Chersaecia dextrorsa ( Benson, 1860 )

Páll-Gergely, Barna, 2018, Systematic revision of the Plectopylinae (Gastropoda, Pulmonata, Plectopylidae), European Journal of Taxonomy 455, pp. 1-114 : 20-24

Chersaecia dextrorsa ( Benson, 1860)

Figs 7 E–H, 9–10, 11A–B

Helix (Plectopylis) refuga var. dextrorsa Benson, 1860: 246–247 [“Phye-thán in the Tenasserim Valley”]. Helix (Plectopylis) brachydiscus Godwin-Austen, 1879 a: 1–3 , pl. 1, fig. 1–1e [“Mulé-it, east of

Moulmein, Tenasserim”]. Syn. nov.

Helix (Plectopylis) refuga var. dextrorsa – Hanley & Theobald 1870: 7, pl. 13, fig 9. — Godwin-Austen 1875b: 613.

Helix (Plectopylis) dextrorsa – Godwin-Austen 1875a: 44 [“Phié Thaw”]. — Nevill 1878: 70.

Helix (Plectopylis) brachydiscus – Tryon 1887: 162, pl. 36, figs 32–34.

Helix (Plectopylis) refuga var. dextrorsa – Tryon 1887: 164, pl. 35, fig. 2 [“Austen”].

Plectopylis dextrorsa – Gude 1896a: 156, fig. 21a–c.

Plectopylis brachydiscus – Gude 1896a: 154–155, fig. 18a–c.

Plectopylis (Chersaecia) dextrorsa – Gude 1899d: 148; 1899e: 175; 1914b: 110–111, fig. 52.

Plectopylis (Chersaecia) brachydiscus – Gude 1899d: 148; 1899e: 175; 1914b: 108–110, fig. 51.

Chersaecia brachydiscus – Páll-Gergely et al. 2015c: 10.

Chersaecia dextrorsa – Páll-Gergely et al. 2015c: 10.

Diagnosis

A medium-sized, dextral, more or less flat species, usually with periostracal folds on the upper periphery of the body whorl, parietal callus strongly developed, apertural fold present, main plica short (not reaching apertural fold) or long (in contact with apertural fold); parietal wall with a single lamella and an intermediate plica attached to its lower end; palatal plicae free, most of them divided in the middle.

Material examined

Holotype

MYANMAR: 1 shell, holotype of P. dextrorsa (D = 16 mm), Tenasserim Valley ( NHMUK 1906.2.2.144).

Syntypes

MYANMAR: 12 shells, syntypes of P. brachydiscus, Tianoba , Mulé-it Range, Tenasserim, Limborg leg. ( NHMUK 1903.7.1.752).

Additional material

MYANMAR: 2 shells, Moulmein, coll. Möllendorff ( SMF 150118); 7 shells, Tavoy, Tenasserim, coll. Möllendorff ( SMF 150119); 3 shells, Tenasserim ( NHMUK 1888.12.4.1557–1559); many eroded, whitish shells, Tenasserim, Limborg leg. ( NHMUK); 2 juvenile shells, Tenasserim ( NHMUK 1893.4.7.1–2); 7 shells, Tavoy, Tenasserim ( SMF 150119).

THAILAND: 4 shells, Siam, Ponsekai ( SMF 150120); 4 shells, Siam, Ponsekai, coll. Möllendorff ( SMF 150120); 31 shells, Kanchanaburi Province, Erawan National Park, Erawan Falls, 14°22′27.91″ N, 99°8′45.43″ E, 150 m a.s.l., 14 Sep. 1985, F.G. Thompson leg. ( UF 76507); 17 specimens (preserved in ethanol; 4 specimens in separate vials; 2 specimens, PL003 and PL004, used for molecular study), same data as for preceding ( UF 76385); 8 shells, Kanchanaburi Province, 15.3 km NW of Sangkhla Buri, 660 m, 9 May 1988, K. Auffenberg leg. ( UF 346181).

Description

SHELL (based on the type series of brachydiscus ). Dextral, flat or almost flat, light to dark brown; whorls 6–6.75 (n = 5), separated by a deep suture; in face of deep suture, upper surface seems to be flat due to flat whorls; protoconch irregularly wrinkled with extremely fine granular structure which is sometimes not visible, or visible only on a small part of the protoconch; teleoconch ribbed and spirally striated, giving the sculpture a reticulated appearance; the riblets may be covered by deciduous folds of the periostracum on fresh shells; on dorsal surface of whorls there are 8–9 spiral lines, and much denser riblets; sculpture also visible on ventral side and inside umbilicus; periostracal filaments longer near the sutures, whereas juvenile/subadult specimens possess long periostracal filaments on periphery of body whorl; peristome white, conspicuously thickened and slightly reflected; parietal callus slightly S-shaped; apertural fold prominent, approximately half as long as parietal callus, free from main plica.

Five shells were opened. Parietal wall with a short vertical lamella, connected to intermediate plica, which extends beyond lamella in posterior direction; intermediate plica ca half as long as main plica; main plica free from lamella and usually from apertural fold, although almost reaching each other in old specimens; two opened shells had a short lower plica under lamella. Palatal wall with six plicae, first three more or less straight, sometimes with additional posterior small denticles; second usually with a dichotomously divided posterior end; fourth and fifth divided, their posterior portions descending downwards; anterior portion of the fifth plica can be missing; sixth curved.

MEASUREMENTS (in mm). D = 16, H = 5.8 (holotype); D = 16.8–19.7, H = 5.9–7.2 (type series of brachydiscus , n = 4); D = 14.8–19.3, H = 5.7–7.6 (Thai specimens, n = 5).

CHARACTERS OF THE GENITAL SYSTEM. Three specimens from sample UF 76385 were dissected. Right ommatophoral retractor passes between penis and vagina-vas deferens; penis reduced, very small, ovoid, internally with parallel, serrate folds; retractor muscle inserts on proximal end of penis; vas deferens extremely long, cylindrical and cable-like, internally with parallel, oblique folds; vagina internally slightly serrate longitudinal folds, very long, as thick as vas deferens; weak fibres attach vagina to body wall; bursa copulatrix about as long as vagina; diverticulum of two specimens contain spermatophores; end of spermatophores were found in stalk of bursa copulatrix; spermatophores formed by irregularly packed slender, reddish thread; inner wall of diverticulum with parallel folds.

RADULA. Centrals with small, triangular cusps; endocones of laterals rhomboid; ectocones of first laterals larger than centrals; ectocones of marginals undivided, endocones of marginals divided by shallow incision.

Differential diagnosis

The most similar species in terms of shell characters is Chersaecia reversalis sp. nov., which is, nevertheless, strikingly different in terms of genital anatomy. Some populations of C. dextrorsa have a long parietal main plica that is continuous with the apertural fold, whereas in C. reversalis sp. nov. the apertural fold is always separate from the parietal plica. Chersaecia reversalis sp. nov. has a sculpture characterized by a smooth basal surface and fine, reticulated dorsal surface with small periostracal filaments. In contrast, C. dextrorsa has a strong sculpture all over the shell and elevated periostracal ribs with long filaments on the periphery of the body whorl of fresh, usually subadult specimens. Chersaecia densegyrata sp. nov. has a flatter shell, denser whorls, and two vertical plicae on the palatal wall (with a horizontal plica below and three above). See also under Chersaecia austeni .

Distribution

The species is distributed in western Thailand (Kanchanaburi and southern Tak Provinces) and eastern Burma ( Fig. 12 ).

Remarks

The populations discussed here under the name Chersaecia dextrorsa show a variability in the following traits: (1) distance of the apertural fold from the parietal callus; (2) relationship of the apertural fold to the main plica (connected or separated); (3) shell shape (flat or slightly elevated spire); (4) shape of the body whorl (rounded or bluntly shouldered); (5) presence of absence of periostracal folds. Table 2 summarizes the differences between the examined populations. None of these differences are sufficient for species- or subspecies-level distinction. Moreover, the first two traits are probably agedependent. Namely, mature specimens (i.e., shells with strongly thickened peristome) of the type series of Plectopylis brachydiscus have the apertural fold and the main plica almost reaching each other, and those of sample UF 76507 have the apertural fold and the parietal callus connected. I therefore synonymize P. brachydiscus with P. dextrorsa .

Only the holotype of Helix (Plectopylis) refuga var. dextrorsa was found in the NHM. This specimen is an intact shell but I was not able to observe the parietal wall. More material was probably examined by previous authors because the inner plicae and lamellae have already been described ( Gude 1914b).

The shells from Sangkhla Buri are larger than other Thai specimens and have an additional short plica (probably homologous with the lower plica) below the lamella. In the type series of P. dextrorsa , the additional short plica is present in some shells only.

The spermatophores of C. dextrorsa are formed by an irregularly folded thread-like structure, whereas the threads in species of Plectopylis are regularly, longitudinally packed.

Figures

Fig. 7. Parietal (A, C, E, G, J, K, M) and palatal (B, D, F, H, I, L, N) plication in species of Chersaecia Gude, 1899. A–B. C. austeni (Gude, 1899) (1903.06.01.1014.1). C–D. C. densegyrata sp. nov. (UF 347223). E–F. C. dextrorsa (Benson, 1860) (UF 346181). G–H. Chersaecia dextrorsa (UF 76507). I–J. C. brachyplecta (Benson, 1863) comb. nov. (after Gude 1914b). K–L. C. smithiana (Gude, 1897) comb. nov. (NHMUK 1888.12.4.1555–1556, two different specimens). M–N. C. reversalis sp. nov., paratype from type locality (UF 347046). Arrows below the letters indicate the direction of the aperture. Not to scale.

Fig. 12. Distribution of plectopylid species in Thailand, Laos and Myanmar. Legends: circle = Chersaecia shanensis (Stoliczka, 1873); filled square = Chersaecia reversalis sp. nov.; empty square = Chersaecia dextrorsa (Benson, 1860); empty rhomb = Hunyadiscus tigrina sp. nov.; filled rhomb = Hunyadiscus saurini Páll-Gergely, 2016; triangle, top up = Plectopylis crassilabris sp. nov.; triangle, top down = Naggsia laomontana (L. Pfeiffer, 1862); empty triangle, top down = Plectopylis bensoni Gude, 1914.