Gyraulus takhteevi, Sitnikova, Tatiana & Peretolchina, Tatiana, 2018

Sitnikova, Tatiana & Peretolchina, Tatiana, 2018, Description of a new species Gyraulus (Pulmonata: Planorbidae) from the land thermal spring Khakusy of Lake Baikal, ZooKeys 762, pp. 1-12 : 1

Gyraulus takhteevi sp. n.

Gyraulus cf. borealis : Sitnikova and Takhteev 2006: 143 (record from thermal spring Khakusy)

Type locality.

Thermal spring Khakusy (East Siberia).

Types.

Holotype (dry) registration number in ZIN collection (St. Petersburg, Russia) is 522-2015 (1), 3 paratypes (dry) registration number is 522-2015 (2) with a label: 'East-northern shore of Lake Baikal, thermal spring Khakusy, pebbles, water temperature 23-25 °C, #0957, col. T. Sitnikova, 09.08.2009'. Collections of the Limnological Institute SB RAS (Irkutsk, Russia): 2 paratypes (dry) under numbers 901 and 902 with the label: 'Khakusy, shallow springs at a depth down to 3 cm, water temperature 33 °C, col. V. Takhteev, #57, 07.10.2004'; 20 paratypes (in alcohol) under number 1101 with a label 'Khakusy, #1526, 08.06.2015, col. T. Peretolchina’ and 4 paratypes (dry) and 2 paratypes (in alcohol) under number 1102 collected 3 July 2003, #0344, T. Sitnikova.

Etymology.

The species name ‘takhteevi’ is in honor of the Russian zoologist and hydrobiologist Prof. V.V. Takhteev (Irkutsk State University) who investigates biota of thermal springs in East Siberia.

Description.

Shell (Fig. 1 A–E) brown or green-brown, discoidal, pseudodextral, small, up to 5.0 mm of diameter at 4.0 whorls, smooth with fine growth lines on rounded last whorl, spire convex with rounded whorls, two last whorls of an umbilicus almost flat. Index a/b 0.36 - 0.57. Height of last whorl (a) less than aperture width. Aperture oval rounded. Species occurs in two morphs differing in spire width, narrow (Fig. 1 A–C) and wide (Fig. 1 D–E); all small individuals (less than three whorls) have a narrow spire; at 3.25 - 4.0 whorls the portion of morph 2 (with wide spire) is approximately 1/3 or 1/2 of total number of examined adult specimens. The designated holotype belongs to the dominant morph 1 (with a narrow spire). Sizes of the holotype and paratypes are presented in Table 2.

Radular teeth. The formula of the radula is 10 (9) –1– (9) 10. The central radular teeth are bicuspid with two equal-sized cusps. Two first lateral teeth usually bicuspid, sometimes with third small cusp. Other teeth have three rounded cusps, and only young (not working or worn) tricuspid teeth have three long sharp cusps. Mesocone is flanked (Fig. 2). Both morphs have identical radular morphologies and formulae.

Reproductive system. Seminal vesicles have thickened bend before joining with hermaphrodite duct, which is thin up to carrefour. Prostate consists of 22-26 diverticula, bursa copulatrix is oval in shape, its length including duct more than ½ length of oviduct. Length of copulatory organ almost equal to length of prostate (Fig. 3). Phallotheca twice as long as preputium length in morph with narrow spire and 1.7-1.8 times in morph with wide spire (Fig. 4A, B). Preputium slightly turned-up. Seminal pore lies near the proximal end of the thickening. Length of the stylet (Fig. 5A) varies from 189 to 260 mµ (n = 5).

Egg mass (= cluster or syncapsula) is a round transparent sac less than 1 mm in size, consisting of 4-5 eggs (Fig. 4F).

Differential diagnosis.

The new species (especially morph 2 with wide spire) is similar to the Palaearctic species G. acronicus (Fig. 1F, G) in shell shape, and differs from it in the small size of adults, which do not reach 6-7 mm diameter. Additionally, the new species differs from G. acronicus in having shorter preputium, the elongated oval shape of the bursa copulatrix, smaller number of prostate folds, and the smallest stylet length. A preputium of G. acronicus is 1.3-1.7 times shorter than the phallotheca (Fig. 4C, D); the bursa copulatrix has an elongate club-shaped (Meier-Brook, 1964, 1983) or a wide rounded shape, and length of copulatory organ is less than prostate length ( Glöer and Vinarski 2009; own data). The length of stylet of G. acronicus is more than 300 mµ (Fig. 5B). The size of the egg mass of G. takhteevi sp. n. is less than that of G. acronicus (its length up to 2.05 mm) and consists of fewer eggs (4-5 eggs vs. 5-7 eggs in G. acronicus ) ( Beriozkina and Starobogatov 1988).

Distribution and ecology.

Snails similar to morph 1 of the new species were also found in the thermal spring Frolikha located near to Khakusy (ca. 20 km north); all of them were young and were not examined in detail. The number of gastropods in the thermal spring Khakusy in March 2016 was 1,706 individuals/m2 at water temperature +29 °C in a locality 2 m far from the main source; a minimal number of snails (59 indv./m2) was registered at water temperature +10 °C in a small pond downstream of the main source (Epova et al. 2017). The proportions of G. takhteevi sp. n. and Lymnaea were about fifty-fifty.

The confinement of morphs to different sites of the thermal spring has not been confirmed. The population of G. takhteevi consists mainly of young snails, in which the morph with a narrow spire dominated: in July 2003 28 of the 39 collected snails were juvenile, with adults presented by six snails with shells of narrow spire (morph 1) and five individuals with wide spire (morph 2). In June 2015 there were 16 individuals of morph 1 and eleven specimens of morph 2, with more than 50 young snails; in March 2016 39 young specimens were collected and 16 adult snails of morph 1 and six individuals of morph 2.

The cultivation of the new species under summer conditions of ephemeral waterbodies (sand, pebbles, water temperature +20-24 °C, food items of vegetable fodder) was not successful, and all snails died in two weeks. It is worth noting that specimens of G. acronicus under the same conditions lived more than one year and reproduced, attached their egg masses to pebbles. According to the interruption lines on shells, the snails live up to 5 years.

Sequences analysis.

A total 14 COI (620 bp long) and 14 16S (500 bp long) sequences were produced. Inspection of the sequences revealed the existence of two unique haplotypes for COI among G. takhteevi sp. n. These haplotypes weakly differ from each other (in 1-2 nucleotide substitutions). Both morphs (shells with narrow or wide spire) are part of the haplotype 1, while haplotype 2 consists of the morph with wide spire shell (Fig. 6). There are no genetic differences between 16S nucleotide sequences obtained.

The haplotype of G. acronicus from Krestovka River shares the same clade as Gyraulus sp., sequence of which is retrieved from GenBank (KC495834), which differs from that of G. takhteevi sp. n. with 5-6 nucleotide substitutions (~ 1%).