Mercuria midarensis, Boulaassafer, Khadija, Ghamizi, Mohamed & Delicado, Diana, 2018

Boulaassafer, Khadija, Ghamizi, Mohamed & Delicado, Diana, 2018, The genus Mercuria Boeters, 1971 in Morocco: first molecular phylogeny of the genus and description of two new species (Caenogastropoda, Truncatelloidea, Hydrobiidae), ZooKeys 782, pp. 95-128 : 102-105

Mercuria midarensis sp. n.

Type material.

Holotype, MNCN 15.05/200019H (Ethanol 80%), a small ditch 7 km from Midar, Northern Morocco, 34°54.5795'N, 3°34.0292'W, 03/06/2015, leg. K.B., M.G., D.D., T.H. Paratypes MNCN 15.05/200019P, UGSB 17921, and MHNM 18 ZTMH12 (from the same lot).

Other material.

MOROCCO. MHNM 18 ZTMH20, UGSB 17921, UGSB 17922, a small ditch 7 km from Midar, 03/06/2015 (34°54.5795'N, 3°34.0292'W); MHNM 18 ZTMH13, UGSB 19933, Selouan River, S of Nador, 30/04/2016 (35°4.6117'N, 2°55.485'W); MHNM 18 ZTMH14, UGSB 19939, Ouzej River, Al Aaroui, 30/04/2016, (35°0.3634'N, 2°59.5133'W); UGSB 19935, Cherarba ponds W of Saidia, 28/04/2016 (35°6.3116'N, 2°20.75'W); MHNM 18 ZTMH15, UGSB 19934, Izerouan River, 20 km W of Nador, 12/05/2015 (35°9.8333'N, 3°6.6'W); MHNM 18 ZTMH16, UGSB 19940, Mariouari River, 30/04/2016 (35°18.36'N, 2°58.6483'W); MHNM 18 ZTMH17, UGSB 19938, Messoussate River, Selouan, 02/05/2016 (35°3.81'N, 2°54.383'W). SPAIN. MHNM 18 ZTMH18, UGSB 19937, Rio de Oro, Melilla, 18/05/2015, (35°17.2483'N, 2°56.6283'W).

Diagnosis.

Shell ovate-conic, whorls 4-5; periostracum whitish; body whorl large, convex, occupying more than three-quarters of total shell length; aperture ovate, complete; umbilicus narrow, not covered by the inner lip; protoconch microsculpture granulated; operculum dark orange to dark brown; central radular tooth formula (3)4 –C–4(3)/1– 1; bursa copulatrix elongate, with a short duct; one seminal receptacle elongate without duct; penis gradually tapering to strap-like, light to dark grey pigmented; penial appendix similarly pigmented, shorter than penis, base wide, medially positioned on inner edge of penis; nervous system extremely elongated (mean RPG ratio = 0.70), slightly pigmented.

Description.

Shell ovate-conic, whorls 4-5, height 3-4.7 mm (Figure 10 A–J; Suppl. material 1: Table 1). Periostracum whitish. Protoconch ca. 400 µm wide, whorls 1.5; nucleus ca. 125 µm wide; protoconch microsculpture granulated (Figure 10 K–L). Teleoconch whorls shouldered, separated by deep sutures. Body whorl large, occupying three-quarters of total shell length. Aperture ovate, complete, sometimes in contact with the body whorl; inner lip thicker than outer lip; peristome margin straight (Figure 10 I–J). Umbilicus narrow, not covered by the inner lip.

Operculum as for genus, dark orange to dark brown, surrounded by a thin and transparent border, whorls 2, muscle attachment area oval and located near the nucleus (Figure 10 M–N). Radula length intermediate, ca. 900 µm long (23% total shell length), 7.5 times longer than wide (Figure 11A, F; Suppl. material 1: Table 2); 53-67 rows of teeth. Central tooth formula (3)4 –C–4(3)/1– 1; central cusp V-shaped, long (Figure 11 B–D, G–I). Lateral tooth formula (2)3 –C– 3(2); central cusp tongue-shaped (Figure 11J). Inner marginal teeth with 12-14 long, pointed, cusps. Outer marginal teeth bearing 18-20 cusps (Figure 11 E–J, Suppl. material 1: Table 2).

Head and tentacles dark brown pigmented; eye lobes and snout margin unpigmented; pigmentation lighter on neck (Figure 12 G–H). Ctenidium well developed, with 21-26 gill filaments, occupying most of pallial cavity. Osphradium elongate, positioned opposite middle of the ctenidium (Figure 12A). Stomach slightly longer than wide; style sac shorter than stomach, surrounded by an unpigmented intestine (Figure 12B, Suppl. material 1: Table 3). Glandular oviduct from two to three times longer than wide. Albumen gland shorter than capsule gland. Bursa copulatrix elongate, from two to three times longer than wide. Bursal duct very short. Renal oviduct unpigmented, coiled. Seminal receptacle elongate, without duct (Figure 12 D–F, J, Suppl. material 1: Table 4). Prostate gland bean-shaped, approx. two times longer than wide (Figure 12I). Seminal duct entering the posterior region and pallial vas deferens emerging close to its anterior edge. Penis gradually tapering to strap-like, attached well behind right eye, base large and slightly pigmented. Penis tapering, light grey pigmented. Penial appendix light grey, shorter than penis, two times longer than wide, base wide, medially positioned on inner edge of penis (Figure 12 G–H, K–L; Suppl. material 1: Table 5). Terminal gland large, occupying the whole distal end of the appendix. Nervous system slightly pigmented, extremely elongated (mean RPG ratio 0.70; Suppl. material 1: Table 6); cerebral ganglia almost equal in size and shape (Figure 12C).

Etymology.

This species is named midarensis after Midar, the nearby city where this species was collected for first time.

Distribution.

Mercuria midarensis sp. n. is distributed mostly in spring-fed and riverine habitats of northeastern Morocco and the surroundings of the Spanish city of Melilla.

Remarks.

The mean uncorrected sequence divergence within Mercuria midarensis was 1.8%, ranging from 0%-3.4%. Despite their geographic proximity, M. midarensis populations were genetically resolved into two geographically separate groups: the northern populations, i.e. those from the Mariouari River, Rio de Oro, and Izerouan River basins; and the southern ones, i.e. ditch in Midar (type locality), Selouan River, Ouzej River, and Cherarba Ponds. Although mean genetic distances between these two groups of populations were relatively high ranging from 2.6% to 3.4% for COI, we found no consistent morphological differences to consider them distinct. However, we did observe shell variability within each group. For most localities, shell length mostly ranged from 3.0 mm to 4.5 mm, with the body whorl occupying 75-82% total shell length. As exceptions, specimens from the Mariouari and Selouan river basins had larger shells (4.0-5.5 mm and 3.6-4.8 mm, respectively) and a body whorl occupying 60-75% total shell length (Figure 10C, F). Although female genitalia morphology and morphometry were similar in all populations of the species, we detected variability in penis shape and size. Both the southern populations and the population from the Rio de Oro basin (within the northern group) are characterized by a long penis (2.1-3.2 mm), 2 to 3 times longer than the appendix and 1.5 to 3 times longer than the head. In contrast, northern populations have a shorter penis (0.6-2.1 mm), which is 2.6 to 4 times longer than the appendix. Moreover, in the population from the Izerouan River basin, the penis is 1.4 to 1.8 times longer than the head while it is shorter than the head in the population from the Mariouari River basin.

Mercuria bakeri and M. tingitana have more elongate, tall-spired shells than most of the populations of M. midarensis sp. n., except for specimens from the Mariouari and Selouan rivers. Mercuria midarensis sp. n. differs from M. bakeri in its shorter seminal receptacle, shorter bursal duct, and larger penial appendix, and from M. tingitana in its longer seminal receptacle, larger penis, and fewer cusps on the radular teeth. The mean genetic distance between M. bakeri and M. midarensis sp. n. was 6.8% while between the latter and M. tingitana it was 8.5%.

Ecology.

Most specimens were found in small ditches or river tributaries attached to stones or simply in the sediment. Mercuria midarensis sp. n. co-occurs with other gastropod species such as Melanopsis praemorsa , Galba truncatula , Ancylus fluviatilis , and Physella acuta .