Opisthorchophorus, Beriozkina et Starobogatov, 1995

Opisthorchophorus View in CoL confusus Andreeva, sp. nov.

Figs 2 View FIG ; 3 A–D View FIG

Zoobank registration: urn:lsid:zoobank.org:act:

0ED7D867-0CED-4CB2-9980-C967EEDA67A0

? = Opisthorchophorus baudonianus sensu Beriozkina et al. [1995] , non Gassies, 1859 (partim).

? = Opisthorchophorus hispanicus sensu Beriozkina et al. [1995] , non Servain, 1880.

Type locality. Russia, Western Siberia, Tomsk Region, vicinity of Tomsk City, a temporary waterbody in the Tom’ River floodplain. Collected by M.P. Miroshnichenko, 06.06.1953. Approximate coordinates: 56°29’5”N, 84°57’E GoogleMaps .

Type series. The holotype (accession No. 1/503- 2023) and ten paratypes (accession No. 2/503-2023) are kept in ZIN. 5 paratypes are deposited in RMBH. Their accession numbers are: MByt-40.1 ( Omsk Region, Atachka River, 1 spec.), Mbyt-42.1 ( Omsk Region, Tyukalka River , 1 spec.), Mbyt-44-1, 44-2, 44-3 ( Tyumen Region, Bolshoy Balyk River , 3 spec.) .

Additional material. See Supplementary Table S1 for details.

Etymology. After Latin confusus – ‘confounded, confused’. The etymology reflects the fact that this species has been confused with other species of the Bithyniidae and has remained unrecognized until now.

Holotype dimensions at 5.5 whorls (mm). Shell height 11.7; shell width 8.3; spire height 5.7; body whorl height 9.0; body whorl height above the aperture 3.0; aperture height 6.0; aperture width – 4.6.

Description. Shell massive, low-conical or broadly conical ( Fig. 2 View FIG ), yellow-brown, light brown or vitreous-transparent, shiny, thin-walled. Shell sculpture is represented by longitudinal densely arranged longitudinal ribs and spiral lines. Spiral lines may consist only of conchiolin and are easily erased when washing samples or cleaning shells from fouling. Adult shells are up to 11.7 mm high and 8.3 mm wide ( Table 2). The last whorl massive, its height is 0.73-0.82 of shell height; elevation of the body whorl above the aperture is 0.23-0.28 shell height. Rapidly increasing whorls (up to 5.5 in number) are uniformly convex, almost stepped, separated by a deep suture. The umbilicus is open. The spire height is 0.43-0.54. The tangent line of the whole shell is not straight but with two breaks, the tangent line of the spire is slightly concave. The apical angle of shells with 5 whorls and more is 61–65°, in those with less than 5 turns – 63–79°. The aperture is round-oval with a pronounced angle in its upper part and thin and fragile margins. The aperture height is 0.47–0.55 of the shell height, the aperture width is 0.71-0.88 of its height. The operculum is oval with an obtuse angle in the upper part and a large nucleus located almost in its centre. The growth lines on the operculum are weakly developed. Shells of males and females do not differ externally, and no significant differences were found in their morphometric indices.

The soft body, as a rule, is lightly coloured, with a well-defined spotted pattern.

The penis is massive, curved, with a short outgrowth, the length of which is much shorter than the length of the penis. The distal end of the penis varies in length and shape: it may be uniformly or irregularly thickened along its entire length and pointed at the end to varying degrees.

Variability. The variability in shell shape and proportions is moderate; at 4.0-4.5 whorls it is close to trochoid, and at higher whorl numbers it becomes low-conical or broadly conical. The rate of whorl increase alters with age; it is highest in juvenile specimens. The most variable are such plastic characters as the shell height and width and the body whorl height ( Table 2). Shells larger than 8 mm in height are rarely found in the collections; the typical shell height is 4-7 mm. Shell apexes are often corroded, which causes problems with measurements and calculation of morphometric indices. Shell coloration varies considerably; yellow-brown and colourless shells are predominantly found in the samples, but light brown and white shells also occur, as well as shells with non-uniform coloration. Shells may be covered with fouling which hides both the coloring and the suture depth. The shape and proportions of the penis of males in the samples examined are quite variable ( Fig. 3 View FIG ).

Differential diagnosis. Opisthorchophorus confusus sp. nov. differs from representatives of the Bithynia tentaculata complex by convex whorls, which are separated by deep suture (in Bithynia snails, whorls are flattened or weakly convex). From species O. abakumovae and O. troschelii the new species can be distinguished by lower spire and broadly conical shell with relatively large body whorl. Shell of O. abakumovae is narrowly conical, almost turreted, tall, whorls increase very fast, the spire height is about half of the shell height. In O. troschelii , the shell is conical, massive, whorls increase fast, the spire height is not less than one third of the shell height. The presence of a distinctive sculpture on shell surface differs O. confusus sp. nov. from all other species previously assigned to the genus Opisthorchophorus , including O. valvatoides . In other species of Opisthorchophorus , the shells are smooth or slightly striated, only with thin growth lines. In O. confusus sp. nov., longitudinal ribs and sometimes spiral lines are clearly visible (if shells are not heavily worn). The new species is well distinguished from representatives of Boreoelona by shell shape and proportions; shell of O. confusus sp. nov. is broadly conical, while Boreoelona sibirica has a turriculate shell, with high spire; the whorls in O. confusus sp. nov. shell increase slowly compared to B. sibirica . O. confusus sp. nov. is also well distinguished from the co-occuring Boreoelona contortrix by shell shape. Shell of O. confusus sp. nov. has slowly increasing whorls and a relatively large, strongly inflated body whorl, whereas in B. contortrix shell is conical with rapidly increasing whorls turns and a low, less inflated body whorl.

The penis of O. confusus sp. nov. is rather massive with a shortened distal end compared to the penises of other species of the genus Opisthorchophorus and is similar in shape and proportions to the penis of B. sibirica , which has a more elongated distal end.

Egg-clusters. These are aggregates of unbranched yellowish transparent strands with even margins and slightly bumpy surface, consisting of densely packed large polygonal egg capsules separated from each other ( Fig. 4 View FIG ). The egg-clusters laid by females from the Atachka River were single-rowed, supplemented at the ends by a second row of egg capsules; those from the Maly Uchug River were regular doublerowed ( Fig. 4 View FIG ). The egg-masses are deposited on the substrate at a small distance from each other. There were from 4 to 6 egg capsules in the clutch of one female; the total number of egg capsules laid by one individual depended on the size of the animal: for example, 22 egg capsules were laid by individual with 7.5 mm shell height, and 50 egg capsules were laid by a snail of 9.0 mm shell height. The number of egg capsules in a clutch varied from 2 to 43. The size (length) of egg capsules from the Atachka River ranged from 1.3 to 1.6 mm; from the Maly Uchug River – 1.2 to 1.7 mm. The egg capsule width varied from 1.3 to 1.7 mm in the Atachka River; and from 1.3 to 2.1 mm in the Malyi Uchug River. The main index of egg capsules from the Atachka River ranged from 0.87 to 1.23; from the dam on the Malyi Uchug River from 0.88 to 1.75. It should be noted that egg capsules are flattened due to their dense packing, even if they were arranged in one row. The regularly ellipsoid or spherical shape was preserved only in egg capsules laid two or three at a time.

The double-row egg-clusters of Opisthorchophorus confusus sp. nov. are morphologically similar in arrangement of egg-capsules and their packing to clutches of “ Opisthorchophorus baudonianus ” from waterbodies of Western Siberia [ Andreeva, Lazutkina, 2004] and species of the genus Boreoelona from waterbodies of the Russian Far East [ Prozorova, 1992].

Distribution. The species was found in samples from waterbodies of the central regions of European Russia, the Urals, Western Siberia and central Kazakhstan. Probably, its range also covers northern areas of European Russia, northern areas of Kazakhstan and the Baikal Region (see Discussion). The sequenced specimens were collected in the southern part of Western Siberia ( Omsk Region) and in the southern part of European Russia ( Krasnodar Krai).

Ecology. Typical habitats of Opisthorchophorus confusus sp. nov. are floodplain waterbodies, as well as shallow zones of small rivers with slow current. The species is found in thickets of macrophytes, on snags and submerged artificial objects at depths up to 1.5 m, together with bithyniids of the genera Bithynia , Opisthorchophorus , and (only in Siberia) Boreoelona . More detailed data on the coexistence of O. confusus sp. nov. are presented in Supplementary Info, Table S3.

The snails of this species serve as the first intermediate hosts of the Opisthorchis felineus trematodes in Western Siberia. Using PCR method, Andreeva et al. [2024] have detected the presence of the opisthorchid parthenitae in three specimens of O. confusus sp. nov. collected from the Bolshoy Balyk River ( Tyumen Region) .