Mercuria balearica ( Paladilhe, 1869 )

Miller, Jonathan P., Delicado, Diana, García-Guerrero, Fernando, Khalloufi, Noureddine & Ramos, Marian A., 2023, Morphology and taxonomic assessment of eight genetic clades of Mercuria Boeters, 1971 (Caenogastropoda, Hydrobiidae), with the description of five new species, European Journal of Taxonomy 866 (1), pp. 1-63 : 21-26

Mercuria balearica ( Paladilhe, 1869)

Figs 11–14 ; Supp. file 2: Tables S10–S13

Amnicola balearica Paladilhe, 1869: 113 .

Mercuria balearica – Boeters 1988: 207, figs 106–107.

Pseudamnicola balearica – Quintana 2019: 19, fig 1a.

Revised diagnosis

Shell ovate-conic; aperture obliquely broad ovate; protoconch microsculpture granulated; central radular tooth formula (4)3-C-3(4)/1-1; glandular oviduct 2.5 times as long as wide; albumen gland longer than capsule gland; bursa copulatrix pyriform to elongate; seminal receptacle elongate, with a short duct; penis darkly pigmented, as long as the penial appendix; penial appendix unpigmented or slightly pigmented at the junction with the penis, rounded, small, base narrow, medially positioned on the inner edge of the penis; nervous system pigmented, elongated (mean RPG ratio = 0.53).

Type material (not examined)

Syntype FRANCE • 1 spec. (sex unknown); UM.PLD.005 ( Breure & Audibert 2017).

Type locality

According to the original description ( Paladilhe 1869), the type locality is Port Mahon, Minorca, Balearic Islands [Port-Mahon (Illes Balears)].

Material examined

SPAIN – Islas Baleares • 30 specs; Minorca, Colarsega River in Port Mahon ; MNCN 15.05/94744 • 30 specs; Minorca, stream near Sant Joan de Carbonell ; MNCN 15.05/94745 • 30 specs; Minorca, Cala en Porter ; MNCN 15.05/94746 • 30 specs; Minorca, Son Bou, Barranc de Bec ; MNCN 15.05/94747 . – Málaga • 30 specs; Ardales, El Chorro Spring ; MNCN 15.05/94732 • 30 specs; spring in Montecorto ; MNCN 15.05/94731 , MNCN 15.05/94737 • 30 specs; trough in Churriana ; MNCN 15.05/94733 • 30 specs; Venta El Pilar Spring ; MNCN 15.05/94734 , MNCN 15.05/94738 • 30 specs; trough in Los Granados ; MNCN 15.05/94735 • 30 specs; stream in Montecorto; MNCN 15.05/94736 . – Cádiz • 30 specs; Alozaina, Valentín Spring ; MNCN 15.05/94739 . – Jaén • 30 specs; Fontanar, stream in Caño de la Rambla ; MNCN 15.05/94740 • 30 specs; Peal de Becerro, Cinco Caños Spring ; MNCN 15.05/94743 . – Granada • 30 specs; Mojácar, Arabic Spring ; MNCN 15.05/94741 ). – Murcia • 30 specs; spring in Sierra de la Muela ; MNCN 15.05/94742 .

Additional locality information is provided in Supp. file 1: Table S1.

Description

SHELL. Ovate-conic, whorls 4–5, height 2–3.5 mm, width 1.4–2.6 mm ( Fig. 11A–F ; Supp. file 2: Table S10); periostracum yellowish to brown; protoconch with 1.5 whorls, ca 350 µm wide, nucleus less than 200 µm wide; protoconch microsculpture granulated ( Fig. 12A–C ); teleoconch whorls convex, separated by a deep suture; body whorl large, convex, occupying about two-thirds of the total shell length; aperture obliquely broad ovate, complete; inner lip thicker than outer lip; aperture margin straight, inner lip touching the shell wall; umbilicus narrow, not covered by the inner lip ( Fig. 11A–F ).

OPERCULUM. As for the genus, orange to brown, sometimes yellowish, about two whorls; muscle attachment oval, located near the nucleus ( Fig. 11G–H ).

RADULA. Length intermediate, ca 800 µm long (35% of total shell length), containing about 65 rows of teeth. Central tooth formula (4)3-C-3(4)/1-1, central cusp V shaped, cutting edge slightly concave ( Fig. 12D ). Lateral tooth formula (3)4-C-4(3), central cusp V shaped and slightly longer than the central tooth one. Inner marginal teeth with 14–18 cusps; outer marginal teeth with 15–17 cusps ( Fig. 12E–F ). Radular data were collected from the following specimens: MNCN 15.05/94745 – stream near Sant Joan de Carbonell, Minorca; MNCN 15.05/94747 – Barranc de Bec, Son Bou, Minorca; MNCN 15.05/94738 – Venta El Pilar Spring, Málaga; MNCN 15.05/94741 – Arabic Spring, Mojácar, Granada; MNCN 15.05/94743 – Fuente de los Cinco Caños Spring, Peal de Becerro, Jaén.

PIGMENTATION AND ANATOMY. Animal slightly darkly pigmented, although unpigmented specimens were also found ( Fig. 11D ); head and tentacles black, pigmentation lighter on eye lobes, snout and neck; snout about as long as wide, approximately parallel-sided, with medium distal lobation. Ctenidium occupying almost the total length of the pallial cavity; 21–27 gill filaments; filaments broad, triangular, fused at the base by an epithelium ( Fig. 13D ). Osphradium elongate, more than 3 times as long as broad, positioned opposite to the middle of the ctenidium. Stomach almost as long as wide with two nearly equally sized chambers (Supp. file 2: Table S11); style sac longer than wide, with the unpigmented intestine surrounding its distal part before continuing on as a straight rectum ( Fig. 13E ).

MALE GENITALIA. Prostate gland bean-shaped, about 2 times as long as wide (Supp. file 2: Table S13), connected by the posterior vas deferens to a convoluted seminal vesicle and the testis ( Fig. 13C ). Penis darkly pigmented, gradually tapering, attached to the neck behind the right eye; penial appendix and distal end of the penis nearly equal in size; penial appendix unpigmented or slightly pigmented at the junction with the penis, rounded, small, base narrow, medially positioned on the inner edge of the penis ( Fig. 13B ).

FEMALE GENITALIA. Glandular oviduct 2.5 times as long as wide; albumen gland longer than capsule gland ( Fig. 13A ); bursa copulatrix pyriform ( Fig. 14B–D ) to elongate ( Fig. 14A ), ca 2 times as long as wide (Supp. file 2: Table S12), lying against the albumen gland; bursal duct shorter than bursa length; renal oviduct unpigmented, coiled, with three loops; coils can be wide open ( Fig. 14E ) or highly coiled ( Fig. 14F–G ); seminal receptacle elongate, with a short duct, positioned on the distal part of the renal oviduct just above the junction with the bursal duct ( Fig. 14E–G ).

NERVOUS SYSTEM. Pigmented, elongated (mean RPG ratio = 0.53; see Supp. file 2: Table S15); cerebral ganglia approximately equal in size; pleuro-supraoesophageal connective ca 9 times as long as pleuro-suboesophageal one ( Fig. 13F ).

Ecology and distribution

Mercuria balearica presents broad ecological plasticity, inhabiting springs, streams and ponds with conductivities ranging from 179 to 2750 µS/cm. Population densities were low. During our field surveys, we found evidence of local extinctions in the regions of Andalusia and Minorca (e.g., extirpated populations in the towns of Zuheros, Arriate, Alozaina, Gaucín, Ferreries), suggesting that the species was more abundant in the late twentieth century. This species occurs sympatrically with Pseudamnicola meloussensis Altaba, 2007 , Theodoxus aff. fluviatilis , Melanopsis spp. and Potamopyrgus antipodarum .

The species was previously cited from springs, streams, small rivers and ditches in the southern Iberian Peninsula and on the Balearic Islands of Ibiza and Minorca ( Boeters 1988; Glöer et al. 2015). We confirmed the species in Minorca (Ibiza was not included as a field site) and in several other springs and streams in south-eastern Iberia ( Fig. 1 ).

Remarks

The taxonomy of the species has undergone several revisions, with the most recent change occurring in 2019, when Quintana (2019) placed the species in the genus Pseudamnicola Paulucci, 1878 . He had found an abundant population living in Colarsega River at Port-Mahon ( Minorca Island), which is the type locality of M. balearica . He sent a subsample to the MNCN in Madrid so that we could review the classification. We confirmed that all the specimens received belong to Pseudamnicola meloussensis and communicated to Quintana that more exhaustive sampling would be necessary to find specimens of M. balearica . However, he thought that if all the specimens found in this locality are Pseudamnicola , then M. balearica and P. meloussensis are probably the same species and they should be renamed as Pseudamnicola balearica , the species described by Paladilhe (1869) (as Amnicola balearica ), with P. meloussensis consequently being a junior synonym of P. balearica . However, during a more detailed sampling of this population in 2018, we were able to collect a few specimens of M. balearica living together with P. meloussensis . The DNA-based sequences ( Miller et al. 2022) and shell morphology ( Fig. 11C ) of these specimens correspond to those of a member of Mercuria , confirming their placement in this genus (and their continued presence at the type locality).

Our morphological observations (this study) and DNA results ( Miller et al. 2022) confirm the previous assignments of other Iberian populations to M. balearica ( Boeters 1988; Glöer et al. 2015). Populations from the stream near Sant Joan de Carbonell, Minorca (MNCN 15.05/94745), Venta El Pilar Spring, Málaga (MNCN 15.05/94738) and Fuente Valentin Spring, Cádiz (MNCN 15.05/94739) accounted for most of the morphological variation observed between Minorcan and Iberian populations. Sequence divergence of COI among these populations was low (<1.2%) ( Miller et al. 2022).

Intraspecific morphological differences for this species are observed mainly in shell, penis and female genitalia features. Specimens from Minorca are generally larger in size and present globose to high-spired shells, whereas the Iberian specimens have only globose shells (Supp. file 2: Table S10). The specimens from Los Granados Trough in Málaga (MNCN 15.05/94735) have the smallest shells. All the specimens have a pigmented epithelium, though some specimens from Arabic Spring in Mojácar (MNCN 15.05/94741) are unpigmented. Variation in the male reproductive organs is as previously described for the genus: the penis is either slightly longer or shorter than the appendix, and the appendix is usually rounded and unpigmented (only the population in Peal del Becerro, Jaén presents a slightly pigmented appendix). The bursa copulatrix varies from pyriform in most of the populations to elongate in the specimens from Venta El Pilar Spring, Málaga (MNCN 15.05/94734). Populations from Minorca have both morphotypes. Females from Venta El Pilar Spring (MNCN 15.05/94734), the stream in Montecorto (MNCN 15.05/94736), Fuente Valentín Spring (MNCN 15.05/94739) and Arabic Spring (MNCN 15.05/94741) have a bursa copulatrix that surrounds the albumen gland, which is located beneath it (see Fig. 14C ) rather than parallel to it (the typical orientation in other members of the genus).

Mercuria balearica can be distinguished from M. similis by its smaller, less globose shell, less convex body whorl, shorter tapering triangular penis with a rounded unpigmented penial appendix that is slightly larger than or equal in size to the penis, pyriform bursa copulatrix and higher number of cusps on the central and lateral teeth of the radula. Mercuria balearica resembles M. carrillorum sp. nov. and M. felixi sp. nov. in shell shape (ovate-conic) and colour but differs from these congeners by having a shorter, more pigmented penis with a larger rounded penial appendix. Also, their distribution areas do not overlap. Sequence divergences for COI indicated that M. balearica is most closely related to M. tensiftensis and M. felixi (5.2% mean divergence with both species) and most distantly to M. midarensis and M. melitensis ( Paladilhe, 1869) (8.6% mean divergence with both species) ( Miller et al. 2022).

Figures

Fig. 11. Shells and operculum of Mercuria balearica (Paladilhe, 1869). A–B, G–H. Stream at Sant Joan de Carbonell, Minorca, Spain. C. Colarsega River, near the Port of Mahon (type locality). D. Fuente Árabe Spring, Mojácar, Granada, Spain (unpigmented specimen). E. Venta El Pilar Spring, Málaga, Spain. F. Spring, Alozaina, Cádiz, Spain. G. Operculum, inner side. H. Operculum, outer side.

Fig. 12. Protoconch and radula of Mercuria balearica (Paladilhe, 1869). A–C. Venta El Pilar Spring, Málaga, Spain. A. Details of the protoconch. B. Details of the protoconch nucleus. C. Protoconch microsculpture. D–F. Stream at Sant Joan de Carbonell, Minorca. D. Overview of radular teeth rows. E. Rows of lateral and inner marginal radular teeth. F. Detailed view of the outer marginal teeth.

Fig. 13. Anatomy of Mercuria balearica (Paladilhe, 1869). A, D, F. Stream at Sant Joan de Carbonell, Minorca, Spain. B, C, E. Venta El Pilar Spring, Málaga, Spain. A. Female genitalia. B. Penis and head. C. Prostate gland. D. Ctenidium and osphradium. E. Stomach. F. Perioesophageal ring.

Fig. 14. Variation of the bursa copulatrix of Mercuria balearica (Paladilhe, 1869). A. Elongate, stream at Sant Joan de Carbonell, Minorca, Spain. B. Pyriform, Venta El Pilar Spring, Málaga, Spain. C–D. Pyriform, Fuente Valentín Spring, Alozaina, Cádiz, Spain. E–G. Detailed views of the seminal receptacle (Sr) and renal oviduct loops.

Fig. 1. Phylogenetic clustering and geographic location of the studied populations of Mercuria Boeters, 1971 throughout the western Palaearctic region. The tree topology on the left has been modified from Miller et al. (2022) to show the species clades (i.e., coloured clades) of the populations described here. One population (grey dot) represents the distribution of M. maceana (Paladilhe, 1869). Scale bar below topology: expected change per site.