Psilochasmus slavaukrainii Achatz, Morton, Orlofske & Tkach, 2025

Psilochasmus slavaukrainii Achatz, Morton, Orlofske & Tkach sp. nov.

Fig. 4 a, b View Figure 4

Type material.

Holotype: HWML -218107 , labeled Anas clypeata , small intestine, Skadovsk District , Kherson Region, Ukraine, 9 Nov 2002, coll. V. V. Tkach . Paratypes (3 slides), labeled identical to holotype: HWML -218108 . Vouchers ( juveniles; 1 slide): HWML -218109 , labeled Tadorna ferruginea , small intestine, Skadovsk District , Kherson Region, Ukraine, 30 Oct 2002, coll. V. V. Tkach .

Description.

Based on four adult specimens (measurements of holotype are given in text; measurements of entire series are given in Table 2 View Table 2 ). Body elongate, 3,219 × 759, widest near level of testes; forebody cylindrical; hindbody with strong lateral expansion with narrow muscular, retractable tail-like process,. Body length to width ratio 4.2. Ratio of body width at level of testes to body width at level of ventral sucker 1.7. Tegument armed. Forebody length represents 35 % of body length. Oral sucker subterminal, oval, 259 × 206. Ventral sucker protuberant with deep cavity, consisting of strongly muscular portion and extensive surrounding sub-tegumental rim, 312 × 426. Oral sucker to ventral sucker width ratio 0.5. Prepharynx not observed. Pharynx muscular, elongate-oval, 143 × 100. Oral sucker to pharynx length ratio 1.8; oral sucker to pharynx width ratio 2.1. Esophagus muscular, bifurcating anterior to level of ventral sucker, 482. Ceca thin-walled, extending posterior to level of posterior testis.

Testes tandem, in posterior half of body, lobate. Anterior testis 269 × 238; posterior testis 345 × 199. Cirrus-sac elongate, slender, reaching level of ovary or anterior to it, 421 × 80. Internal seminal vesicle unipartite, tubular, with broader proximal part. Pars prostatica indistinct. Genital pore immediately anterior to anterior margin of ventral sucker.

Ovary oval, median or submedian, pretesticular, posterior to level of ventral sucker, 128 × 128. Mehlis’ gland between level of ovary and anterior testis. Uterine seminal receptacle present. Laurer’s canal not observed. Vitellarium distributed throughout most of hindbody length, absent in tail; most follicles lateral to gonads, uterus and cirrus-sac. Vitelline reservoir between level of ovary and anterior testis (not readily observed in holotype). Eggs few, 5 in holotype, 84–101 × 50–66.

Excretory pore positioned near tip of tail. Excretory bladder not readily observed.

Taxonomic summary.

Type host: Anas clypeata (L.) ( Anseriformes : Anatidae ).

Other host (only juveniles collected): Tadorna ferruginea (Pallas) ( Anseriformes : Anatidae ).

Site: small intestine.

Locality: Skadovsk District, Kherson Region, Ukraine; 46°26'38.3"N, 32°01'44.9"E.

Representative DNA sequences: PX 118873 (ITS region + 28 S); PX 114584 ( nad 1).

Etymology.

This species is named in honor of the national salute in the country in which it was collected.

Diagnosis.

These digeneans clearly belong to Psilochasmus based on the 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. The body of P. slavaukrainii sp. nov. has a strong lateral expansion immediately posterior to the level of the ventral sucker that is absent in properly relaxed mature congeners. It is noteworthy that the original description of P. agilis by Travassos (1921) did not mention such a lateral expansion. However, Szidat (1957) illustrated ‘ younger’ P. agilis specimens (referred to as P. oxyurus ) that exhibited this trait. At the same time, the mature adult stage illustrated by Szidat (1957) had only a slight widening of the body.

Psilochasmus slavaukrainii sp. nov. is a much smaller digenean compared with P. agilis . For instance, the body length of mature specimens is only 2,076 –3,219, whereas the body length of mature P. agilis exceeds 4,000. The forebody of the new species represents 30–36 % of the body length, whereas in P. agilis it is approximately 25 %. The new species is distributed in the Palearctic, whereas P. agilis is restricted to the Neotropics, and possibly Nearctic (see Discussion below). Psilochasmus slavaukrainii sp. nov. and P. cf. agilis differ by 2.1 % in the partial 28 S sequences and 16.8–17.2 % in the partial nad 1 sequences (Table 3 View Table 3 ).

The oral sucker to pharynx width ratio of P. slavaukrainii sp. nov. (2.1–2.7) is greater than in P. longicirratus (1.6 based on the original drawing). The new species is shorter ( 2,076 –3,219) compared to P. longicirratus ( 3,740 –5,000). Furthermore, the suckers and pharynx of P slavaukrainii sp. nov. are much smaller (oral sucker 192–259 × 195–206; ventral sucker 312–389 × 330–426; pharynx 103–143 × 73–100) compared to P. longicirratus (oral sucker 340 in diameter; ventral sucker 640 in diameter; pharynx 255 × 204). However, the testes of the new species are typically larger (anterior testis 217–269 × 222–238; posterior testis 221–345 × 157–208) compared to P. longicirratus (testes 170 in diameter). The eggs of the new species (84–101) are much smaller than those of P. longicirratus (116–124).

Psilochasmus slavaukrainii sp. nov. and P. oxyurus are very similar morphologically. The oral sucker to pharynx length ratio is generally smaller in the new species (1.4–1.9, mean 1.8) compared to P. oxyurus (1.8–2.3, mean 2.0 in our material). The post-testicular field is longer in the new species (27–30 %, mean 28 % of body length) compared to P. oxyurus (23 % of body length based on the illustration of Braun (1902) (14–25 %, mean 19 % of body length in present material). Despite being similar morphologically, these species differ by 2.0 % in the 28 S, 11.5 % in the 5.8S + ITS 2, and 16.4–16.8 % in the partial nad 1 sequences (Table 3 View Table 3 ).

Psilochasmus slavaukrainii sp. nov. has a well-developed esophagus (182–482 long), whereas the ceca of P. skrjabini bifurcate almost immediately posterior to the pharynx. The new species is much smaller in body length ( 2,076 –3,219 in the new species vs 6,750 in P. skrjabini ), oral sucker width ( 195–206 in P. slavaukrainii sp. nov. vs 400 in P. skrjabini ), ventral sucker size (312–389 × 330–426 in P. slavaukrainii sp. nov. vs 780 × 600 in P. skrjabini ) and pharynx size (103–143 × 73–100 in P. slavaukrainii sp. nov. vs 200 × 250 in P. skrjabini ).

Psilochasmus slavaukrainii sp. nov. lacks a distinct additional muscular sphincter at the anterior end of the pharynx, whereas such a structure is present in P. sphincteropharynx . The pharynx of P. slavaukrainii sp. nov. (103–143 × 73–100) is also smaller than P. sphincteropharynx (150–162 × 160–170). Psilochasmus slavaukrainii sp. nov. has a smaller body ( 2,076 –3,219) than P. sphincteropharynx ( 4,200 –4,250). The oral sucker of these species is similar in size, or slightly smaller in the new species, whereas the ventral sucker is noticeably larger in the new species (312–389 × 330–426) compared to P. sphincteropharynx ( 270 in diameter). Eggs are somewhat shorter in P. slavaukrainii sp. nov. (84–101) compared to those of P. sphincteropharynx (105–110).

It is worth noting that the specimen of P. oxyurus illustrated by Bykhovskaja-Pavlovskaja (1962: fig. 68), from the mallard Anas platyrhynchos L. (collected from an unknown locality in the former Soviet Union) appears to be essentially identical to P. slavaukrainii sp. nov.

No variation was detected in ribosomal loci (ITS 1 + 5.8S + ITS 2 + 28 S) between the two isolates of the new species, whereas a 0.4 % variation was observed between two isolates in the partial nad 1 sequences (Table 3 View Table 3 ).