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Metacestode infestation of Semibalanus balanoides (L., 1767) and Balanus crenatus Bruguiеre, 1789, collected in the Barents and White Seas and in the northern part of the Sea of Okhotsk in 2020 and 2021, respectively, was studied. A total of 313 S. balanoides from Mogilnaya Bay of Kildin Island (Barents Sea) and isolated mature wrinkled barnacles B. crenatus, two and four specimens from the Pechora Sea and Kandalaksha Bay of the White Sea, respectively, were examined in 2020. Metacestodes Fimbriarioides intermedia (Fuhrmann, 1913) (Cyclophyllidea, Hymenolepididae) were found in 1.0 ± 0.6% of S. balanoides in the Barents Sea with an invasion intensity (I. I.) of 2-5 specimens, and in one of two B. crenatus from the Pechora Sea (I. I. 15 specimens). For the first time, in both B. crenatus from the Pechora Sea, taken from the valves of the mussel Mytilus edulis, metacestodes Microsomacanthus sp. I (I. I. 13 and 20 specimens) with proboscis hooks 38-41 (39.4 ± 0.1) µm long and blades of 9.5-11 (10.7 ± 0.1) µm were obtained. One of four B. crenatus from the Kandalaksha Bay was infected with another Microsomacanthus sp. II (I. I. 19 specimens) with proboscis hooks 44.0-49.5 (45.7 ± 0.5) µm long and blades 14.0-16.0 (14.8 ± 0.07) µm long. A total of 362 S. balanoides were collected and dissected in Gizhiginskaya Bay of the northern part of the Sea of Okhotsk in 2021, of which 8.0 ± 1.4% were infected with F. intermedia metacestodes (I. I. 1-19 specimens). Study results of the infestation of S. balanoides on the Koni-Pyagin coast of the Sea of Okhotsk (according to the collections of 2006-2007) were supplemented and clarified. Description of metacestodes and the taxonomic affiliation of cysticercoids Microsomacanthus spp. are given., (© 2024. Pleiades Publishing, Ltd.)

Two cysticercoids, belonging to ascocercus type, namely euascocercus and multicercus, were found in haemocoele of dragonfly larvae of the genus Aeshna from the lakes of the Magadan Province. The cysticercoid of Schistotaenia srivastavai Raush, 1970 (euascocercus) is formed of the outer (exocyst) and inner (endocyst) envelopes, containing the scolex and larval strobila. The outer and inner surfaces of the exocyst are represented by the tegument covered with microvilli. The microvilli of the outer tegument are restricted by the surface layer, consisting of granular and fibrillar material, and possess different structures at different stages of post-embryonic development. The multicercus of Mircia shigini (Konyaev et Gulyaev, 2006) is able to multiply asexually by the endogenous budding. The daughters' individuals are formed in the envelope of the multicercus that represents the tegument bearing microvilli. These microvilli are also restricted by the surface layer. The morphology and development of each individual cysticercoid of the multicercus is similar to those of euascocercus. The production of a great amount of cysticercoids, and the presence of the surface layer resembling the laminated layer of Echinococcus, relates multicercus to hydatid cysts.

Morphogenesis of the true ascocercus (euascocercus or euascocysticercoid) was studied in Schistotaenia srivastavai, a relatively rare parasite of the Red-necked Grebe Podiceps griseigena, and of intermediate hosts, damselfy and dragonfly larvae, in the Okhotsk-Kolyma region. Stages of postembryonic development were reconstructed by the material from spontaneously infected dragonflies' larvae, which corresponded to the published data on the development of metacestodes from the genus Tatria s. l. studied by Mrazek (1927) and Rees (1973). The euascocercus is the most widespread morphological modification of ascocysticercoids among Schistotaeniidae. According to Gulyaev (1989) and our data, the species of the family (the genus Mircia) have also a polycephalic modification of the ascocercus, or the multicercus. It represents a maternal individual that is filled with numerous small filial cysticercoids during the postembryonic development. These cysticercoids are formed of individual buds in the outer wall; later on, they are gemmated into the primary cavity of the maternal individual. Consequently, each daughter individual possesses its own single-layer exocyst, homological to the inner layer of the two-layered exocyst of the other ascocerci. Supposedly, exocyst's outer layer of monocephalic ascocerci is homologous to the outer wall of multicercus' maternal individual. Finally, diagnostic features of the third modification of ascocerci, namely megalocercus, described in the uniquely large metacestode Dioecocestus asper, corresponds to the characteristic of the metacestode Schistotaenia tenuicirrus, studied by Boertje (1975). S. tenuicirrus differs from D. asper in the spiral configuration formed on the entire surface of the endocyst (in D. asper, only on its dorsal side), and in a low number of proglottids in the larval strobile (D. asper possesses a multisegmental strobile). The reason, why so large protective envelopes (exo- and endocysts) develop in S. tenuicirrus with relatively small size of prospective body, remains unclear. Thus, life cycles of Schistotaeniidae represent all three modifications of the ascocerci: the true ascocercus (Shistotaenia, Tatria, Ryjikovilepis, Joyeuxilepis), the multicercus (genus Mircia), and the megalocercus (S. tenuicirrus).

The megalocercus of Dioecocestus asper (Mehlis 1831) from the haemocoele of dragonfly larvae possesses two envelopes: outer (exocyst) and inner (endocyst) ones. The exocyst contains the large endocyst and larval strobila with scolex attached to the latter. Outer and inner surfaces of these envelopes are organized as the tegument and have some structural differences. The exocyst is covered with slender microvilli. Its outer tegument contains numerous mitochondria; the inner one is filled with lipid droplets released into the exocyst's cavity. The well-developed protonephridial (excretory) system consisting of flame cells, collecting ducts and canals is the unique feature of the exocyst, noted for the first time. Thick (more, then 50 microm) distal cytoplasm of the outer tegument of the endocyst is the place of accumulation of uniform globules looking like a hyaloid layer. This outer layer together with underlying fibrous layer (up to 20 microm), apparently, protect the scolex and larval strobila during the transfer through feather clump in the stomach of grebes, definitive hosts of D. asper. Muscle cells of both envelopes retain their synthetic activity even in the fully developed metacestode. Probably, they are the main structural element, which produces fibers of the extracellular matrix and maintains the integrity of protective envelopes of the megalocercus.

Investigations were undertaken using light and transmission electron microscopy to clearly delineate the morphology of the cercomer, i.e. the protective envelopes and tail appendage, in cysticercoids of Microsomacanthus paraparvula, which develop in the haemocoel of the caddisworm Grensia praeterita (Insecta: Trichoptera). Two protective envelopes, the exocyst and endocyst, were identified. The non-cellular exocyst is found to consist of granular material and of thin, dense membrane-like layers, which are located parallel to each other. The exocyst of the mature metacestode tightly adjoins the outer surface of the endocyst, containing prospective parts (the scolex and the neck), except for the areas at its poles. A long tail appendage is located outside the exocyst. Evidence was found to indicate the existence of active synthetic processes occurring in the tail appendage. Non-cellular exocysts are widely distributed within metacestodes of the families Hymenolepididae and Dilepididae, and, presumably, are formed by means of glandular secretions from the oncosphere, given the early appearance of non-cellular exocysts in ontogeny.

Metacestodes Dioecocestus asper (Cyclophyllidea) habe been found in larvae of dragonflies Aeshna spp. (Odonata, Anisoptera) from the lakes of the Upper Kolyma and northern Okhotsk seaboard basins (3.8 +/- 0.46 % of n = 1730 and 0.09 +/- 0.09 % of n = 1065, respectively) for the first time. Thus, participation of dragonflies in lifecycle of Dioecocestidae has been proved. Larvagenesis of D. asper (from spontaneously infested hosts) have been retraced from the stage of primary lacuna up to fully-developed metacestode. Process of development of the metacestode D. asper includes two invaginations. The first one follows the start of exogenous formation of undifferentiated anlage of the cystoscolex; as the result, the latter settles to the bottom of the forming two-layered exocyst homologous to the cercomer. Subsequent development of the prospecive part and the endocyst occurs in the closed amniotic cavity of the sacciform exocyst, the back pole of which preserves embryonic hooks and the front pole possesses the invaginational channel. At the stage of early scolexogenesis, the endocyst with the anlage of the strobila and the scolex separates from the exocyst and the excretory atrium forms on the back pole of endocyst. At the stage of late scolexogenesis, the formation of the larval strobila (differentiation of the sex) is completed and the second invagination starts. Strong retractors pull bottom of the stribila into the endocyst; simultaneously, the front part of it is also turned inside. The invaginational pore of the endocyst of fully-developed metacestode D. asper remains open; the scolex with the most part of the strobila is located outside the endocyst in the cavity of the exocyst. The encystment of the metacestode occurs when the temperature rises up to 40 degrees C (i. e., reaching the temperature of the definitive host). Morphogenesis of the uniquely large (among the cyclophyllid metacestodes developing in the invertebrate intermediate host) metacestode D. asper lasts for about two--three years and is associated with the developmental terms of the intermediate host, the larva of the Aeshna dragonfly, in climatic conditions of the Far North-East of Asia. Early stages of morphogenesis of D. asper were noted at the dragonfly larvae of the age of 0+ (at September) and 1+ (at June), and fully-developed metacestodes, only at elder ages. Owing to the extremely large size of D. asper metacestodes we refer them as a specific modification of cysticercoids: a megalocercus (Megalocercus). Common features were noted in the structure and morphogenesis of D. asper megalocercus and ascocerci of Schistotaeniidae. High morphological similarity of these metacestodes and their development in relative taxonomic groups of intermediate hosts, larvae of amphibiotic insects (the dragonfly larvae, and occasionally the larvae of mayflies or water-bugs), prove their belonging to the same philogenetically related group of metacestodes Cyclophyllidea, emerging independently from other groups of cysticercoids. We suggest naming this morpho-ecological group of larvocysts as ascocerci (or ascocysticercoids). The use of the name "ascocercus" as nominal nomenclature for group of larvocysts supposes the change of the name "ascocercus Schistotaeniidae" to "euascocercus".

The pygmaeus-species group is composed of close related species from the genus Microphallus in which metacercariae develop inside daughter sporocysts without encystment. Infection of periwinkles Littorina (Neritremna) spp. with intramolluscan stages of a new species of this group (Microphallus kurilensis sp. nov.) was recorded on the coasts of Sakhalin and Kuril islands, north of the Sea of Okhotsk and Chukchi Peninsula (the Bering Sea). Application of molecular methods allowed us to establish that M. kurilensis metacercariae are conspecific with one of the morphotypes of microphallid adults obtained from the intestine of the Pacific common eider (Somateria mollissima v-nigrum), which was shot in the north of the Sea of Okhotsk (Galaktionov, Olson, and Blasco-Costa, in press). The adults of the same morphotype were recorded in the Pacific common eider from the northwestern part of the Bering Sea (Chukchi Peninsula). In the course of experimental infection of the slaty-backed gull Larus schistisagus chicks with metacercariae of M. kurilensis, few microphallid adults were obtained. These adults were identical in their morphology with specimens of the microphallid morphotype from the Pacific common eider, which had been identified as M. kurilensis based on molecular data. Morphological description of metacercaria and adult of M. kurilensis and list of their differences from the same developmental stages of other species from pygmaeus-group are provided. It is concluded that M. kurilensis is transmitted in the host system including periwinkle Littorina (Neritrema) and seaducks (predominately, Pacific common eider). Most probably, distribution of M. kurilensis is not limited by the north Asiatic coast but expanded to the North American coast of the Pacific Ocean.

The genus Microsomacanthus was created by Lopez-Neyra (1942). Its type species, Taenia microsoma Creplin, 1829, was described from the common eider Somateria mollissima and remained a collective species until Fuhrmann (1913) revealed its diagnostic features on the original material. Simultaneously he described two new species of "microsoma-Gruppe", Microsomacanthus jaegerskioeldi (Fuhrmann, 1913) and M. diorchis (Fuhrmann, 1913). Fuhrmann stressed that under the name of Hymenolepis microsoma Cohn (1901) gave a description of quite different species (a parasite of the freshwater duck Anas penelope). Much earlier Leuckart (1879) used the same name for the denomination of a certain larval form from freshwater snails, without any experimental verification. Fuhrmann (1932), in conflict with himself, synonymized Taenia microsoma Creplin, 1829 and Hymenolepis microsoma sensu Cohn, 1901 and thus caused new inadequacy. Spassky et Spasskaja (1954) confirmed the validity of the genus Microsomacanthus Lopez-Neyra, 1942 and supplemented its diagnosis with such significant criteria as the number of rostellar hooks (10) and the absence of any accessory structures in the copulatory apparatus. Intermediate hosts of M. microsoma proved to be gammarids (Belopolskaya, 1952). Nevertheless Yamaguti (1959) synonymized a larval form from pond snails (Monocercus lymnaei Villot, 1883) with Microsomacanthus microsoma, meanwhile Hymenolepis microsoma sensu Cohn, 1901 was shown to be a synonym of Microsomacanthus compressa (Linton, 1892). The life cycle of the latter species includes copepods as intermediate hosts and freshwater gastropods as storage (reservoir) ones. Schmidt (1986) and Czaplinski and Vaucher (1994) chose M. compressa as an illustrative example of the genus Microsomacanthus instead of its type species. The diagnostic features of the genus remain vague, so the list of its synonyms attained as much as 17. Fuhrmann's material on M. microsoma, as well as on the two other species of "microsoma-Gruppe", deposited in the Museum of Natural History (Geneva) was reinvestigated. The diagnostic features of the group formulated by Fuhrmann (1913) were confirmed and enlarged. Large assemblage of species corresponding to M. microsoma by morphology (small gonads, early resorption of testes, formation of eggpacket in the uterus) and biology (gammarids as intermediate hosts) is established. This group is taken as the nominotypical subgenus Microsomacanthus (Microsomacanthus). For the antithetic group which life cycle corresponds to that of M. compressa (a species with comparatively large gonads, with testes persisting till the maturation of female gonads and eggs disseminating one by one from the uterus) we erect a new taxon, Microsomacanthus (Leucartcohnacanthus subgen. nov. Microsomacanthus paracompressa (Czaplinski, 1956) is selected as its type species. Representatives of both subgenera of the genus Microsomacanthus may have rostellar hooks of the same length. Literary data on their larval forms were often based on misidentification.

Microsomacanthus diorchis (Fuhrmann, 1913) is redescribed and illustrated on the basis of the type-material and new findings from common eider Somateria mollissima captured in Iceland and specimens from the same host species from the Barents, White and Bering Seas. A lectotype is designated and an amended diagnosis is provided. The main differentiating features of M. diorchis are the size and shape of rostellar hooks and the cirrus, the well-marked delay in the antiporal testis development and the bow-shaped uterus. This parasite is shown to be specific to S. mollissima for both Atlantic and Pacific populations of the host. Hymenolepis (Microsomacanthus) somateriae of Bishop & Threlfall (1974) [nec M. somateriae Ryzhikov, 1965] is recognised as a synonym of M. diorchis (Fuhrmann, 1913). The taxonomic position of the species described as Aploparaksis murmanica Baylis, 1919 from common eider is discussed.

Investigation of the helminth fauna was carried out in different amphipode species at Bering Sea coast of Chukchi Peninsula in July-September 2004. More than 3700 amphipode specimens of the following three species were examined: Lagunogammarus setosus Dementieva, 1931 (n = 2772 specimens) (Gammaridae), Eogammarus barbatus Tzvetkova, 1965 (n = 471), and Spinulogammarus subcarinatus (Bate, 1862) (n = 509) (Anisogammaridae). Numerous metacestodes of four hymenolepidid species from the genus Microsomacanthus Lopez-Neyra, 1945 were found, namely gull parasites M. ductilis (Linton, 1927) and M. lari (Yamaguti, 1940) comb. n., and eider parasites M. minimus Ryjikov, 1965 and M. somateriae Ryjikov, 1965. Relative selectivity of cestodes in the choice of intermediate host is revealed: M. ductilis and M. somateriae are found on L. setosus only, while M. minimus and M. lari are found in both species of anisogammarids. Brief descriptions and figures of metacestodes are given; data on extensiveness and intensity of the intermediate hosts invasion in six collection localities are provided. The greatest extensiveness of invasion is recorded in L. setosus (E. I. in M. ductilis was 1.98 %, in M. somateriae - 1.84 %). The identification of metacestodes is confirmed by the finding of mature cestodes of all above species in two Pacific eider specimens (Somateria mollissima var. nigrum) and three gull specimens (one Larus argentatus and two L. hyperboreus) captured in the area where the largest sample of Gammaridae was collected.

Microsomacanthus jaegerskioeldi (Fuhrmann, 1913) is redescribed and illustrated on the basis of type-material from Somateria mollissima captured in the vicinity of Göteborg (Sweden) and specimens from the same host from the Barents Sea, off Iceland and in the East Siberian Sea. A lectotype is designated and an amended diagnosis is provided. Previous reports of this parasite in various hosts and localities are discussed, and the geographical distribution and host range of the species are updated.

A new species of the genus Microsomacanthus Lopez-Neyra, 1942 is described. The material was collected from eider ducks (Somateria mollissima, S. fischeri, and S. spectabilis) in the Chaunskaya lowland (North-Western Chukotka). Microsomacanthus parasobolevi sp. n. differs from the closest species of this genus (M. polystictae Regel, 1988, M. sobolevi Spassky et Jurpalova, 1964; M. strictophalius Tolkatchieva, 1966) by the structure and size of the copulatory organ. Wide distribution of the new species was detected based on the cestodes collections from eider ducks of the Eastern Chukotka, Lena River mouth (in the museum of the Institute of the Parasitology RAS) and Iceland (Karl Skirnisson's cestode collection of the common eider). Furthermore, this species was reported once of the common eider in Newfoundland and Labrador, but erroneously identified as Hymenglejns (Microsomacanthnis) formosoides Spasskaja et Spassky, 1961 by Bishop and Threlfall (1974).

Four species of the genus Echinatrium Spassky et Jurpalova, 1965 are redescribed. The material was collected from ducks of the Chaunskaya lowland (Chukotka). Morphological study of genital apparatus in cestode species found was carried out. According to the priority rule, it is suggested that the genus Monosaccanthes Czaplinski, 1967 is a junior synonym of the genus Echinatrium.

There are 5 species of the genus Dicranotaenia recorded in the North-west of Chukotka: Dicranotaenia coronula (?) (Dujardin, 1845), D. fallax (Krabbe, 1869), D. parvisaccata (Shepard, 1943), D. sacciperum (Mayhew, 1925) and D. clangulae sp. n. Brief characteristic of the new species. Rostellar hooks 24-28, their total length 17-20 mkm, blade 10-12 mkm, guard processus 7-9 mkm, basis 13-15 mkm. Blade is parallel to basis, significantly longer than guard processus, hook form of aploparaksoid type. Cirrus pouch with s-like bend and distal enlargement (as in D. fallax). Cirrus almost cylindrical, 115-136 mkm in length, diameter at base 20-23 mkm, distal diameter 13-16 mkm. Internal acessory sac (SAI) situated anterior and dorsal from base of cirrus, gradually attenuate to distal end, length of SAI 27-30, diameter at base 30-32. Descriptions of adult forms and metacestodes of four other species are given.

It was found out, that the cestode Microsomacanthus paraparvula Regel, 1994 being a common parasite of diving ducks in Chukotka uses a caddisfly Grensia praeteria (Trichoptera) as an intermediate host in its life cycle. Mature fragments of the cestode have been collected from droppings of the experimentally infected nestling of the kittiwake Rissa tridactyla (non-specific host) and used for the fine morphology study of embryonic shells and for an infection of intermediate hosts.

Microsomacanthus paraparvula sp. n. differs from all other species of the genus by extremely small length of proglottids and genital organs. The size and general shape of the hooks closely resemble those of M. parvula (Kowalewski, 1904). However, the new species performs twice as many proglottids for the same specimen size. The dimensions and topography of the internal organs are also different, the cirrus sac size being one third of that in M. parvula. Finally, the new species represents a distinctly different life cycle using the caddisfly as the intermediate host.

8 species of cestodes belonging to the family Hymenolepididae were recorded from Polysticta stelleri (Pall.) in the Chaun lowland. A new species of hymenolepidids, Microsomacanthus polystictae sp. n., differing from close (by the length of proboscis hooks) species of the genus in the structure and size of the copulatory organ is described. Brief diagnosis of the new species: cestodes small, specimens ending with hermaphrodite proglottids 2.44 to 2.91 mm long. Proboscis hooks 0.038 to 0.042 mm long, blade 0.011 to 0.012 mm long. Strobila of maximum length with 84 proglottids. In young male proglottids the aporal testis half as large as poral and median ones, in well-developed proglottids testes 0.020 to 0.026 X 0.023 to 0.032 mm. Bursa of cirrus 0.150 to 0.190 X 0.020 to 0.028 mm. Cirrus 0.107 mm long, basis 0.008 to 0.011 mm wide, median part 0.012 to 0.014 mm wide, apical part 0.004 mm wide. Cirrus armed with spines 0.003 to 0.004 mm long. Ovary bilobate, yolk gland compact. Spermatheca small, situated medially, in front of poral ovary lobe. Copulative part of vagina sacciform, 0.086 to 0.118 mm long, 0.023 to 0.037 mm maximum width. Distally and proximally vagina with muscular sphincters. Uterus sacciform, with 35 to 40 embryos. There were no mature "eggs" in the material.