MERCURIINAE, 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 : 229-230

Subfamily MERCURIINAE n. subfam.

TYPE GENUS. — Mercuria Boeters, 1971: 175 .

DIAGNOSIS. — Small to medium-sized Hydrobiidae (height 2-6 mm), shell ovate-conical with pointed apex, rounded whorls, deep suture and narrowly perforate umbilicus; milky coloured, especially at the base. Operculum corneous, subspiral, slightly concave, with eccentric nucleus, external surface with rough spiral scratches directed to the outer edge; reddish, transparent, nuclear region thickened and more intensely coloured. The foot is provided with lateral footwings on both sides of the operculum. Tentacles long and filiform, little pigmented. Mantle dark or blackish, mantle edge with pallial tentacle, which in some species may be reduced to a knob-like protuberance. Ctenidium well developed with 20-30 filaments. Radula taenioglossate with acute mesocones in central and lateral teeth. Intestine coil around style sac tight; intestine in pallial cavity straight. Female genitalia: renal oviduct coiled, gonopericardial duct present; one seminal receptacle distally positioned to renal oviduct, and lying against funnel-like basis of bursa copulatrix at junction with bursal duct; bursa copulatrix distally positioned to renal oviduct, and lying against albumen gland, ventral wall of pallial oviduct closed. Male reproductive organ bilobed, penial appendix bulbous without distict glandular structures, penis proper and branching region pigmented with black. Habitat: Coastal, often fluctuating waterbodies, springs and their outflow, tide areas of rivers, brooks, coastal lakes and swamps.

DIFFERENTIATING FEATURES

Versus Nymphophilinae Taylor, 1966 : according to Hershler et al. (2003: 362) the monophyletic North American Nymphophilinae are characterised by the unique presence of surficial glandular fields on the penis which are completely absent in all anatomically studied species of Mercuria . Nymphophilinae have a multispiral operculum, e.g., Nymphophilus minckleyi Taylor, 1966 , the monotype of Nymphophilus Taylor, 1966 , has an operculum with about 6 whorls ( Taylor 1966: 200, fig. 20; Thompson 1979: 42, 46). In Mercuria , in all species the number of whorls of the operculum is less than 2 (e.g., Adam 1940: 4, fig. 2B). In the radula of Nymphophilinae the central tooth often tends to form a broadened and rounded mesocone; especially in Nymphophilus minckleyi the central tooth and the lateral tooth bear a large blunt-edged mesocone ( Thompson 1979: 42, 44, fig. 10), whereas in Mercuria the mesocones of these teeth are acute ( Adam 1940: 4, fig. 2A; Giusti 1979: 9, figs 5A, B, 11, fig. 6C).

Versus Pseudamnicolinae Radoman, 1977: in representatives of the Pseudamnicolinae the shell is brownish while in Mercuria it is milky. In males of Pseudamnicola such as P. lucensis (Issel, 1866) , the type species, the penis is simple without any lobe ( Boeters 1971: 178, fig. 3). In Mercuria the verge is bifurcated, composed of the penis and a lobe (e.g., Boeters 1971: 178, fig. 7). In females the duct of the seminal receptacle (RS1 sensu Radoman 1983) branches off the renal oviduct at a distance of the coiled section of the renal oviduct and closely in front of the duct of the bursa copulatrix ( Boeters 1971: 178, fig. 4). In Mercuria the receptaculum seminis (RS2 sensu Radoman 1983) is positioned at the distal end of the coiled section of the renal oviduct ( Boeters 1971: 178, fig. 8).