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POPULATIONS OF A. PELEDINA The present evidence suggests that A. peledina from Alaska is distinct, to some degree, from European samples. However, Siberia and the Russian Far East have not been adequately sampled genetically for A. peledina. Accordingly, it is difficult to determine the genetic variability and population structuring of this species. Coupled with increased taxon sampling, additional genetic sampling of Alaskan populations could help to indicate whether they are a unique species or population. Sampling of quickly evolving nuclear loci or, ideally, next generation sequence data (e.g. RADSeq) would be useful for determining whether gene flow exists between the Alaskan and Nordic localities. Unfortunately, COI, the most common marker for determining differences between leech species and populations (de Carle et al., 2017; Tessler et al., 2018c; Mack et al., 2019), and additional nuclear loci did not amplify for these samples, potentially leading to some issues with missing data. Furthermore, given that no external morphological differences were noted between samples of Nordic and Alaskan A. peledina, we refrain from formal species or population delimitation analyses at this time. Nevertheless, the fact that the Alaskan population is sister to, and genetically divergent from, the Nordic samples suggests that this is not an invasive or nonnative species that was translocated only in recent times by humans, which would have been plausible given that the first records of this species in Alaska came from the 1970s (Holmquist, 1974; Hauck et al., 1979) and that it has not officially been reported since then, despite the clear importance of these American animals. The Nordic populations are fairly similar genetically, despite being sampled from multiple (albeit geographically close) countries. The maximum genetic distance at the COI locus is 1.52%, which is below the average value (~2.4%) typically reported for species of Hirudinea (Kvist, 2015; de Carle et al., 2017; Anderson et al., 2020; Mack et al., 2019; Iwama et al., 2019). However, the countries sampled are all in relatively close proximity. It would be most useful to add samples from central and eastern Russia. Unfortunately, a 12S sequence for A. peledina in the Baikal region of Russia from a recent publication was not made publicly available (Bolbat et al., 2019). MORPHOLOGY Our morphological examination and comparison of Paracanthobdella and populations of Acanthobdella help to characterize these species further. The scanning electron micrographs (Figs 6–9) and morphometry of facial hooks (N = 1280) help to accentuate the main external differences between the two species: (1) the presence or absence of a cup-shaped depression between rows of chaetae (anterior sucker); and (2) the chaetal dimensions and shape (Fig. 3; Table 3). The differences in both these characteristics become more notable as the species mature. The deep cupshaped anterior sucker, which is viewed as the most important distinguishing feature of P. livanowi, develops gradually through ontogeny from a flat state characteristic of juvenile individuals of the species. In A. peledina, the anterior end does not form a clearly separated sucker even in fully grown specimens, but a deep cavity appears between chaetae concomitantly with the growth of the animal. The shape of the chaetae differs between species: in A. peledina, chaetae are bent at a right angle and the breadth of the chaetae is similar in all rows, whereas in P. livanowi the angle is obtuse and chaetae in rows 4 and 5 have substantially greater breadth (Fig. 3; Table 3). The well-developed prostomium, which has been considered as another distinguishing feature of the species and the genus, was found to be less prominent and conspicuous than presented in previous studies (Epstein, 1987). Other studies have examined the internal morphology of these species (Bielecki et al., 2014); known differences from this work and others are summarized in Table 1. Acanthobdella peledina has the same morphology across the Nordic and Alaskan populations examined here and seems to be indistinguishable from Siberian populations (I. A. Kaygorodova & P. Świątek, unpublished scanning electron microscopy data) in other studies (Kaygorodova et al., 2012). Nevertheless, although we did not find differences between A. peledina from Alaska and Eurasia, it is entirely possible that detailed internal examinations might unearth differences, given that these populations appear to be divergent genetically. CONCLUSION AND FUTURE DIRECTIONS Our results help to shed light on the hook-faced fish worms (Acanthobdellida): an ancient lineage that is most closely related to leeches, and demonstrate that Acanthobdellida species and populations have diverged fairly recently. It is even possible that there are multiple species within A. peledina. Specifically, the American and Nordic populations appear to be distinct genetically and are likely to be isolated reproductively. However, there are important gaps to fill in the knowledge of the populations of this species before definitive action is taken on determining whether they represent the same species. Those gaps are as follows: (1) adding specimens from localities for central and eastern Russia; (2) obtaining additional genetic data (i.e. COI and additional nuclear data) for Alaskan samples; and (3) looking for internal morphological differences between populations., Published as part of Carle, Danielle Božena De, Gajda, Łukasz, Bielecki, Aleksander, Cios, Stanisław, Cichocka, Joanna M., Golden, Heidi E., Gryska, Andrew D., Sokolov, Sergey, Shedko, Marina Borisowna, Knudsen, Rune, Utevsky, Serge, Świątek, Piotr & Tessler, Michael, 2022, Recent evolution of ancient Arctic leech relatives: systematics of Acanthobdellida, pp. 149-168 in Zoological Journal of the Linnean Society 196 on pages 165-166, DOI: 10.1093/zoolinnean/zlac006, http://zenodo.org/record/7037718, {"references":["de Carle D, Oceguera-Figueroa A, Tessler M, Siddall ME, Kvist S. 2017. Phylogenetic analysis of Placobdella (Hirudinea: Rhynchobdellida: Glossiphoniidae) with consideration of COI variation. Molecular Phylogenetics and Evolution 114: 234 - 248.","Tessler M, Weiskopf SR, Berniker L, Hersch R, Mccarthy KP, Yu DW, Siddall ME. 2018 c. Bloodlines: mammals, leeches, and conservation in southern Asia. Systematics and Biodiversity 16: 488 - 496.","Mack J, de Carle D, Kvist S. 2019. Prey, populations, and the Pleistocene: evidence for low COI variation in a widespread North American leech. Mitochondrial DNA. Part A, DNA Mapping, Sequencing, and Analysis 30: 749 - 763.","Holmquist C. 1974. A fish leech of the genus Acanthobdella found in North America. Hydrobiologia 44: 241 - 245.","Hauck AK, Fallon MJ, Burger CV. 1979. New host and geographical records for the leech Acanthobdella peledina Grube 1851 (Hirudinea, Acanthobdellidae). Journal of Parasitology 65: 989.","Kvist S. 2015. Does a global DNA barcoding gap exist in Annelida? Mitochondrial DNA. Part A, DNA Mapping, Sequencing, and Analysis 27: 2241 - 2252.","Iwama RE, Oceguera-Figueroa A, de Carle D, Manglicmot C, Erseus C, Miles NMS, Siddall ME, Kvist S. 2019. Broad geographic sampling and DNA barcoding do not support the presence of Helobdella stagnalis (Linnaeus, 1758) (Clitellata: Glossiphoniidae) in North America. Zootaxa 4671: 1 - 25.","Bolbat AV, Sorokovikova NV, Kaygorodova IA. 2019. Assessing efficiency of the mitochondrial 12 S marker fragment for the use in reconstruction of the phylogeny of acanthobdellids. Russian Journal of Genetics 55: 1554 - 1558.","Epstein VM. 1987. Tip KOLCHATYE CHEVRI - Annelida [Phylum RINGED WORMS - Annelida]. In: Bauer ON, ed. Opredelitel parazitov presnovodnykh ryb fauny SSSR [Identification key to parasites of the freshwater fishes of the fauna of the USSR]. Leningrad: Akademiya Nauk SSSR, Zoologicheskii Institut, Izdatelstvo Nauka, 340 - 372.","Bielecki A, Cichocka JM, Jelen I, Swiatek P, Plachno BJ, Pikula D. 2014. New data about the functional morphology of the chaetiferous leech-like annelids Acanthobdella peledina (Grube, 1851) and Paracanthobdella livanowi (Epshtein, 1966) (Clitellata, Acanthobdellida). Journal of Morphology 275: 528 - 539.","Kaygorodova IA, Dzyuba EV, Pronin NM. 2012. Leech-like parasites (Clitellata, Acanthobdellida) infecting native and endemic eastern Siberian salmon fishes. The Scientific World Journal 2012: 652827."]}

Species: Paracanthobdella livanowi (Epstein, 1966) NOTES ON OUR TAXONOMIC SCHEME Here, we present a taxonomic scheme for hookfaced fish worms (Acanthobdellida). Despite its historical significance, we do not retain the family Paracanthobdellidae for P. livanowi, because both morphological and molecular analyses suggest that the two acanthobdellidan species share many characteristics. Initially, Acanthobdellidae and Paracanthobdellidae were differentiated based on a few morphological differences that were viewed as taxonomically important (Epstein, 1987). Specifically, members of Paracanthobdellidae were said to possess a primitive prostomium and well-developed anterior sucker. Although the two-family system seemed well supported at the time, we have found that the prostomium of P. livanowi is not more conspicuous or developed than the anterior region of A. peledina. Moreover, the area between rows of chaetae in the cephalic extremity of the latter can be deepened such that it resembles a shallow sucker. It should also be noted that juvenile individuals of P. livanowi do not bear well-developed anterior suckers; this character is only common to large-bodied individuals of the species. The sum of this evidence suggests that the morphological differences between the two acanthobdellidan species, although pronounced, are not sufficient to warrant two families. We therefore classify both Acanthobdella and Paracanthobdella under the single family Acanthobdellidae. We establish a new suborder Acanthobdelliformes to match better the taxonomy erected for Hirudinea in prior work (Tessler et al., 2018a), which divided Hirudinida (leeches) into five suborders (Americobdelliformes, Erpobdelliformes, Hirudiniformes, Glossiphoniiformes and Oceanobdelliformes). Acanthobdelliformes is defined by the presence of chaetae on each of five contiguous segments in the anterior body region, and 31 segments (mid-body ones are quadrannulate with annulus a3 being subdivided) (Sawyer, 1986; Purschke et al., 1993; Bielecki et al., 2014). Although previous studies have reported different numbers of segments for each acanthobdellidan species (e.g. 29 for Acanthobdella and 30 for Paracanthobdella; Bielecki et al., 2014), this discrepancy is attributable to presumed differences in the number of segments that comprise the posterior sucker, which should be substantiated by a careful morphological analysis in the future. The higher taxonomy (class through order) follows our prior classification scheme (Tessler et al., 2018a). Others have constructed alternative schemes, but we feel ours to be internally more consistent and phylogenetically appropriate. Alternative classifications include subclass Acanthobdellidea Livanow, 1905 (Archihirudinea Lukin, 1956 is an equivalent synonym) and subclass Acanthobdelliones (Epstein, 1987). It is important to note that the two acanthobdellidan species were originally in the same genus (Epstein, 1966). However, in the late 1980s, P. livanowi was given its own genus and even family (Epstein, 1987). Ultimately, the decision to classify a monophyletic lineage of two species into one vs. two genera and/or families is subjective. We have decided to retain the genus-level classification proposed by Dr Epstein (rather than lump them) to honour his contributions to the study of Hirudinea and accentuate the differentiation of the anterior sucker, chaetae and internal anatomy that separates these species. This is also in concordance with the suggestions in the most recent, broad morphological comparison paper (Bielecki et al., 2014); see Table 1 for some of the differences between these species., Published as part of Carle, Danielle Božena De, Gajda, Łukasz, Bielecki, Aleksander, Cios, Stanisław, Cichocka, Joanna M., Golden, Heidi E., Gryska, Andrew D., Sokolov, Sergey, Shedko, Marina Borisowna, Knudsen, Rune, Utevsky, Serge, Świątek, Piotr & Tessler, Michael, 2022, Recent evolution of ancient Arctic leech relatives: systematics of Acanthobdellida, pp. 149-168 in Zoological Journal of the Linnean Society 196 on pages 161-162, DOI: 10.1093/zoolinnean/zlac006, http://zenodo.org/record/7037718, {"references":["Epstein VM. 1966. Acanthobdella livanowi sp. n. - a new species of the ancient leeches (Archihirudinea) from Kamchatka. Doklady Akademii Nauk SSSR 168: 955 - 958.","Epstein VM. 1987. Tip KOLCHATYE CHEVRI - Annelida [Phylum RINGED WORMS - Annelida]. In: Bauer ON, ed. Opredelitel parazitov presnovodnykh ryb fauny SSSR [Identification key to parasites of the freshwater fishes of the fauna of the USSR]. Leningrad: Akademiya Nauk SSSR, Zoologicheskii Institut, Izdatelstvo Nauka, 340 - 372.","Tessler M, de Carle D, Voiklis ML, Gresham OA, Neumann JS, Cios S, Siddall ME. 2018 a. Worms that suck: phylogenetic analysis of Hirudinea solidifies the position of Acanthobdellida and necessitates the dissolution of Rhynchobdellida. Molecular Phylogenetics and Evolution 127: 129 - 134.","Sawyer RT. 1986. Leech biology and behaviour. Wotton-under- Edge: Clarendon Press.","Purschke G, Westheide W, Rohde D, Brinkhurst RO. 1993. Morphological reinvestigation and phylogenetic relationship of Acanthobdella peledina (Annelida, Clitellata). Zoomorphology 113: 91 - 101.","Bielecki A, Cichocka JM, Jelen I, Swiatek P, Plachno BJ, Pikula D. 2014. New data about the functional morphology of the chaetiferous leech-like annelids Acanthobdella peledina (Grube, 1851) and Paracanthobdella livanowi (Epshtein, 1966) (Clitellata, Acanthobdellida). Journal of Morphology 275: 528 - 539."]}