Psilochasmus urbeni Achatz, Morton, Orlofske & Tkach 2025, sp. nov.

Psilochasmus urbeni Achatz, Morton, Orlofske & Tkach sp. nov. Fig. 4 c, d Type material. Holotype: HWML -218110, labeled Aythya marila, small intestine, Stump Lake, Nelson County, North Dakota, USA, 6 Nov 2006, coll. V. V. Tkach. Paratypes: HWML -218111 (hologenophore), Aythya affinis, small intestine, Lake Winnibigoshish, Itasca County, Minnesota, USA, 1 Nov 2007, coll. V. V. Tkach; UWSP–PARA (3 paratypes): Aythya affinis, small intestine, Green Bay, Lake Michigan, Oconto County, Wisconsin, USA, 24 Nov 2019, coll. S. A. Orlofske. Description. Based on 5 specimens (measurements of holotype are given in text; measurements of entire series are given in Table 2). Body elongate, cylindrical, widest near level of testes, 6,711 × 1,307, with narrow, retractable, muscular tail-like process. Body length to width ratio 5.1. Ratio of body width at level of testes to body width at level of ventral sucker 1.3. Tegument armed. Forebody length represents 35 % of body length. Oral sucker subterminal, subspherical, 422 × 417. Ventral sucker protuberant with deep cavity, consisting of strongly muscular portion and extensive surrounding sub-tegumental rim, 785 × 830. Oral sucker to ventral sucker width ratio 0.5. Prepharynx not observed. Pharynx muscular, subspherical, 287 × 290. Oral sucker to pharynx length ratio 1.5; oral sucker to pharynx width ratio 1.4. Esophagus muscular, bifurcating immediately anterior to level of ventral sucker, 675. Ceca thin-walled, extending to near posterior end of body. Testes tandem, in posterior half of body, lobulated. Anterior testis 705 × 436; posterior testis 901 × 408. Cirrus-sac elongate, slender, reaching level of ovary or anterior to it, 1,690 × 226. Internal seminal vesicle unipartite, tubular, with broader proximal part. Pars prostatica indistinct. Genital pore immediately anterior to level of ventral sucker. Ovary subspherical, median or submedian, pretesticular, posterior to level of ventral sucker, 232 × 238. Mehlis’ gland between level of ovary and anterior testis. Uterine seminal receptacle present. Laurer’s canal not observed. Vitellarium distributed throughout most of hind body length, absent in tail, with most follicles lateral to gonads, uterus and cirrus-sac, confluent posterior to testes. Vitelline reservoir (not readily observed in holotype) between level of ovary and anterior testis. Eggs numerous, 42 in holotype, ≤ 137 in paratype, 90–104 × 62–70. Excretory pore not observed. Excretory bladder not readily observed. Taxonomic summary. Type host: Aythya marila (L.) (Anseriformes: Anatidae). Other hosts: Aythya affinis, Melanitta americana (Swainson). Site: small intestine. Type locality: Stump Lake, Nelson County, North Dakota, USA; 47°53'15.5"N, 98°18'11.3"W. Other localities: Lake Winnibigoshish, Itasca County, Minnesota, U. S. A.; 47°27'14.8"N, 94°16'30.9"W; Green Bay, Lake Michigan, Oconto County, Wisconsin, USA; 44°48'51.2"N, 87°52'54.7"W; Monroe County, Florida. Specimens deposited: The type series consists of 5 fully mature specimens (one is a hologenophore). Representative DNA sequences: PX 118875 (5.8S + ITS 2 + 28 S), PX 114586 (nad 1). Etymology. This species is named for Bruce Urben (Wisconsin Waterfowl Association) and his family for their leadership in waterfowl conservation, support of wetland habitat restoration, and donation of numerous birds for parasitology research. Diagnosis. Psilochasmus urbeni sp. nov. belongs to the genus based on its echinostome-like body that lacks a cephalic collar and the presence of a protrusible, retractile, muscular tail-like process at the posterior end of the body. This new species has an oral sucker that is only slightly wider than the pharynx (oral sucker to pharynx width ratio 1.1–1.5, mean 1.3), whereas that of other Psilochasmus spp. is typically much wider than the pharynx (oral sucker to pharynx width ratio 1.6–2.8 based on the present study, original descriptions and illustrations). The body length of P. urbeni sp. nov. (6,000 –7,020) is greater than P. agilis (4,500), although the suckers and ovary in the two species are similar in size. The esophagus of P. urbeni sp. nov. (537–923) is much longer compared to P. agilis (270 based on the original illustration). Psilochasmus urbeni sp. nov. and P. cf. agilis differ by 0.1 % in the 28 S and 9.4–9.8 % in the partial nad 1 sequences (Table 3). Psilochasmus urbeni sp. nov. is much longer than P. longicirratus (body length 6,000 –7,020 in the new species vs 3,740 –5,000 in P. longicirratus). Despite the difference in body length, all structures and organs overlap in size, except for testes; the testes of the new species are much larger (anterior testis 327–810 × 248–617; posterior testis 612–951 × 234–528) compared to P. longicirratus (testes 170 in diameter). The eggs of P. urbeni sp. nov. (87–113, mean 99) are generally smaller than in P. longicirratus (116–124). Similar to the previous comparisons, the body length of P. urbeni sp. nov. (6,000 –7,020) is greater than in P. oxyurus (2,415 –4,189 in new material). The esophagus of P. urbeni sp. nov. (537–923) is much longer than in P. oxyurus (328–456 in new material). Both suckers and the pharynx are larger in the new species compared to P. oxyurus (Table 2). The oral sucker to pharynx length ratio is noticeably smaller in P. urbeni sp. nov. (1.1–1.5, mean 1.4) than in P. oxyurus (1.8–2.3, mean 2.0 in new material). These species differ by 0 % in the 28 S, 0.2 % in the 5.8S + ITS 2, and 6.3 % in the partial nad 1 sequences (Table 3). Psilochasmus urbeni sp. nov. has a well-developed esophagus (537–923 long), whereas the cecal bifurcation of P. skrjabini is situated essentially immediately posterior to the pharynx. The oral sucker of P. urbeni sp. nov. (318–455) is also much longer than that of P. skrjabini (200). The body of P. urbeni sp. nov. lacks the strong lateral expansion immediately posterior to the level of the ventral sucker that is present in P. slavaukrainii sp. nov. Psilochasmus urbeni sp. nov. is also a much larger digenean compared to P. slavaukrainii sp. nov. in most regards, including body, sucker, and pharynx sizes as well as esophageal length (Table 2). For example, the body length of P. urbeni sp. nov. is more than twice that of P. slavaukrainii sp. nov. (6,000 –7,020 vs 2,076 –3,219). These species differ by 2 % in the 28 S, in 11.5 % in the 5.8S + ITS 2, and 15.3–16.2 % in the partial nad 1 sequences (Table 3). Psilochasmus urbeni sp. nov. lacks a distinct additional muscular sphincter at the anterior end of pharynx, which is present in P. sphincteropharynx. Otherwise, P. urbeni sp. nov. is a larger species as compared with P. sphincteropharynx. The body length of P. urbeni sp. nov. (6,000 –7,020) is noticeably greater than that of P. sphincteropharynx (4,200 –4,250). Both suckers and the pharynx are larger in the new species (oral sucker 318–455 × 284–417; ventral sucker 402–785 × 520–830; pharynx 224–301 × 194–376) compared with P. sphincteropharynx (oral sucker 240–270 × 220–230; ventral sucker 270 in diameter; pharynx 150–162 × 160–170). Intraspecific variation of 0.2–0.9 % was detected among nad 1 sequences of P. urbeni sp. nov. (Table 3).