Mercuria corsensis, Boeters & Falkner, 2017

Boeters, Hans D. & Falkner, Gerhard, 2017, The genus Mercuria Boeters, 1971 in France (Gastropoda: Caenogastropoda: Hydrobiidae). West-European Hydrobiidae, Part 13, Zoosystema 39 (2), pp. 227-261 : 249-251

Mercuria corsensis n. sp.

( Figs 11F ; 13 A-F; Table 8)

TYPE LOCALITY AND MATERIAL EXAMINED. — Only the type series is known. France. Corsica, Bonifacio, spring East of the road towards Couvent de la Trinité about 500 m South of the road Ajaccio- Bonifacio [UTM NL08] ( BOE 0916 ), leg. Boeters IX.1979 ; the locality is already mentioned by Caziot (1903: 325) as the finding place of an enigmatic “ Valvata ” which was not refound.

ETYMOLOGY. — Derived from Corsis, greek-latin, a variant of Corsica, combined with the suffix –ensis which indicates the origin.

DISTRIBUTON AND ECOLOGY. — In France only at the southern tip of Corsica at La Trinité. Found only in a single spring together with representatives of the Limnaeidae Lamarck, 1799 and the Planorbidae Rafinesque, 1815 . The ecology of the closely related M. zopissa in Sardinia is described as follows: “Vive in ruscelli e torrenti, tra la vegetazione della riva o attaccata ai ciottoli del fondo. Alcune popolazioni si rinvengono in tazze sorgentizie (Monte dei Sette Fratelli, Sarrabus)” [Lives in brooks and torrents, between the vegetation of the borders or sticking on gravels of the ground. Some populations live in spring ponds (at Monte dei Sette Fratelli, Sarrabus)] ( Giusti & Pezzoli 1980: 23).

DESCRIPTION

Shell

Shape of shell broadly conical to slightly ovoid with rounded apex and convex sidelines; height 3.10 mm [2.85-3.50 mm] and diameter 2.20 mm [2.05-2.30 mm] (n = 8); last whorl about 51.8% [47.2-56.1%] (n = 8) of total height of shell; umbilicated; shell colour transparent milky, shell surface usually with dark-brown or ochreous iron-manganese coating; 3.75-4.0 whorls, slightly swollen and convex, separated by a shallow suture; last whorl towards aperture neither ascending nor descending; aperture obliquely broad ovate, forming at the top a blunt and nearly rectangular angle; peristome continuous, usually touching the shell wall over a short and slightly convex bent distance; outer margin simple, basal margin and columellar edge faintly reflexed, columellar edge slightly thickened.

Measurements

See Table 8.

Operculum

Pale reddish.

Animal

External characters. Pallial tentacle not seen. Mantle strongly blackish pigmented. Ctenidium with 23 to 24 gill filaments.

Male copulatory organ. Penis short but slender and rather whip-like, not extending beyond the elliptical appendix or only slightly longer. Penis blackish pigmented, appendix merely at its branching off.

The female genital tract. Not examined.

DIFFERENTIATING FEATURES

Compared to shells of the Sardinian M. zopissa ( Paulucci, 1882) ( Fig. 11 G-H; Fig. 13 G-H) the shells of M. corsensis n. sp. with a heigth of 3.10 mm [2.85-3.50 mm] and a diameter of 2.20 mm [2.05-2.30 mm] (n = 8) are slightly larger and more ovate, which is reflected by a ratio of height of aperture to total height of c. 52% instead of 42% (females) and 44% (males)in M. zopissa . For example, the shell of a male of M. corsensis n. sp. showed the following measurements: height 2.90 mm, diameter 2.05 mm and height of aperture to total height of 51.6%. Further, whereas in M. corsensis n. sp. the last whorl before the aperture neither descends nor ascends, in M. zopissa the last whorl often descends clearly immediately before the aperture, as can be deduced, for example, even from the frontal view in Fig. 11H. A character which discriminates Mercuria corsensis n. sp. as well as M. zopissa from all other Mercuria species is the shallow suture in combination with the ovate shape.

REMARKS

The new species has been considered by Falkner et al. (2002: 79) as conspecific with Mercuria zopissa ( Paulucci, 1882) from Sardinia, but a new evaluation of its characters led us to acknowledge it as a distinct species. Judging from the nevertheless existing striking morphological similarities, M. zopissa and M. corsensis n. sp. must be considered as closely related sister species. This means they form a group which is differentiated with a significant gap against all other Mercuria -species of France and Spain. As they are exclusively confined to Corsica and Sardinia it can be assumed that their evolution is correlated to the well known Tertiary plate tectonics and the complex palaeogeographic history of the two islands (as for example outlined with instructive maps in de Jong 1998: 113-118). They must share a common ancestral species which lived on the Corso-Sardinan microplate when it became separated from the South European continental margin. As the distance between the northernmost occurrences of M. zopissa and the type locality of M. corsensis n. sp. is about 220 km, the split of the two sister species must predate the younger pleistocene glaciation cycles, otherwise the large gap in the distribution areas can not be explained. In all circumstances today’s occurrences of both species must be considered as relicts of former larger distribution areas on both islands.

TAXA UNCERTAIN, INSUFFICIENTLY KNOWN OR CONFUSED

Figures

FIG. 11. — A-C, Mercuria meridionalis (Risso, 1826), France, Var, Foux de Draguignan, leg. Boeters 27.IX.1969, BOE 2414 ex 0261 (A, B); BOE 2151 ex 0261 (C); A, ♀; B, ♂; C, ♂, pallial tentacle, copulatory organ and pedal lobe clearly to be seen; D, E, Mercuria sp., presumable topotypes of Bithinia [sic] moutonii Dupuy, 1849; D, France, Alpes-Maritimes, Cannes (i.e. Mandelieu-la-Napoule [Népoule sensu Dupuy]), ♀ (MNHN-LOC); E, France, Var, Grasse (MNHN-JOU); F, Mercuria corsensis n. sp. France, Corse-du-Sud, Bonifacio, Couvent de la Trinité, leg Boeters IX.1979 (holotype MNHN-IM-2000-31951 ex BOE 0916); G-H, Mercuria zopissa (Paulucci, 1882), Italy, Sardinia, Monte dei Sette Fratelli, leg Giusti, Topotypes ex SMF 252038/7; G, globular shell; H, slender shell; I-J, Pseudamnicola emiliana (Paladilhe 1869); I, Neotype ex Topotype; France, Hérault, ruisseau [d’eaux douces] près de Balaruc (MNHN-IM-2000-32541 ex de Folin); J, de Folin’s label [“determ. par Paladilhe”, handwriting de Folin]. Scale bars: A, B, D-I, 3 mm; C, 1 mm.