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Cirrodrilus japonicus (Pierantoni, 1912) has not been identified since it was originally described from specimens removed from preserved Japanese crayfish that had been deposited in the Natural History Museum in Hamburg, Germany. A morphological comparison of the published description and recently discovered syntypes of C. japonicus from the Hamburg Museum with syntypes and newly collected material of Cirrodrilus ezoensis (Yamaguchi, 1934), supported the view that preservation artifacts in C. japonicus caused Prof. Yamaguchi to believe his well-preserved specimens were member of a separate species. Therefore, C. ezoensis becomes a junior synonym of C. japonicus.

Crayfishes and other invertebrates were collected during 2010 to 2016 from 519 sites located in the river basins of the Adour, Charente, Dordogne, Garonne, Loire, Seine, Rhône and Sélune in France. North American species included Pacifastacus leniusculus at 255 sites, Faxonius limosus at 206 sites, Procambarus clarkii at 56 sites, and the endemic Austropotamobius pallipes at two sites. However, branchiobdellidans were only recorded from 100 sites with 23 of these being sampled more than once, resulting in a total of 127 collections. The widely distributed western North American, P. leniusculus carried four of its endemic branchiobdellidan species: Cambarincola gracilis, C. okadai, Triannulata magna and Xironogiton victoriensis. X. victoriensis was found at the majority of sites, with C. okadai, C. gracilis, and T. magna at fewer locations. Although F. limosus was the second most numerous crayfish species collected, it did not carry any of its endemic North American branchiobdellidan species. However, it was found cohabiting with P. leniusculus at four sites but carried X. victoriensis at only one of these. European A. pallipes only occurred at two sites with individuals harboring X. victoriensis, although no cohabiting exotic crayfish were found. Crayfish were also absent from three sites where free-living X. victoriensis were recovered from substrate samples. Procambarus clarkii appeared at sites scattered across the country, while specimens with C. mesochoreus were restricted to the Adour drainage. This study has shown the widespread distribution of exotic branchiobdellidans in the wild and the virtual extinction of endemic species in France. In addition, we have provided the first European record of T. magna, and the first record of C. mesochoreus in France; the latter being only the second recording in Europe. With this comprehensive survey of their crayfish hosts in France, monitoring future exotic range expansions and endemic contraction or extinction can be traced. These data will be available to authorities for future planning in maintaining healthy freshwater bodies by reducing the damaging effects caused by exotic crayfishes.

The branchiobdellidan slide collection of Professor Hideji Yamaguchi was rediscovered in 1995; however, all of the written records associated with it appear to have been lost. The slide collection has been examined, cataloged and deposited in the Invertebrate Collection of the Hokkaido University Museum, Sapporo, Japan (ICHAM). The collection was found to contain whole mounted specimens of all 17 species described by Prof. Yamaguchi, together with specimens of three other Oriental species described after his death. In addition, mounted specimens of some of these species were designated syntypes. Many sectioned and stained specimens were also cataloged, but those were not identified to species at this time.

Over the last 130 years demand for crayfish has exceeded regional supplies around the world, so stocks, primarily from North America, have been imported to satisfy this need. These demands are human based and include gastronomy, sport fishing bait, food for rearing animals, educational aides, ornamentation in waterbodies, and more recently a significant increase in pet crayfish sales. The three most common commercial species from North America are Pacifastacus leniusculus, Procambarus clarkii and Orconectes limosus, although four other species are increasing in importance as they become more widely distributed. All of these crayfish in their endemic range have been reported to carry branchiobdellidan annelids. Therefore, when the crayfish are translocated, their ectosymbionts likely accompany them. Eighty-six potentially transportable branchiobdellidans are recognized in this paper, along with the distribution of six species known to have been translocated. Moreover, branchiobdellidans endemic to the translocation regions have adopted introduced crustacean hosts, which demonstrates that branchiobdellidan host species specificity is not as restrictive as many researchers believe. On the evidence to date, these translocated branchiobdellidans appear to have the same relationship and cause the same amount of damage, if any, as those on their endemic crayfish hosts. The geographical distribution of endemic branchiobdellidan—crayfish species associations are unique to each of four disjunct regions as defined by Bănărescu: Euro-Mediterranean, East Asian, western North American and eastern North American; the latter includes eastward drainages from Canada to Costa Rica.

Worldwide biodiversity is suffering a “biogeographical homogenization” due to biological invasions. The translocation of freshwater crayfish around the world for aquaculture and the pet trade is also spreading their symbionts, so that all of the association's members have the potential to form invasive populations. In Europe, the signal crayfish, Pacifastacus leniusculus, is often concomitant with the branchiobdellidan Xironogiton victoriensis and through repeated translocations their ranges are expanding. In this study, we report the first observation of X. victoriensis on a native European crayfish population of Austropotamobius pallipes. This work is an alert to the authorities and decision makers on the risks to native species when aquaculture projects introduce alien crayfish species and their symbionts into an area. Future studies are needed to evaluate the impact of this non-native ectosymbiont on native crayfish populations.

A new species of ectosymbiotic branchiobdellidan, Cirrodrilus iwakiensis sp. nov., is described from Cambaroides japonicus (de Haan, 1849), the endemic freshwater Japanese crayfish, collected in Aomori Prefecture, northern Honshu Island, Japan. The anatomical features used to identify C. iwakiensis are compared with those of its closest congener, C. nipponicus (Yamaguchi, 1932), as well as C. aomorensis (Yamaguchi, 1934) and C. tsugarensis Gelder and Ohtaka, 2000 distributed in the same region.

Invasive alien species present a global threat to biodiversity, particularly where pathogens and symbionts are involved. Branchiobdellidans are clitellate annelids with an obligate ectosymbiotic association primarily on astacoidean crayfish. There are several examples of branchiobdellidan species adopting a geographically exotic host where endemic and exotic crayfishes cohabit the same water body. The first records of a western North American branchiobdellidan, Xironogiton victoriensis , adopting the eastern North American crayfish, Procambarus clarkii , in 2 river basins in Spain provide further evidence of the ectosymbionts’ tolerance to adopt an exotic host. Given worldwide translocations of these and other commercial crayfish species, limnologists and agency managers need to be alert for further introductions of X . victoriensis and other branchiobdellidans. Impacts of these exotic ectosymbionts on habitat and biota at a new location are unknown, as are their consequences on native biodiversity.

Despite the importance of aquatic symbionts in community organization, often little is known about the relative role of host and environmental characteristics on symbiont abundance. We investigated the importance of environmental and host physical characteristics on the abundance of the branchiobdellidan, Branchiobdella kozarovi , on a population of narrow-clawed crayfish, Astacus leptodactylus , in the Islamic Republic of Iran. Crayfish from four sites in the Aras Dam Reservoir were sampled on 12 occasions during 2009. Abundance of B. kozarovi was modeled against water temperature, crayfish length, mass, and sex after controlling for temporal and spatial effects. The most parsimonious model indicates that B. kozarovi abundance was primarily related to water temperature but also increased linearly with host body length. We found that omitting water temperature dramatically decreased model performance while omitting host physical characteristics decreased model performance marginally. We recommend that ecologists consider both environmental and host physical characteristics when monitoring aquatic symbionts such as branchiobdellidans, as both types of factors will provide insight into community dynamics and guide future management strategies.

Distribution of two ecologically similar but usually spatially separate ectosymbionts, branchiobdellidans (Annelida) and scutariellids (Platyhelminthes), on atyid shrimp (Neocaridina spp.) is reported from 18 localities in five Provinces of southeastern China. Prevalence was determined for the branchiobdellidan, Holtodrilus truncatus, found at seven locations, the scutariellid, Scutariella japonica, present at every site, and where cohabitation occurred. Both ectosymbionts showed a microhabitat predilection for the host's branchial chambers and instances of cohabitation occurred at all seven locations where H. truncatus were collected, although not on every shrimp. On-site observations of live hosts supporting both ectosymbionts showed that neither H. truncatus nor S. japonica reacted aggressively or defensively towards the other when in close proximity. Instances of imported Chinese Neocaridina spp. into central Honshu Island, Japan, almost certainly came from areas in southeast China identified in this study. These imported populations are predicted to spread northwards into the area where endemic Japanese branchiobdellidans occur.

Two species of North American branchiobdellidans, Cambarincola gracilis Robinson, 1954 and Cambarincola okadai Yamaguchi, 1933, have been reported in Europe for the first time. These branchiobdellidans together with Xirogoniton victoriensis Gelder and Hall, 1990 were found on signal crayfish, Pacifastacus leniusculus (Dana, 1852), collected from the Lot and Tarn River drainages in southern France. Specimens of X. victoriensis were also reported on the same host in the Mayenne River drainage in northeastern France. Brief morphological descriptions of the three alien branchiobdellidan species are given. These introductions are briefly discussed and compared with similar alien ectosymbiotic associations found in Japan.

Northern crayfish, Orconectes virilis (Hagen, 1870), were collected from 89 sites across Alberta, Saskatche - wan, Manitoba, Montana, North Dakota, and Minnesota. The entocytherid ostracod Thermastrocythere riojai (Hoff, 1943) was found on O. virilis at 45 of the 89 sites, distributed primarily in the eastern and southern portion of the study area. These observations of T. riojai greatly extend the known range of the species. The widespread distribution of T. riojai suggests that the dearth of entocytherid records from other parts of Canada is a result of nontargeted sampling rather than true absence. In addition, we report on observations of 3 noteworthy associations of oligochaetes with their crayfish hosts. RESUMEN.—Se colectaron especimenes del acocil Orconectes virilis (Hagen, 1870) en 89 sitios a lo largo de Alberta, Saskatchewan, Manitoba, Montana, North Dakota y Minnesota. Encontramos el ostracodo entociterido Thermastrocythere riojai (Hoff, 1943) en O. virilis en 45 de los 89 sitios, distribuidos principalmente en la parte sur y la parte este del area de estudio. Estas observaciones de T. riojai amplian considerablemente el area de distribucion conocida de esta especie. La distribucion extensa de T. riojai sugiere que la escasez de registros de entociteridos en otras partes de Canada es el resul- tado del muestreo inespecifico y no de una ausencia verdadera. Ademas, reportamos observaciones de 3 asociaciones sobre- salientes entre los oligoquetos y los acociles huespedes.

Branchiobdellidans, or crayfish worms, are species-rich in the Appalachian area of the southeastern United States. Even so, the Great Smoky Mountains National Park (GSMNP) in the southern Appalachians has only one record of a branchiobdellidan species. As part of the All Taxa Biodiversity Inventory, we undertook a general survey of the branchiobdellidans in the GSMNP. Crayfish were collected from a total of 15 sites distributed across 11 watersheds in the GSMNP We identified a total of 10 species of branchiobdellidans: 1 species of Bdellodrilus, 1 species of Oedipodrilus, 2 species of Pterodrilus, 1 species of Xironodrilus, and 5 species of Cambarincola. The composition and number of species per site ranged from 0 to 6, and individual branchiobdellidan species occurrence ranged from 1 to 11 sites. Our results suggest that multiple factors affect distribution and occurrence on certain hosts. Study of specimens of Cambarincola holostomus Hoffman, 1963 enabled us to add new morphological characters ...

A re-description of Cambarincola bobbi Holt, 1988 is provided based on observations made during recent collections in the Cumberland River Watershed in Tennessee. The branchiobdellidan Pterodrilus robinae, a new species, is described, along with a refinement for identification of five additional Pterodrilus species. Diversity, sympatry, and range extensions of branchiobdellidans from three areas of the Cumberland River Watershed are reported.

Orconectes virilis (northern crayfish) were collected from 67 sites in Alberta, Saskatchewan, Manitoba, and far-western Ontario, Canada, and yielded 2 species of branchiobdellidans, Cambarincola vitreus and Cambarincola chirocephalus. This is the first report of branchiobdellidans in the Prairie Provinces. Cambarincola vitreus was distributed across the study area, but C. chirocephalus appeared to be restricted to southeastern Saskatchewan, southern Manitoba, and the site in western Ontario. Neither branchiobdellidan species was observed on crayfish in the Beaver River or the South Saskatchewan River and associated tributaries upstream (west) of Saskatoon, Saskatchewan, despite multiple sampling at these locations.

Preserved specimens of the crayfish Cambarus bartonii, Orconectes limosus, and Orconectes virilis from 27 locations in New Brunswick, and O. limosus from Freshwater Lake, Cape Breton Highlands National Park, Cape Breton Island, NS, Canada, were examined for branchiobdellidans. Three species of branchiobdellidans—Bdellodrilus illuminatus, Cambarincola fallax, and Xironogiton instabilis—were identified on C. bartonii. Only branchiobdellidan cocoons were recorded on O. limosus, and only X. instabilis was recovered from O. virilis in New Brunswick. The introduction of O. limosus and O. virilis into New Brunswick does not appear to have simultaneously introduced non-native branchiobdellidans. No branchiobdellidans were found on introduced O. limosus from Freshwater Lake, NS. This work provides the first records of branchiobdellidans occurring in the Maritime Provinces of Canada, and establishes the eastern limit for these ectosymbionts in North America. In addition, one specimen of C. bartonii was fou...

Symbionts are well known to have benefi cial and/or pathogenic effects on crayfi sh health, and as such assert direct impacts on freshwater ecosystems and commercial astaciculture. Several reviews (e.g., Vogt 1999, Evans and Edgerton 2002, Edgerton et al. 2002, Longshaw 2011, citations therein) provide valuable resources of information on crayfi sh symbionts ranging from viruses to arthropods. Although a useful starting point for interested researchers, these reviews may no longer accurately refl ect many recent shifts in our understanding of branchiobdellidan taxonomy, biology and hostsymbiont interactions. Branchiobdellidans, or crayfi sh worms (Fig. 23.1), comprise an order in which all known species are obligate ectosymbionts of freshwater crustaceans, primarily astacoidean crayfi shes. No free-living members are known (although see Holt 1973a, Timm 1991). Consequently, our understanding of branchiobdellidan ecology and evolution is inextricably intertwined with that of their crayfi sh hosts. Recent studies on branchiobdellidans have illuminated several errors and inaccuracies in earlier works. Consequently, our aim for this chapter is to provide an overview of current knowledge of branchiobdellidans, with a particular focus on aspects that are of interest to astacologists. For additional detail we direct readers to Gelder and Williams (2015).

Branchiobdellidans are leech-like, obligate ectosymbionts primarily associated with astacoidean crayfish. The branchiobdellidan-host symbiosis is common among freshwater habitats in North and Central America, the Euro-Mediterranean, and East Asia. Branchiobdellidan taxonomic diversity varies across the distribution of the order, with more than two-thirds of the 22 total genera and approximately 140 total species found in North and Central America. No free-living members of Branchiobdellida are known, although individuals have been observed independent of a host in the benthic zone. The ectosymbiont association is obligate as embryonic development – within a cocoon – will only occur on a live host. Branchiobdellidans are opportunistic omnivores with diet varying with habitat, season, etc. Species display high morphological convergence, perhaps as a function of a shared symbiotic nature, with ecological adaptability varying widely across the taxon.

This is the first article of branchiobdellidans on blue crabs, Callinectes sapidus, in Chesapeake Bay, MD, USA. The ectosymbionts consisted of sympatric populations of Cambarincola mesochoreus and Cambarincola pamelae living on the ventral body surface and in the gill chambers of blue crabs. The occurrence of branchiobdellidans in the upper bay region was documented at ten sites during August and late September 2003. Branchiobdellidans were found on blue crabs following a rainy summer when salinity levels fell below 3 ppt. The blue crabs move into the freshwater estuaries during the summer, where they enter the habitats of crayfishes; however, the mechanism of transfer from one crustacean host to another is not known.

This is the first report of two North American branchiobdellidans, Sathodrilus attenuatus Holt, 1981, and Xironogiton victoriensis Gelder and Hall, 1990, on the signal crayfish, Pacifastacus leniusculus (Dana, 1852) introduced into Japan from the Columbia River system, northwestern North America. Signal crayfish from 12 localities in eastern and northern Hokkaido, Japan, were examined and each supported S. attenuatus. In addition, an individual of this species was found on preserved material from Ishikawa Prefecture, central Honshu. All of these branchiobdellidans reported in Hokkaido most probably came from the original population of signal crayfish introduced into Lake Mashu, Hokkaido, Japan, in 1930. It is suggested that the use of non-pathogenic branchiobdellidans, when present, provides an easy method for tracing the spread of crayfishes around Japan and could also be applied in other countries and continents. Specimens of X. victoriensis were only found on crayfish in a stream at Akashina in Nagano Prefecture, central Honshu, Japan. Although the signal crayfish appears to be displacing the endemic Japanese crayfish, C. japonicus, no native branchiobdellidans were found on any of the introduced signal crayfish examined.

To investigate the phylogenetic relationships of leeches, branchiobdellidans, and acanthobdellidans, whole nuclear 18S rDNA and over 650 bp of mitochondrial cytochrome c oxidase subunit I were acquired from 101 annelids, including 36 leeches, 18 branchiobdellidans, Acanthobdella peledina, as well as 28 oligochaetes and combined with homologous data for 17 polychaete outgroup taxa. Parsimony analysis of the combined aligned dataset supported monophyly of leeches, branchiobdellidans, and acanthobdellidans in 100% of jackknife replicates. Monophyly of the oligochaete order Lumbriculida with Acanthobdellida, Branchiobdellida, and Hirudinea was supported in 84% of jackknife replicates. These results provide support for the hypotheses that leeches and branchiobdellidans are sister groups, that acanthobdellidans are sister to them, and that together with the family Lumbriculidae they all constitute a clade within Oligochaeta. Results support synonymy of the classes Clitellata and the more commonly used Oligochaeta. Leeches branchiobdellidans, and acanthobdellidans should be regarded as orders equal to their closest relatives, the order Lumbriculida.

Crayfishes from 81 sites in New England yielded six species of branchiobdellidans — Bdellodrilus illuminatus, Cambarincola fallax, C. mesochoreus, C. philadelphicus, Pterodrilus missouriensis, and Xironogiton instabilis. This is the first report of the distribution of branchiobdellidans across the region. The Royal Ontario Museum, Toronto, Ontario, Canada, supplied branchiobdellidans, subsequently identified as C. mesochoreus, that had been removed from the gills of preserved marine crabs, Cancer borealis and Callinectes sp., reportedly collected from the Gulf of Maine, USA. Over 100 specimens of Hyas araneus, Cancer irroratus, C. borealis, and Carcinus maenas were collected at irregular intervals since 1978 from the coasts of northern Massachusetts, and Damariscotta estuary, Maine, USA and St. Andrews, New Brunswick, Canada, in an unsuccessful attempt to find a marine branchiobdellidan.

Using 18S rDNA, mitochondrial cytochrome c oxidase subunit I and morphological characters, the Branchiobdellidae (Annelida, Clitellata) were shown to form a monophyletic group distinct from the leeches using two distant ‘oligochaetes’ as outgroups. The study used 20 branchiobdellid species from 14 genera in four subfamilies with these representing each of the taxon’s three distributional regions in the Holarctic realm. No monophyletic groups were found using the gene sequence data that related to either geographical regions or currently recognized subfamilies. However, two monophyletic groups were strongly supported; the two European species of Branchiobdella and the combination of Sathodrilus attenuatus and Xironogiton victoriensis. The latter pair is taxonomically diverse, but sympatric on Signal crayfish, Pacifastacus leniusculus, in California, USA.

Different hypotheses have been proposed on the phylogenetic relationships of branchiobdellidans and aphanoneurans among the Annelida based on the anatomical and embryological characters. The 18S ribosomal RNA gene sequences have been analyzed from representatives of the three major taxa of the Annelida plus the branchiobdellidans and aphanoneurans to assess their phylogenetic relationships to each other. In this preliminary study, all of the phylogenetic analyses show the branchiobdellidans as a sister group to the leeches, rather than the oligochaetes. The position of the aphanoneurans is stable as an independent taxon that evolved after the polychaetes branched from the evolutionary stem, but before the ancestral oligochaetes emerged.

Branchiobdellidans, or crayfish worms, are ectosymbiotic clitellate annelids associated primarily with freshwater crayfishes. The main objectives of our study were to infer a molecular phylogeny for the North American Branchiobdellida, examine its congruence with morphology-based hypotheses of relationships at the subfamily and genus level, and use our dataset to assess consistency of GenBank-archived branchiobdellidan sequences. We used nucleotide sequence data from two mtDNA genes (COI and 16S rDNA) and three nuclear genes (28S rDNA, 18S rDNA, and ITS1) to estimate phylogenetic relationships among 47 described and one undescribed species of Branchiobdellida. We recovered a monophyletic branchiobdellidan clade with generally short branch lengths, suggesting that a large portion of the taxon has likely undergone a recent and rapid radiation in North America. Results from our phylogenetic analyses indicate that current taxonomic groupings are largely unsupported by the molecular data. All four subfamilies are either paraphyletic or polyphyletic, and only three of seven sampled non-monotypic genera were monophyletic. We found a high rate (49%) of inconsistency in GenBank-archived sequences, over 70% of which can be attributed to field- or laboratory-based error.

Specimens of Austropotamobius pallipes were found to support Branchiobdella italica, B. parasita, B. astaci, and B. hexodonta, either alone or in a combination, in Piedmont, Liguria, Lombardy, Emilia and Friuli‐Venezia Julia. The American Cambarincola mesochoreus occurs on the introduced Lousiana red swamp crayfish, Procambarus (S.) clarkii, also in Piedmont. A taxonomic key to the species reported in northern Italy is given.

Publisher Summary This chapter focuses on freshwater annelids of North America. Freshwater annelids (oligochaetes, branchiobdellidans, leeches, and acanthobdellids) can be found in both standing and flowing water and are often an important component of the aquatic food web. There are currently over 200 oligochaete species reported from North America that inhabit freshwater and estuarine bodies, including those in the lesser known groundwater and organic mud habitats. Most oligochaetes feed by ingesting sediment, but several North American genera prey on other worms or on a variety of small invertebrates. Branchiobdellidans or crayfish worms are leech-like ectosymbionts living primarily on astacoidean crayfish. 15 genera and 107 species of Branchiobdellidan are reported from North America. Leeches (Hirudinida), an important component of the benthos of most freshwater lakes, ponds, and quieter flowing sections of streams and rivers, can also be found. There are 76 leech species currently known from North America, most of which prey on chironomids, oligochaetes, amphipods, and molluscs. The chapter describes the ecology, life history, anatomy, morphology, ecology, evolution, and physiology of oligochaetes, branchiobdellidans, leeches, and acanthobdellids found in North America.

Xironogiton victoriensis n.sp. is described from the crayfish Pacifastacus leniusculus leniusculus on Vancouver Island, British Columbia, Canada. Collections of P. l. leniusculus from Goldstream River, and Colquitz Creek, Victoria, and Burdette Creek, Burnaby, Vancouver, in southwestern British Columbia yielded Cambarincola fallax, C. gracilis, C. montanus, and Xironogiton kittitasi. Specimens of each species of Xironogiton have been examined and a taxonomic key for the genus has been constructed. The information was also used in a Wagner analysis of Xironogiton which showed the genus to be a monophyly.

Analyses using PAUP (phylogenetic analysis using parsimony) were conducted on a matrix of 24 taxa and 26 characters. The taxa included generalised descriptions of a haplotaxid and a lumbriculid oligochaete as outgroups with a hypothetical branchiobdellidan ancestor and 21 genera as ingroups. The branchiobdellidan taxon is a monophyly, and using anatomical characters only, is composed of three suprageneric assemblages. The taxonomic rank of the taxon and assemblages cannot be determined until they have been compared with cladistic analyses on other closely related clitellate groups. In the meantime the currently accepted ordinal rank of Branchiobdellida and the major branchiobdellidan families should continue to be used, with the latter reflecting the assemblages of the genera in our analyses: Branchiobdellidae (including Holtodrilus n.gen., Sinodrilus n.gen., and Xironodrilus), Bdellodrilidae (including Caridinophila and Hidejiodrilus n.gen.), and Cambarincolidae. An analysis with two zoogeographical characters in the matrix caused the genera to be divided into their respective North American and Eurasian groups; however, the North American Triannulata appeared constantly in the latter group. The genera in the Branchiobdellidae, as defined by us, are found in both geographical regions, and predictably the assemblage was split in two.

The spermatozoa of six species belonging to the branchiobdellidan family Branchiobdellidae (i.e. Xironogiton victoriensis, Cirrodrilus kawamurai, Ankyrodrilus legaeus, Xironodrilus formosus, Branchiobdella kobayashii, Branchiobdella orientalis) were studied and compared to the other sperma-tozoa already described in the group. A parsimony analysis was performed on the spermatozoal data of the species examined, as well as on their somatic characters. The results of the two analyses were contrasted and a further parsimony analysis was run on the matrix comprising both sets of characters. The study of sperm ultrastructure confirmed the genera recognized with traditional somatic characters and the monophyly of the branchiobdellidans. Xironodrilus was proved to be the sister species of Ankyrodrilus and its inclusion into the family Branchiobdellidae was supported. Evolutionary hypotheses on intergeneric differences in the family consistent with its biogeography can be suggested by the cladograms: Xironogiton is an early offshoot of branchiobdellidan lineage migrating to North America and probably radiating only in recent times; Branchiobdella kobayashii has a spermatozoon completely different from that of the other species of the genus, thus suggesting a complex story for this widespread taxon with a disjunct distribution.

The zoogeographical distribution of branchiobdellidans (Annelida) and temnocephalidans (Platyhelminthes), ectosymbiotic on freshwater crustaceans, is reviewed and revised. In the areas where the Holarctic branchiobdellidans and Gondwanian temnocephalidans overlap, distributions of species and their respective hosts are reviewed in detail. Specimens of the branchiobdellid, Branchiobdella astaci, and the scutariellid temnocephalidan, Bubalocerus pretneri, were allowed to meet in vitro on the dorsal carapace of a crayfish, Austropotamobius pallipes. This usually resulted in the scutariellid being ingested by a branchiobdellid. Specimens of B. astaci were placed in contact with the host of B. pretneri, Troglocaris sp., a cave-dwelling shrimp and the branchiobdellid immediately moved away from the shrimp.

Annelids and arthropods have long been considered each other's closest relatives, as evidenced by similarities in their segmented body plans. An alternative view, more recently advocated by investigators who have examined partial 18S ribosomal RNA data, proposes that annelids, molluscs, and certain other minor phyla with trochophore larva stages share a more recent common ancestor with one another than any do with arthropods. The two hypotheses are mutually exclusive in explaining spiralian relationships. Cladistic analysis of morphological data does not reveal phylogentic relationships among major spiralian taxa but does suggest monophyly for both the annelids and molluscs. Distance and maximum-likelihood analyses of 18S rRNA gene sequences from major spiralian taxa suggest a sister relationship between annelids and molluscs and provide a clear resolution within the major groups of the spiralians. The parsimonious tree based on molecular data, however, indicates a sister relationship of the Annelida and Bivalvia, and an earlier divergence of the Gastropoda than the Annelida-Bivalvia clade. To test further hypotheses on the phylogenetic relationships among annelids, molluscs, and arthropods, and the ingroup relationships within the major spiralian taxa, we combine the molecular and morphological data sets and subject the combined data matrix to parsimony analysis. The resulting tree suggests that the molluscs and annelids form a monophyletic lineage and unites the molluscan taxa to a monophyletic group. Therefore, the result supports the Eutrochozoa hypothesis and the monophyly of molluscs, and indicates early acquisition of segmented body plans in arthropods.

The use of spermatozoan ultrastructural morphology to produce a meaningful phylogeny requires a critical number of descriptions from representative species in a taxon. Branchiobdella spermatozoan morphology is considered to be aberrant within the Clitellata. The description of the spermatozoon of B. kozarovi has extended the information on interspecific variation in this genus. The spermatozoa of this species have an acrosome (16 μm long), nucleus (4.5 μm long), midpiece (1.5 μm long), and tail (158 μm long), with a total length of about 180 μm. The acrosome is a thick helix with about 15 gyres, contains an acrosome tube and acrosome vesicle which has branches penetrating into the central subacrosomal space. The acrosomal “button” is located on the anterior surface, with the “cork” and secondary tube at the posterior. The rod-shaped nucleus has a concavity in both the anterior and posterior surfaces. The midpiece has seven, parallel mitochondria twisted about the central axis. The centriolar remnant with an indistinct ninefold symmetry in cross section is located posterior to the mitochondria. The prominent central sheath extends from the mitochondria to the end of the flagellum, which has no end piece. The axoneme has glycogen granules located along its length, and an helical marginal fiber. A review of descriptions of spermatozoa of Branchiobdella shows some inconsistencies. The observations of two anterior nephridial pores and an eversible penis completes the anatomical characters of B. kozarovi used in current branchiobdellidan taxonomy.

The body wall muscles in five species of branchiobdellidans are all arranged in the oligochaete pattern and the muscle fibres are obliquely striated. The structure of the circular muscle fibres do vary to some degree. The longitudinal muscle fibres in Ankyrodrilus legaeus, Branchiobdella kozarovi, and Xironogiton instabilis all are round circomyarian and thus double-obliquely striated. These species represent three of the four genera composing the family Branchiobdellidae. Although Bdellodrilus illuminatus and Cambarincola fallax, from the families Bdellodrilidae and Cambarincolidae, respectively, also possess a few round circomyarian fibres, most are ‘polyplatymyarian’ comparable to single-obliquely striated fibres. A similar division of branchiobdellidan families is obtained based on the number of anterior nephridial pores. The muscular structure in the branchiobdellidans shows both similarities and differences with the leeches and the lumbriculid oligochaetes. One phylogenetic explanation for this is that the branchiobdellidans separated from the common clitellate ancestor before the oligochaetes and leeches became recognizable taxa.

A brief review of the taxonomic history and integrity of selected North American branchiobdellidans is given to clarify their current status. The zoogeographical distribution of branchiobdellidans on the North American continent is presented based on published reports and information in the Catalog of the National Museum of Natural History, Smithsonian Institution, Washington, DC. These data are tabulated by species with locations referenced to zoogeographical subregions, political units of provinces, states, or countries, and source citations. Distributional anomalies of certain taxa are discussed.