspecific fish and postcyclic transmission might play in the life cycle of the Asian fish tapeworm, Bothriocephalus acheilognathi (Cestoda: Pseudophyllidea) Yamaguti, 1934. Young-of-the-year (YOY) bonytail chub (Gila elegans) were exposed to copepods infected with B. acheilognathi and subsequently fed to subadult bonytail chub. Within 1 wk after consumption of the YOY chub, subadults were necropsied and found infected with gravid and nongravid tapeworms. This study provides evidence that postcyclic transfer of B. acheilognathi can occur. Postcyclic transmission may be an important life history trait of B. acheilognathi that merits consideration when studying the impact and distribution of this invasive and potentially pathogenic tapeworm.

The 11 nominal species of the Deropristiidae, belonging to the genera Deropristis (three species), Pristicola (one species), Skrjabinopsolus (five species) and Cestrahelmins (two species) were re-evaluated for a phylogenetic and biogeographical analysis of the group. Skrjabinopsolus elongatus (Madhavi, 1974) (= S. indicus Gupta & Ahmad, 1976, S. kurotchkini Parukhin, 1976), S. sanyaensis Shen, 1990, Deropristis paurosoma Wang, 1989 and Cestrahelmins Fischthal, 1957, do not belong to the Deropristiidae. The genus Opisthodiplomonorchis Madhavi, 1974 is resurrected for its type-species, O. elongatus (= S. elongatus), and D. paurosoma is also transferred to this genus. Species of Opisthodiplomonorchis occur in polynemid fishes of the Indo-Pacific. Five nominal species thus comprise the Deropristiidae, i.e. S. armatus Ivanov in Ivanov and Mrygin, 1937, S. manteri (Cable, 1952), D. hispida (Abildgaard, 1819), D. inflata (Molin, 1859) and P. sturionis (Little, 1930), which corresponds to the original composition of the family. A comparison of Skrjabinopsolus from acipenserids in North America and Europe revealed variability in morphological characters considered diagnostic in the past (body size and position of the ovary and testes). Based on the morphology and biology, the two species of Skrjabinopsolus can be considered incipient species. A phylogenetic analysis of the redefined Deropristiidae indicates that Deropristis and Pristicola may form a monophyletic group with Skrjabinopsolus as the sister taxon. The biogeography of the Deropristiidae suggests an origin and distribution associated with the formation of the North Atlantic basin. Freshwater and marine/estuarine origins for the family are equally parsimonious possibilities. A freshwater origin can be traced at least to Cretaceous Laurasia followed by a drift-vicariance separation and subsequent association with North Atlantic drainages. Support for a marine ancestry of the Deropristiidae is indicated by the biogeography of its sister taxa (outgroups in this study) and of its hosts, and is also consistent with an Atlantic origin. The deropristiids evidently originated and speciated within the Acipenseridae in the Atlantic region, with a host switch into Atlantic Anguilla spp.

The historical biogeography of sturgeons is explored using information from palaeogeography, palaeontology and phylogenetic interrelationships. The integration of information from these diverse sources indicates that sturgeons reached a wide Laurasian distribution in the Cretaceous and Tertiary by freshwater and coastal dispersal routes across land connections and along newly forming continental margins. The fossil record also suggests a considerable degree of morphological stasis and also supports an estuarine habit, and perhaps diadromy, as an old and conserved life history trait. While a ‘centre of origin’ for sturgeons remains elusive, phylogenetic relationships indicate that diversification appears to have been associated with fragmentation of biota, and of landmasses and basins, by late Tertiary geological and climatic phenomena, such as orogeny and unequal glaciation over North America, the desiccation of central Asia and alteration of its drainages, and the formation of discrete Ponto-Caspian basins by the fragmentation of the Paratethys. Amphi-oceanic distributions of certain species (Acipenser medirostris Ayres) and sister taxa (e.g. A. oxyrhynchus Mitchill and A. sturio L.) are explained by coastal dispersal and subsequent vicariance by geological (sea-floor spreading and development of new continental margins) and climatic (Pliocene cooling) changes during the Tertiary. An hypothesis is developed for the relationships of the North American sturgeons and their potential relationships with the Siberian sturgeon A. baeri. Late Tertiary climatic and geological phenomena are hypothesized as mediators of vicariance and subsequent diversification of these acipenserids. It appears that although acipenserids are a geologically old group, the historical biogeography of surviving lineages is best explained by more recent geological and climatic changes.

Patterns, processes, and some hypotheses regarding the structure of helminth communities in freshwater fishes have been investigated using sturgeons (Acipenseridae), a widely distributed group of basal actinopterygian fishes. Analyses at the infracommunity and component-community levels for one species, Acipenser fulvescens, indicated occasionally dense, but isolationist, gut helminth communities with moderate species richness and a high degree of predictability conferred by host-specific helminths that dominated the communities. Largely predictable helminth communities are also characteristic of acipenserids from Siberian and Ponto-Caspian drainages and basins. Although host specificity was the major determinant of helminth community diversity in most acipenserids, exceptions were found in sturgeons of the Aral Sea drainages, where non-host-specific parasites dominated the helminth communities. Gut complexity and evolutionary age were both found to be poor predictors of community richness in sturgeons and other basal actinopterygians. The stochastic component of the gut helminth fauna is determined by shared trophic category and sympatry with other major benthivorous fishes, mainly benthic coregonines in Holarctic drainages and benthic ostariophysans (e.g., cypriniforms) elsewhere. Although some predictions of "evolutionarily mature" communities are partially fulfilled, the authors of this study caution that owing to differences in the biology and history of individual species and groups and the diversity of fishes in general, broad generalizations about the factors shaping fish parasite communities will be difficult to make.

Patterns, processes, and some hypotheses regarding the structure of helminth communities in freshwater fishes have been investigated using sturgeons (Acipenseridae), a widely distributed group of basal actinopterygian fishes. Analyses at the infracommunity and component-community levels for one species, Acipenser fulvescens, indicated occasionally dense, but isolationist, gut helminth communities with moderate species richness and a high degree of predictability conferred by host-specific helminths that dominated the communities. Largely predictable helminth communities are also characteristic of acipenserids from Siberian and Ponto-Caspian drainages and basins. Although host specificity was the major determinant of helminth community diversity in most acipenserids, exceptions were found in sturgeons of the Aral Sea drainages, where non-host-specific parasites dominated the helminth communities. Gut complexity and evolutionary age were both found to be poor predictors of community richness in sturgeons an...

Fish parasitology has a long tradition in North America and numerous parasitologists have contributed considerably to the current knowledge of the diversity and biology of protistan and metazoan parasites of freshwater fishes. The Journal of Parasitology has been essential in disseminating this knowledge and remains a significant contributor to our understanding of fish parasites in North America as well as more broadly at the international level. However, with a few exceptions, the importance of fish parasites has decreased during the last decades, which is reflected in the considerable decline of funding and corresponding decrease of attention paid to these parasites in Canada and the United States of America. After the 'golden age' in the second half of the 20th Century, fish parasitology in Canada and the United States went in a new direction, driven by technology and a shift in priorities. In contrast, fish parasitology in Mexico has undergone rapid development since the early 1990s, partly due to extensive international collaboration and governmental funding. A critical review of the current data on the parasites of freshwater fishes in North America has revealed considerable gaps in the knowledge of their species composition, host specificity, life cycles, evolution, phylogeography, and relationships with their fish hosts. As to the key question, "Why so neglected?" this is probably because: (1) fish parasites are not in the forefront due to their lesser economic importance; (2) there is little funding for this kind of research, especially if a practical application is not immediately apparent; and (3) of shifting interests and a shortage of key personalities to train a new generation (they switched to marine habitats or other fields). Some of the opportunities for future research are outlined, such as climate change and cryptic species diversity. A significant problem challenging future research seems to be the loss of trained and experienced fish parasitologists. This has 2 major ramifications: the loss of expertise in identifying organisms that other biologists (e.g., ecologists, molecular biologists, evolutionists) work with, and an incomplete comprehension of ecosystem structure and function in the face of climate change, emerging diseases, and loss of biodiversity.

Truttaedacnitis clitellarius (Ward & Magath, 1917) (Nematoda: Cucullanidae) is redescribed from the lake sturgeon Acipenser fulvescens from North America and compared with T. lebedevi (Skrjabina, 1966) from the Siberian sturgeon A. baeri. Studies on the morphology and morphometrics of the two species reveal considerable overlap with unique shared characters, such as the previously unreported position of the excretory pore at the level of the pseudobuccal capsule and arrangement of caudal papillae. This position of the excretory pore is anteriormost among all cucullanids. Although there is a trend towards smaller size of body and spicules in T. lebedevi, this reflects a long period of geographical separation and likely accounts for the minor morphological variations noted. Consequently, the two species are synonymised. The presence of one species of Truttaedacnitis, T. clitellarius, specific to freshwater acipenserids in North America and Siberia, suggests an ancient origin and slow rate of evolution for this cucullanid.

Freshwater fish helminths have been studied for more that 80 years in Mexico, and currently the inventory of this parasite fauna is nearing completion. Between 1999 and 2006, we studied 958 freshwater fishes belonging to 34 species in 36 localities distributed along 9 states of the Mexican Republic, north of the Transmexican Volcanic Belt. In total, 25 helminth taxa were found, including 9 digeneans, 6 nematodes, 4 monogeneans, 5 cestodes, and 1 acanthocephalan. Of the 25 species of helminths, 12 were found as larval stages while 13 were found as adults. A large number of hosts (582, i.e., 60.7%) were uninfected with helminth parasites. The data we present here is then used in combination with other recently published parasite surveys conducted in northern Mexico to revise our understanding of the biogeographical patterns of the freshwater fish parasite fauna in an area inhabited predominantly by Nearctic freshwater fishes.

Nineteen species of parasites were recovered from lake sturgeon, Acipenser fulvescens, from four major waterways of Central Canada; the Saskatchewan, Nelson, Winnipeg and Rainy River systems, Twelve of these are new host records. Host specific parasites, Crepidostomum auriculatum, Diclybothrium armatum, Spinitectus acipenseri and Truttaedacnitis clitellarius, forming the core parasite species, were recovered with the highest prevalence (≥70%) and were most widely distributed. Polypodium hydriforme was recovered from only stage IV sturgeon oocytes. With the exception of Pomphorhynchus bulbocolli, prevalence and intensity of endohelminth infections were not correlated with sex or age of host but the distribution of non host-specific parasites among sampling sites was determined by the type and relative abundance of food items consumed. The parasites of lake sturgeon are closely correlated to its diet and the species of parasites recovered are more similar to those of freshwater sturgeon in Eurasia than to other species of Acipenser in North America. The present parasite community of lake sturgeon appears to have been shaped by three major factors; the presence of core parasite species which predates geographic isolation, a benthic freshwater diet which has reshaped the parasite community to one comprising freshwater species and a long association with freshwater habitats which is reflected in the reproductive isolation of the lake sturgeon and lastly, the establishment of a host-specific parasite, Spinitectus acipenseri.

Spinitectus acipenseri is described as a new species from the muscular stomach of the lake sturgeonAcipenser fulvescens from Manitoba and Saskatchewan. This new species closely resemblesS. gracilis Ward & Magath, 1917, but the major differences are the arrangement and larger size of spines (circlets and semicirclets of spines reaching to the anus in females ofS. acipenseri). Other differences include total body dimensions of adults (length and width relationships) and a 1:4–1:5 ratio of oesophagus to body length.

Freshwater fish helminth parasites are undoubtedly the most well-known group among vertebrate parasites in Mexico; these parasites include 37 adult and 43 larval (metacercariae) species of trematodes, 62 monogeneans, 15 adult and 18 larval (metacestode) cestodes, 6 adult and 4 larval (cystacanth) acanthocephalans, and 54 adult and 15 larval nematodes (L3). Here, we evaluate the extent of the freshwater fish helminth inventory of Mexico using species accumulation curves and discuss the implications of DNA-based taxonomic methods in our understanding of the helminth biodiversity in freshwater fishes. Future directions for the study of the helminth parasites of freshwater fishes are proposed, particularly the role of DNA-based species delimitation criteria in recognizing parasite species, but also in discovering cryptic species. Species accumulation curves indicate that even though the asymptote has not been reached, a tendency toward stabilization is observed in all taxonomic groups except monogeneans. We suggest, therefore, that the inventory, as conventionally understood, is nearing completion for most groups. We suggest that future survey work aimed at enhancing the biodiversity inventory should be strategic, i.e., it should combine the need to target missing components of the host spectrum with the choice of appropriate drainages based on biogeographic, faunistic, and hydrologic data. We also posit that the future belongs to DNA-based taxonomic approaches that aim to uncover previously unrecognized biodiversity.

Pristicola bruchi n. sp. (Trematoda: Deropristiidae) is described from the spiral-valved intestine of the lake sturgeon, Acipenser fulvescens, from the Wolf River in Wisconsin, United States. It differs from the only other species of the genus, Pristicola sturionis, a parasite of the European Atlantic sturgeon Acipenser sturio, in being smaller and in possessing the following characters: a single row of conspicuous peg-like oral spines instead of 2 rows; vitelline follicles that are dorsally confluent over only a small region and that barely overlap the testes instead of extending beyond the posterior testes; and a shorter hermaphroditic duct. Comparisons are hindered by the fact that the type material of P. sturionis is no longer available. This is the first report of the genus in North America and is, apparently, the first time the genus has been reported in sturgeon anywhere since the description of P. sturionis in 1930. The presence of a species of Pristicola in North America means that all 3 genera of deropristiids, Deropristis, Pristicola, and Skrjabinopsolus, now have 2 described species, 1 in North America and another in Europe, reinforcing the amphi-Atlantic biogeography of the family. This, in turn, supports the contention that the deropristiids had diversified into the 3 generic lineages before the establishment of the North Atlantic, and that the present day distribution was likely effected by historical vicariance processes. The association of species of Pristicola and Skrjabinopsolus with the exclusively freshwater lake sturgeon in the interior of the continent also indicates their considerable geological age.

Species of Allocreadiidae are an important component of the parasite fauna of freshwater vertebrates, particularly fishes, and yet their systematic relationships with other trematodes have not been clarified. Partial sequences of the 18S and 28S ribosomal RNA genes from 3 representative species of Allocreadiidae, i.e., Crepidostomum cooperi, Bunodera mediovitellata, and Polylekithum ictaluri, and from 79 other taxa representing 78 families of trematodes obtained from GenBank, were used in a phylogenetic analysis to address the relationships of Allocreadiidae with other plagiorchiiforms/plagiorchiidans. Maximum parsimony and Bayesian analyses of combined 18S and 28S rRNA gene sequence data place 2 of the allocreadiids, Crepidostomum cooperi and Bunodera mediovitellata, in a clade with species of Callodistomidae and Gorgoderidae, which, in turn is sister to a clade containing Polylekithum ictaluri and representatives of Encyclometridae, Dicrocoelidae, and Orchipedidae, a grouping supported by high bootstrap values. These results suggest that Polylekithum ictaluri is not an allocreadiid, a conclusion that is supported by reported differences between its cercaria and that of other allocreadiids. Although details of the life cycle of callodistomids, the sister taxon to Allocreadiidae, remain unknown, the relationship of Allocreadiidae and Gorgoderidae is consistent with their larval development in bivalve, rather than gastropod, molluscs, and with their host relationships (predominantly freshwater vertebrates). The results also indicate that, whereas Allocreadiidae is not a basal taxon, it is not included within the suborder Plagiorchiata. No support was found for a direct relationship between allocreadiids and opecoelids either.

Paracreptotrematina limi Amin and Myer, 1982 (Trematoda), an intestinal fluke specific to the mudminnow, Umbra limi, is conventionally classified within the papillose Allocreadiidae. Its unusual morphology (lack of identifiable vitellaria, large fully embryonated terminal eggs), assumptions of homology of its 2 atypical muscular oral 'papillae' (lobes) with those of the Bunoderinae, and its unknown life cycle make this classification tenuous. Previous phylogenetic analyses of the papillose allocreadiids, based on morphology, placed P. limi as a basal papillose allocreadiid. We tested this hypothesis with a phylogenetic analysis by using partial sequences of the 28S ribosomal RNA gene and the cytochrome c oxidase subunit I gene from several plagiorchiiform taxa, including reportedly related allocreadiids as well as selected species of Plagiorchiidae, Haematoloechidae, and Macroderoididae. Results of phylogenetic analyses of the 28S rRNA gene fragments by using parsimony criteria support the classification of P. limi as an allocreadiid and place it as a sister taxon to a clade with Allocreadium lobatum Wallin, 1909, Bunodera luciopercae (Müller, 1876) and Crepidostomum cooperi Hopkins, 1931, with Polylekithum ictaluri (Pearse, 1924) basal to all of them. Analysis of the cytochrome c oxidase subunit I gene sequence data from fewer taxa supports the placement of P. limi relative to 3 (A. lobatum, C. cooperi, and P. ictaluri) of the 4 allocreadiid taxa. These results also suggest that the previous conception of the papillose allocreadiids as a monophyletic assemblage that includes P. limi may require a reappraisal.

Auriculostoma n. gen. (Trematoda: Allocreadiidae) is proposed for Auriculostoma astyanace n. sp. from the intestine of the characid fish Astyanax fasciatus in the Atlantic coastal drainages of Nicaragua. The new genus differs from all papillose allocreadiid genera, except Bunoderella, in possessing 2 pairs of muscular oral papillae (instead of 1 or 3), of which the ventrolateral pair is moderately developed and the dorsolateral papillae are long and auricular. Auriculostoma differs from Bunoderella Schell, 1964, in having lateral vitelline follicles, completely separated or confluent only in the posttesticular region, a uterus limited to the pretesticular region or with a few eggs at the level of the testes, and a long cirrus sac that overlaps the acetabulum or usually reaches posteriorly to the ovarian level. Three other allocreadiid species, all from South American freshwater fishes and each of which had previously been placed in Crepidostomum, are transferred to Auriculostoma based on the presence of the diagnostic muscular oral papillae. These include Crepidostomum platense Szidat, 1954, C. macrorchis Szidat, 1954, and C. stenopteri Mane-Garzon and Gascon, 1973. Diagnostic features for Auriculostoma also include mainly pretesticular uterus, lateral vitellaria with variation in posttesticular confluence, and tandem testes. The genus appears to be typically associated with neotropical siluriforms (catfishes) and characiforms (tetras).

A 2-yr, seasonal, parasitological study of 1,435 fish, belonging to 4 species of native fishes and 7 species of nonnative fishes from the lower Little Colorado River (LCR) and tributary creeks, Grand Canyon, Arizona, yielded 17 species of parasites. These comprised 1 myxozoan (Henneguya exilis), 2 copepods (Ergasilus arthrosis and Lernaea cyprinacea), 1 acarine (Oribatida gen. sp.), 1 piscicolid leech (Myzobdella lugubris), 4 monogeneans (Gyrodactylus hoffmani, Gyrodactylus sp., Dactylogyrus extensus, and Ligictaluridus floridanus), 4 nematodes (Contracaecum sp., Eustrongylides sp., Rhabdochona sp., and Truttaedacnitis truttae), 3 cestodes (Bothriocephalus acheilognathi, Corallobothrium fimbriatum, and Megathylacoides giganteum), and 2 trematodes (Ornithodiplostomum sp. and Posthodiplostomum sp.). Rhabdochona sp. was the only adult parasite native to the LCR. Infection intensities of Ornithodiplostomum sp. and B. acheilognathi were positively correlated with length of the humpback chub Gila cypha. Adult helminths showed a high degree of host specificity, except B. acheilognathi, which was recovered from all fish species examined but was most abundant in cyprinids. Abundance of B. acheilognathi in the humpback chub was highest in the fall and lowest in the summer in both reaches of the LCR. There was no major taxonomic difference in parasite assemblages between the 2 different reaches of the river (LC1 and LC2). Parasite community diversity was very similar in humpback chub, regardless of sampling site or time. The parasite fauna of the LCR is numerically dominated by B. acheilognathi and metacercariae of Ornithodiplostomum sp. The richest and most diverse component community occurred in a nonnative species, the channel catfish Ictalurus punctatus, but infracommunity species richness was highest in a native host, humpback chub.

Lissorchis macropharynx (Digenea: Lissorchiidae) is described from the shorthead redhorse, Moxostoma macrole-pidotum (LeSueur) (Catostomidae) from the Assiniboine River drainage of southern Manitoba. The species is morphologically similar to Lissorchis hypentelii in possessing a similar body shape (widest at the ovarian region and a relatively long hindbody), a distinctly trilobed and relatively posteriorly situated ovary, and in the distribution of the vitellaria. It differs from L. hypentelii and all other lissorchiids in possessing a massive pharynx that is generally equal in length to or longer than the oral sucker and almost as deep as the cervical region of the body it occupies, a protruding acetabular region with an aspinose ridged/folded and grooved anterior surface, and a massive cirrus sac that reaches the level of the ovary. Museum specimens of Lissorchis spp. from M. macrolepidotum from other drainages in Manitoba and Wisconsin were also identified as L. macropharynx.

Crepidostomum percopsisi n. sp. is described from the small intestine of the trout perch (Percopsis omiscomaycus) in Dauphin Lake, Manitoba. It is morphologically similar to Crepidostomum isostomum, which has been reported from the trout perch and several other species of fish. It differs from C. isostomum based on the vitellaria confined to the hindbody of the worm, size and shape of the cirrus, size of the testes, and its greater body length. A comparison of our specimens with those illustrated and identified as C. isostomum from trout perch indicates that such specimens are identical to larger specimens of C. percopsisi recovered by us from trout perch in May. To date, C. percopsisi has only been reported from the trout perch of Dauphin Lake, Lake Winnipeg, and Oneida Lake, which suggests host specificity.

All species of Truttaedacnitis Petter, 1974, were examined for a phylogenetic analysis. Morphological studies showed that Truttaedacnitis truttae (Fabricius, 1794) from North America possessed a reduced button-shaped or nipple-shaped caudal mucron that is in contrast to the sharp spike-shaped mucron commonly reported in Eurasian specimens. Histological studies showed that the excretory pore in Truttaedacnitis sphaerocephala (Rudolphi, 1809) was situated near the level of the pseudobuccal capsule between the 2 parts of the ventral cephalic ridge. Cucullanus heterodonti Johnston and Mawson, 1943 is transferred to Truttaedacnitis based on the presence of prominent cephalic plates separated by sutures. An hypothesis regarding interrelationships among Truttaedacnitis spp. was developed using phylogenetic systematics. The anterior excretory pore is a synapomorphy supporting the monophyly of Truttaedacnitis clitellarius (Ward and Magath, 1917) and T. sphaerocephala parasitizing sturgeons. The arrangement of cephalic plates indicates that Truttaedacnitis pybusae Anderson, 1992 and T. truttae may form a monophyletic group and further supports the monophyly of T. clitellarius and T. sphaerocephala. Using unordered character states of different cephalic plates produced a cladogram that suggested that species in holarctic/circumboreal families (Acipenseridae, Salmonidae, Petromyzontidae) may be monophyletic. The systematics and biogeography of the various species of Truttaedacnitis and their host specificity indicate that associations in this parasite genus are very ancient and of a relictual nature, determined largely by past continental and oceanic configurations, and a combination of vicariance and dispersal.

Diclybothrium atriatum n. sp. is described from North American acipenserid fishes, Acipenser fulvescens Rafinesque (lake sturgeon) and Acipenser brevirostrum Le Sueur (shortnose sturgeon). Diclybothrium atriatum is distinguished from other diclybothriids by the presence of a unique preovarial structure that we name the ventral atrial organ. This structure possesses a ventral orifice leading to a deep groove that widens internally as a crypt, bordered by densely staining granular bodies and more deeply situated glandular cells. Previous records of Diclybothrium armatum Leuckart, 1835 examined in this study from North American acipenserids are referable to D. atriatum. Studies on the anatomy of this and other species of Diclybothrium Leuckart, 1835, D. armatum and Diclybothrium hamulatum (Simer, 1929), reveal the presence of paired multichanelled sperm ducts that are shown to be characteristic of the Diclybothriidae Bykhovskii and Gusev, 1950. Paired sperm ducts, while common in Turbellaria with multiple testes, are unusual amongst Monogenea. The endemicity of Diclybothrium atriatum and specificity for A. fulvescens and A. brevirostrum suggests that speciation of this parasite may have occurred following the isolation of Acipenser L. spp. in central and eastern North America.

Eight species of helminths were recovered from 45 summer (June-August) and 29 winter (February) lake sturgeon adults and juveniles, from Lake Winnebago, Wisconsin. Eighteen young-of-the-year (YOY) sturgeon collected from the Wolf River were uninfected. Both summer and winter helminth communities were dominated by host specific parasites such as Crepidostomum auriculatum, Skrjabinopsolus manteri, Capillospirura pseudoargumentosa and Truttaedacnitis clitellarius. Examination of stomach contents indicated that chironomids were the most important food items and comparison with past records on lake sturgeon diet suggests an unchanged use of the benthos of Lake Winnebago. The eutrophic nature of Lake Winnebago is probably responsible for the absence of Hexagenia spp., an important component of the diet of lake sturgeon from other systems, which would explain the absence of a host specific nematode, Spinitectus acipenseri. Although the trophic status of the lake may modify the helminth community, some host specific endo-helminths (C. auriculatum, C. pseudoargumentosa) appear to be transmitted in riverine environments (deeper downstream channels) used as feeding areas by older (2 yr+) sturgeon. Differences in habitat and diet of YOY sturgeon in the Wolf River and the absence of any host specific helminths in these juveniles also indicate that the community of host specific helminths is not recruited until YOYjuveniles enter deeper downstream channels to feed.

Polypodium sp. is reported for the first time from Canadian waters. It occurred in lake sturgeon (Acipenser fulvescens) eggs, from the Nelson River, Manitoba, and from Cumberland House, Saskatchewan. The prevalence of this parasite was 100% in females with eggs of 3.0 mm diameter or more. One, 4, 25, and 179 infected eggs were collected in gonads with eggs of this size class, but infected eggs were not apparent in individuals with smaller eggs. This is the northernmost occurrence of this parasite reported in North America.

Wallinia mexicana sp. n. is described from the Mexican tetra, Astyanax mexicanus (De Filippi) (Characidae Weitzman), from two localities in northern Mexico. The new species can be distinguished from the two congeneric species, described from small-bodied characids in South and Central America, mainly by the posterior extent of the vitelline follicles (halfway between the posterior testis and the end of the caeca), by having a larger oesophagus, testes that are always oblique, and eye-spot remnants. The distinct status of the new species was confirmed by molecular data (28S rRNA gene sequences). Phylogenetic analysis suggests the new species is the sister species of W. chavarriae Choudhury, Hartvigsen et Brooks, 2002 described from characids in northwestern Costa Rica. Additionally, genetic divergence between these congeners reached 3.3%, a value higher than that observed for closely related species pairs of allocreadiids for that molecular marker. Based on these new findings, recently published records of this new species as Magnivitellinum simplex Kloss, 1966 and Creptotrematina aguirrepequenoi Jiménez-Guzmán, 1973 in Astyanax mexicanus from Durango and San Luis Potosi states, respectively, are corrected.