Theodoxus dniestroviensis

Anistratenko, Olga Yu., Degtyarenko, Elena V., Osipova, Diana S., Maksymenko, Yuliia V. & Anistratenko, Vitaliy V., 2022, A neritid gastropod name “ Theodoxus dniestroviensis Put’, 1972 ” is a junior subjective synonym of Th. fluviatilis (L., 1758): decision based on the topotypic specimens study, Ruthenica, Russian Malacological Journal (Cambridge, England: 2003) 32 (2), pp. 61-68 : 62-67

Theodoxus dniestroviensis Put’, 1957

( Fig. 4 A–D )

Theodoxus dniestroviensis Put’, 1957: 98.

Theodoxus dniestroviensis Put’, 1972: 80-82, textfig. 5.

Th [eodoxus] dniestroviensis Put’, 1972. – O. Anistratenko et al., 1999: 19, fig. 4, 8.

Th [eodoxus] dniestroviensis Put’, 1972. – V.Anistratenko, O. Anistratenko, 2001: 108.

Theodoxus dniestroviensis Put’, 1972. – Vinarski, Kantor, 2016: 155 (in the Th. fluviatilis synonymy).

? Theodoxus dniestroviensis Put’, 1972. – V. Anistratenko et al., 2020: 120 (in the Th. fluviatilis synonymy).

Type material. The types of Theodoxus dniestroviensis should be in the NMNH NASU collection; despite major effort, none of the labelled type specimens could be located. Since the “ holotype ” of Theodoxus dniestroviensis was fixed by the author in 1972, but this nominal species taxon was introduced earlier in 1957, this specimen cannot possess holotype status [ ICZN, 1999:Art. 72.4.7 and 73.1.3]. Thus, the entire type series dealt with by the author in both publications should be considered syntypes. To fix the identity of Th. dniestroviensis we choose among the syntypes a single illustrated specimen [Put’, 1972, textfig. 5] and designate it here as the lectotype ( Fig. 4D ).

Type locality. Originally given as “the right bank of the Dniester River, Rukhotyn village, Khotyn District, Chernivtsi Region, Ukraine ” [Put’, 1957, 1972] .

Other material. More than 100 shells labelled as “ Theodoxus fluviatilis ” collected from the type locality by Put’ on May 26, 1950 were retrieved in the NMNH NASU depository. The specimens are considered as topotypes of Theodoxus dniestroviensis here. Additionally, over hundred individuals of Th. fluviatilis from other points of the Dniester River and its tributaries, namely the Seret, Strypa, and Hnizna rivers (localities 2–6) were studied.

Remarks. The species name “ Theodoxus dniestroviensis ” has been considered as established by Put’ in 1972 [e.g. O. Anistratenko et al., 1999; V. Anistratenko et al., 2020]. This dating uncritically relies on the article describing “ Theodoxus dniestroviensis Put’ sp. nova.” in detail morphologically and providing a shell photo [Put’, 1972: 80-82, textfig. 5]. However, another publication by Put’ [1957] was recently found where the nominal name “ Theodoxus dniestroviensis ” was established, and the species description satisfies the ICZN [1999] guidelines applied to species described between 1930 and 1999. In both these publications [Put’, 1957, 1972], the author clearly states that the new species name is introduced, lists the same diagnostic features and indicates the same type locality. Thus the name was made nomenclaturally available in 1957, not in 1972 as it was accepted before.

To ascertain the validity of this name, one has to check whether the shells of “ Th. dniestroviensis ” provide unique diagnostic features or their morphology falls within the range of variability of other species of the genus in this or neighboring region. In fact the type locality of “ Th. dniestroviensis ” is situated within the range of Th. fluviatilis , the most common and widespread species of neritids in Europe.

The identity of the name Th. fluviatilis is determined by its lectotype [O. Anistratenko et al., 1999; Anistratenko, 2005] though the species demonstrates a considerable variation in its shell colour pattern and shape [e.g. Zettler, 2008; Glöer, Pešić, 2015]. That is why a large number of nominal taxa are considered as synonyms of Th. fluviatilis [see V. Anistratenko et al., 2020].

The original description of Th. dniestroviensis was based on a relatively few morphological characters. No peculiarities in the size and/or proportions of the shell are mentioned [Put’, 1957, 1972] or depicted ( Fig. 4D ). The diagnosis is focused only on periostracum colouration and number of light spiral stripes on the last whorl of the shell: these can be accounted from 3–10 up to 15 depending on individuals’ age [Put’, 1957, 1972]. As it is known, these characters are prone to considerable intraspecific variation [ Bandel, 2001; Anistratenko et al., 2017, 2020; Sands et al., 2020] and certainly related to environmental conditions [e.g. Neumann, 1959; Rust, 1997; Zettler, 2007; Glöer, Pešić, 2015].

In general, the great conchological variability makes the reliable identification of some Theodoxus species difficult and raises debates on their validity. For example, there is no complete consensus on the taxonomy and nomenclature of Theodoxus species inhabiting the river basins of the Black Sea region [ Anistratenko et al., 2017, 2020; Wesselingh et al., 2019; Sands et al., 2020].At the same time it is demonstrated that the most widely-distributed in Europe Th. fluviatilis comprises many different variations in shell shape and colour pattern [O. Anistratenko et al., 1999; V. Anistratenko, O. Anistratenko, 2001; Bandel, 2001; Shubrat, 2007; Zettler, 2008; Glöer, Pešić, 2015].

The distribution range of the nominal species “ Theodoxus dniestroviensis ” is limited by the type locality only [Put’, 1957, 1972]. The topotypes of Th. dniestroviensis show a diversity of pattern ( Fig. 4 A–C ) that is registered in Th. fluviatilis populations in many parts of its range [ V. Anistratenko et al., 2020; Sands et al., 2020]. It should be noted that among all available topotypes no shell exactly corresponding in colouration and pattern to the one depicted in the 1972 paper [Put’, 1972: 80-82, textfig. 5] has been found ( Fig. 4D ). However, we found shells with longitudinal stripes in Th. fluviatilis populations from other localities of the Dniester River basin ( Fig. 4 E–K ). “ Theodoxus dniestroviensis ” in Put’s interpretation also shows a clear similarity in pattern to some Th. fluviatilis morphotypes earlier described by Lindholm [1908]: about 10–12 stripes are found in Th. obliteratus -morphotype ( Fig. 4L ) and 5–7 stripes in Th. pulcherrimus -morphotype ( Fig. 4O ). A pattern of broken bands forming the spiral rows of large light spots is also commonly registered e.g. in Th. lacrymans -morphotype ( Fig. 4M ) and Th. alboguttatus -morphotype ( Fig. 4N ). It is important to note that the descriptions of Th. alboguttatus , Th. lacrymans , and Th. pulcherrimus morphotypes [ Lindholm, 1908] were based on specimens collected near Odessa ( Table 1): in the ZIN catalogue these samples are erroneously labelled as “ Black Sea, Odessa ” but these molluscs do not inhabit the Black Sea and are evidently originated from the neighboring limans. Meanwhile, the only shell of Lindholm’s Th. obliteratus -morphotype, that makes up the type series of this taxon, was collected by A. Brauner in Dniester River near Rybnitsa, Moldova ( Table 1), i.e. not far from the type locality of “ Theodoxus dniestroviensis ” at least from the same part of the river basin.

Based on conchological features as well as periostracum colouration and patterning it is hardly possible to distinguish Th. dniestroviensis from morphotypes of Th. fluviatilis given the variability in colour and patterns among the topotypic material ( Fig. 4 A–C ). Since the original description contains no other differential characteristics apart the pattern, “ Theodoxus dniestroviensis ” should be interpreted as a local morphotype of the polymorphic species Th. fluviatilis (L., 1758) as it was suggested earlier [O. Anistratenko et al., 1999]. The present study contributes additional data to the recently published overview of the type series of nominal taxa of the genus Theodoxus inhabiting the Ponto-Caspian area [ V. Anistratenko et al., 2020]. It adds knowledge on the river nerites’ variability range as well as sheds more light on understanding how many valid species of this genus exist in a region and what the proper names for them are.

Figures

FIG. 4. Shells ofTheodoxus fluviatilis (Linnaeus, 1758).A–C. Three topotypes of “Theodoxus dniestroviensis”, NMNH NASU, PD, no number, locality 1. D. Lectotype ofTh. dniestroviensis, designated here, locality 1, photo reproduced after Put’, 1972. E. Specimen from Seret River, IZAN #614, locality 2. F. Specimen from Hnizna River, IZAN #616, locality 4. G. Specimen from Strypa River, IZAN #615, locality 3. H, I. Specimens from Dniester River, IZAN #333, locality 6. K. Specimen from Dniester River, IZAN #335, locality 5. L. Th. obliteratus-morphotype, ZIN #5912/1, locality 7. M. Lectotype of Th. lacrymans-morphotype, ZIN #6052/1, locality 8.N. Lectotype of Th. alboguttatus-morphotype, ZIN #6051/1, locality 8.O. Lectotype of Th. pulcherrimus-morphotype, ZIN #6053/1, locality 8. L–O. Reproduced after V. Anistratenko et al., 2020.