Sphaerostoma tamarae, Sokolov & Shchenkov & Frolov & Vlasenkov, 2024

Sphaerostoma tamarae View in CoL sp. nov. ( Figs 1–2 View Figure 1 View Figure 2 )

Diagnosis. The presented species differs markedly from all other congeners in the distribution of the vitelline follicles (restricted to the hindbody vs. extensive in the both fore and hindbody) and the morphology of the ventral sucker (strongly protuberant vs. sessile). In addition, it differs from all Spaerostomum spp., except S. minus , in the uterus length (rather short with several loops vs. extensive, with numerous loops filling the significant volume of the hindbody), and from the mentioned listed species in the body length (≥ 3.2 mm vs. ≤ 1.1 mm) (compare with Looss, 1894; Thapar & Dayal, 1934; Szidat, 1944; Ślusarski, 1958; Kozicka, 1959; Koval, 1971; Lambert & Lambert, 1974).

Description. Body elongate, narrowed in middle third, 4,087 –3,164 X 1,093 –1,007. Tegument unarmed. Oral sucker oval, 287–350 X 301, opening subterminally. Ventral sucker rounded, 385–462 X 448–469, strongly protuberant rather than pedunculate; embedded in one pressure-fixed specimen. Ratio of oral sucker to ventral sucker width 1: 1.49–1.56. Forebody 31.3–33.2% of body length. Prepharynx very short or indistinguishable. Pharynx 161–189 X 154–189. Oesophagus 319–582 long. Intestinal bifurcation in posterior third of forebody. Caeca terminating blindly near posterior extremity of body. Postcaecal space 6.3–9.4% of body length.

Testes two, slightly oblique, intercaecal, approximately in mid-hindbody, with irregular outline, separated: anterior testis 140–228 X 154–210, sinistro-submedian; posterior testis 175–203 X 189–238, median or dextro-submedian. Posttesticular space 20.5–24.2% of body length. Cirrus sac cylindrical, 280–483 X 70–84, with bipartite seminal vesicle, tubular prostatic pars surrounded by prostatiс cells, ejaculatory duct, and short cirrus; proximal portion of cirrus sac dorsal to ventral sucker or entirely in forebody. Genital atrium shallow; common genital pore sinistro-submedian, slightly prebifurcal.

Ovary 3- to 5-lobed, larger than testes, 245–287 X 280–343, median, between testes, contiguous or partially overlapped by ventrally both or one testis. Oviduct, extending in anterior direction. Oötype with Mehlis’ gland anterior to ovary. Seminal vesicle canalicular. Laurer’s canal surrounded by glandular cells, opening dorsally on left side of body distinctly anterior to ovary. Uterus preovarian, intercaecal. Metraterm not differentiated from uterus, with sphincter around distal extremity, opening into genital atrium dorsal to male genital pore. Eggs operculate, deformed in balsam; length of least-deformed eggs 62–74. Vitellarum follicular; follicles in two lateral fields, extending from nearly posterior edge of ventral sucker to postcaecal space; dorsal and ventral to caeca, confluent in posttesticular space or nearly so. Common vitelline reservoir median, anterior to ovary. Excretory vesicle I-shaped, extending to posterior testis; pore terminal.

Material examined. Holotype. Gravid specimen (paragenophore), the Syumnyur River (tributary of Bolon Lake , Amur River basin), Bolon village, Khabarovsk Krai, Russia; 49°52'56''N; 136°7'52''E.; IPEE 1396 View Materials GoogleMaps ; Paratypes. Two gravid specimens (paragenophores) from the same locality; IPEE 1397 View Materials GoogleMaps .

Biology. Host: Rhynchocypris percnura mantschurica (Berg, 1907) ( Cyprinidae ). Site of infection: intestine. Infection indices: 6.5% (n = 62), one trematode specimen per infected host.

Distribution. Russian Far East.

Parasite GenBank NCBI accession number. 28S rRNA gene: PP002118.

Etymology. The specific name honors Tamara Vlasenkova, the mother of the last author.