Pseudobaikalia michelae Sitnikoiva & Kovalenkova

Sitnikova, Tatiana, Kovalenkova, Maria, Peretolchina, Tatiana & Sherbakov, Dmitry, 2016, A new, genetically divergent species of Pseudobaikalia Lindholm, 1909 (Caenogastropoda, Baicaliidae), ZooKeys 593, pp. 1-14 : 4-7

Taxon classification Animalia Littorinimorpha Baicaliidae

Pseudobaikalia michelae Sitnikoiva & Kovalenkova sp. n. Figs 2A, B, C, 3, 4, 5

Etymology.

The species name ' michelae ' is in honour of Ellinor Michel (Natural History Museum, London) who has made a range of studies on gastropods inhabiting ancient lakes.

Type material.

Holotype (dry) and 2 paratypes (dry and in alcohol) from the type locality were deposited in freshwater gastropod collection of Zoological Institute RAS (Saint Petersburg, Russia) and are registered under Nos. 1/522-2015 (holotype) and 2/522-2015 (paratypes). Shells of six paratypes were destroyed for dissections (to study anatomy and nucleotide sequences) with images of them given here (Fig. 2B).

Additional material.

Two shells from a lot No. 1 of Zoological Institute collection with original label ' Leucosia angarensis var. pulla , Baikal, collection W. Dybowski 1875 '; 2 specimens: south-eastern part of Lake Baikal, near Utulik settlement (51°33'13.5"N, 104°05'17.3"E), at the depth of 17-31 m, silty sand; by drag, coll. by authors 07.21.2015 (site 2); 2 shells: the same part of the lake, Murinskaya Bank (51°30'11.3"N, 104°28'39.4"E), 34-39 m depth, stones, sand, silt; by drag, coll. by authors 07.21.2015 (site 3); 7 specimens: central part of the Lake, Barguzin Bay (53°18'34.8"N, 108°44'07.4"E), at a depth of 14 m, sand and stones; by drag, coll. by authors 07.17.2015 (site 4 in Fig. 1).

Type locality.

Kultuk Bay (southern part) of the Lake Baikal (southern Siberia), (51°42'59.9"N, 103°43'23.1"E) from 11 to 27 m depth, sand and stones, sponges (site 1, Fig. 1).

Description.

(Figs 2-5): Shell (Fig. 2A, B) grey-green or light brown, elongate-conic, smooth with 5-6 growth lines, small, up to 6.5 mm high and 3 mm wide; with 4.25-5.75 well rounded whorls, deep suture, without umbilicus; aperture elongate-oval, columellar lip slightly thickened, outer lip thin, simple or slightly rounded, basal lip rounded or slightly elongated. For shell dimensions please see Table 2. Protoconch (Fig. 3C, F) discoidal, about 1.25-1.45 whorls, diameter about 500 μ, surface almost smooth, near suture with slight spiral threads or slightly reticulate. Operculum (Fig. 3A, B) flat, thin, transparent, paucispiral with 5-6 growth whorls, last half whorl weakly frilled, nucleus subcentral, attachment scar elongated oval occupies about 1/3 of operculum width.

Radula (Fig. 4): 580-600 μm length with 46-48 teeth rows, 30-33 of them well-formed. Central teeth square or triangular, about 15.5 μm wide, cutting edge broadly concave; central cusp absent or square formed by the merger of 2-3 cusps; lateral cusps 15-16, thin and long; basal tongue slightly convex or straight. Lateral teeth face rectangular, outer margin with concave bend, central cusp similar to lateral cusps or slightly broader, or merger of 2-3 cusps, inner and outer lateral cusps about 9-10; outer wing rather broad, straight, about two times longer than cutting edge. Inner marginal teeth with approximately 24 cusps and outer marginal teeth with approximately 16 cusps.

Body pigmented black, mantle edge light grey, ctenidium nearly 2 mm in length with 62-64 leaflets, osphradium broadly ovate, slightly narrowed in proximal end, ~ 0.5 mm length, opposite anterior part of ctenidium, a little deeper than mantle fold.

Reproductive system.

Male with small bean prostate gland (Figs 5D, 3D), anterior and posterior vas deferens close together in middle part of prostate, penis light grey, muscular, elongated, with thin glandulous often non-visible fold, gradually tapering with short and small papilla. Female coiled oviduct with one spiral, loop grey pigmented with two 'tube-like evaginations’ (Fig. 4 A–C).

Ecology.

Pseudobaikalia michelae sp. n. was found on heterogeneous (stones, sand, and silt) or soft sediments at depth zone ranging from 11 to 39 m in southern, east-southern and central-eastern parts of Lake Baikal. At the type locality Pseudobaikalia michelae sp. n. was sympatric with three other species of baicaliids: Godlewskia pulchella (Dybowski, 1875), Korotnewia semenkewitschi Lindholm, 1909, and Teratobaikalia duthiersii Dybowski, 1875. Eleven baicaliids including Pseudobaikalia michelae sp. n. and three species of the genus Pseudobaikalia : Pseudobaikalia contabulata , Pseudobaikalia pulla , and Pseudobaikalia zachwatkini were collected together at sites 2 and 4 (Fig. 1); ten baicaliids were registered at site 3 (Fig. 1), including the species, Pseudobaikalia contabulata and Pseudobaikalia zachwatkini .

Remarks.

The genus Pseudobaikalia includes seven mainly shallow water species (Fig. 2 D–K); some of them were found down to 100 or 200 m depth. Two species Pseudobaikalia jentteriana and Pseudobaikalia cancellata were found only in the northern part of the lake, Pseudobaikalia elegantula inhabits the northern and central parts of the lake, and three other species Pseudobaikalia zachwatkini , Pseudobaikalia pulla , and Pseudobaikalia contabulata are found in a range of locations throughout of Baikal, and are sympatric to Pseudobaikalia michelae sp. n.

The shells of Pseudobaikalia jentteriana (Fig. 2E) and Pseudobaikalia pulla pulla (Fig. 2F) are smooth as in the new species, while the other Pseudobaikalia have transverse ribs or lirae. The shell of Pseudobaikalia pulla tenuicosta (from the northern part of Lake Baikal) has slightly raised ribs. Both subspecies of Pseudobaikalia pulla have an operculum equal to aperture size (Fig. 2 F–G), but the operculum of other species including Pseudobaikalia michelae sp. n. is smaller than aperture. The new species is most similar to Pseudobaikalia jentteriana in its size and smooth shell but differs in colour ( Pseudobaikalia jentteriana has light brown shell and body) and in the morphology of the female gonoduct (in Pseudobaikalia jentteriana the oviduct loop is wider and includes 3-4 'tube-like evaginations’); the penis of Pseudobaikalia jentteriana has not been investigated yet ( Sitnikova 1991). The new species is similar to young Parabaikalia elata (Dybowski, 1975) in the shape of shell and aperture (Fig. 2I), but differs in adult shell size and the length of oviduct loop. Parabaikalia elata is partially sympatric to a new species, they co-occur at sites 3 and 4 (Fig. 1), and thus it was included into molecular analyses (Fig. 6, Table 3).

Molecular phylogeny.

Six specimens of Pseudobaikalia michelae sp. n. and thirteen specimens of sister species were analyzed using nucleotide sequences from the mitochondrial genes CO1, mtSSU rDNA and nuclear ATPase α intron segment.

JModelTest selected GTR + I + gamma for each CO1 codon positions separately HKY + I for mtSSU rDNA and HKY nucleotide substitution model for intron dataset as the best fit using the Bayesian Information Criterion.

Sequences of Pseudobaikalia michelae sp. n. cluster in the well-supported clade (posterior probability = 1.00) (Fig. 6). The average mutation distance from the new species to the other Pseudobaikalia species was found to be appropriate for species-level distinction: 6.4 ± 1.3%, 2.1 ± 0.2% and 2.1 ± 0.6% for the mitochondrial CO1 and mtSSU rDNA and the nuclear marker, respectively. Interspecies genetic distances are known for other taxa within this family, which can be non-monophyletic.

The lowest mitochondrial genetic distance between Pseudobaikalia contabulata and Pseudobaikalia jentteriana are comparable to the distances between representatives of subspecies of Pseudobaikalia pulla . They both are about 2% for CO1 and 0.03% for mtSSU rDNA. As for the CO1 data, Pseudobaikalia michelae sp. n. appears to be the sister group to Pseudobaikalia contabulata and Pseudobaikalia jentteriana with 4.8% of base substitutions. The minimum genetic distances in case of the intron of Pseudobaikalia michelae sp. n. is 0.7% to the two species: Pseudobaikalia jentteriana and Pseudobaikalia zachwatkini (Table 3). Moreover, there is an important higher-level character difference in the new species sequence profile as all intron sequences of Pseudobaikalia michelae sp. n. had a relatively large (57 BP) deletion.