Mercuria tachoensis ( Frauenfeld, 1865 )

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 : 15-21

Mercuria tachoensis ( Frauenfeld, 1865)

Figs 8–10 ; Supp. file 2: Tables S6–S9

Amnicola tachoensis Frauenfeld, 1865: 529 .

Mercuria edmundi Boeters, 1986: 126 , fig 4–7. pl. 18a fig. 2.

Mercuria edmundi – Boeters 1988: 209, figs 96–98. — Holyoak et al. 2017: 208, fig. 2.

Mercuria tachoensis – Boeters 1988: 207, figs 99–103.

non M. bayonnensis ( Locard, 1894) – Boeters & Falkner 2017 [in part]: 230, fig. 6p.

non M. anatina ( Poiret, 1801) – Boeters & Falkner 2017 [in part]: 239, fig. 2.

Revised diagnosis

Shell ovate-conic; aperture obliquely and broadly ovate; protoconch microsculpture pitted; periostracum whitish to pale grey; central radular tooth formula (3)4-C-4(3)/1-1; female genitalia with bursa copulatrix elongate, ca 4 times as long as wide; seminal receptacle pyriform; penis darkly pigmented; penial appendix longer than the distal end of the penis, strongly pigmented at the junction with the penis, pigmentation gradually weakens from the junction until two-thirds of the appendix where it is weak; penial appendix base narrow, distally positioned on the inner edge of the penis; nervous system pigmented, elongate (mean RPG ratio = 0.61); cerebral ganglia approximately equal in size.

Type material (not examined)

Syntypes PORTUGAL • 2 specs (sex unknown); NHMW 113526 to 113531 .

Type locality

According to the original description, the type locality is a spring close to the Tagus River [Quellen des Tajo bei Ajuda] near Ajuda, Lisbon, Portugal.

Material examined

PORTUGAL • 30 specs; Mataçaes, Fonte dos passarinhos Spring ; MNCN 15.05/94807 • 30 specs; Coimbra, Fonte dos Amores Spring ; MNCN 15.05/94809 • 30 specs; Coimbra, spring in Jardim da Sereia ; MNCN 15.05/94810 • 30 specs; Leiria, Alpedriz Spring, Nascente do Senhor Jordão ; MNCN 15.05/ 94811 • 30 specs; Leiria, Alpedriz, Das Mouras Spring ; MNCN 15.05/94812 • 30 specs; Leiria, spring in Salir de Matos ; MNCN 15.05/94813 • 30 specs; Leiria, São Gregório da Fanadia, Padre Antonio Spring ; MNCN 15.05/94814 • 30 specs; Santarém, spring along road N114 near the crossing to Moçarria and Vila Nova da Babeca ; MNCN 15.05/94815 • 30 specs; Santarém, Ereira, spring in Rua da Fonte ; MNCN 15.05/94816 • 30 specs; Leiria, spring in São Mamede ; MNCN 15.05/94817 • 30 specs; Lisbon, pond in Pragança ; MNCN 15.05/94818 • 30 specs; Lisbon, Ericeira, Fonte do Cabo Spring ; MNCN 15.05/94819 • 30 specs; Lisbon, Mafra, Fonte dos Tritôes Spring ; MNCN 15.05/94820 • 30 specs; Lisbon, Carvoeira, spring in Valbom ; MNCN 15.05/94821 • 30 specs; Mafra, Ribiera da Maciel Forro , Fonte da Ribera Spring ; MNCN 15.05/94822 • 30 specs; Lisbon, Almargem do Bispo, spring in Vale de Lobos ; MNCN 15.05/94823 • 30 specs; Setúbal, Oleiros, Fonte de Oleiros Spring ; MNCN 15.05/94824 • 30 specs; Baixo Alentejo near Pomarão , Guadiana River ; MNCN 15.05/94825 .

UNITED KINGDOM • 1 spec.; Arundel, stream outflowing from Swanbourne Lake ; UGSB 14156 • 280 specs; Barking, Cuckold’s Haven Wetland , reed belt of Hand Trough Creek ; UGSB 14157 • 30 specs; West Sussex, Burpham, Arun Banks ; MNCN 15.05/94828 .

SPAIN • 30 specs; Asturias, Camoca, Fuente Tebia Spring ; MNCN 15.05/94808 .

FRANCE • 30 specs; Mouguerre, stream near Chemin d’Elizaberry ; MNCN 15.05/94826 • 30 specs; stream near the beach in Bidart ; MNCN 15.05/94827 .

THE NETHERLANDS • 8 specs; Hoogvliet, small stream; UGSB 23178 • 8 specs; Hoogvliet, junction of the running surface in the tidal area; UGSB 23179 • 3 specs; Hoogvliet, Ruigeplaatbosch Park , Oude Maas River ;; wet reed land; UGSB 23177 .

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

Description

SHELL. Ovate to ovate-conic, whorls 4–5, height 2–4 mm, width 2–3 mm ( Fig. 8A–G ; Supp. file 2: Table S6); periostracum yellowish to pale grey, occasionally dark brown; protoconch of 1.5 whorls, ca 350 µm wide, nucleus ca 180 µm wide ( Fig. 9A–B ); protoconch microsculpture pitted ( Fig. 9C ); 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 slightly reflexed; inner lip attached to the body whorl; umbilicus narrow, not covered by the inner lip ( Fig. 8A, C–G ).

OPERCULUM. As for the genus, orange to brown, sometimes yellowish, about two whorls; muscle attachment oval, located near the nucleus ( Fig. 8H–I ).

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. 9D ). Lateral tooth formula (3)4-C-4(3), central cusp V shaped and slightly longer than the central tooth one. Inner marginal teeth with 12–17 cusps; outer marginal teeth with 17–27 cusps ( Fig. 9E–F ). Radular data were collected from the following specimens: MNCN 15.05/94808 – Fuente Tebia Spring, Camoca, Asturias, Spain; MNCN 15.05/94820 – Dos Tritôes Spring, Mafra, Lisbon, Portugal; MNCN 15.05/94821 – spring in Valbom, Carvoeira, Lisbon, Portugal; MNCN 15.05/94824 – Fonte de Oleiros Spring, Oleiros, Setúbal, Portugal; MNCN 15.05/94826 – stream near Chemin d’Elizaberry, Mouguerre, France; MNCN 15.05/94827 – stream near the beach in Bidart, France; MNCN 15.05/94828 – Arun Banks, Burpham, West Sussex, United Kingdom.

PIGMENTATION AND ANATOMY. Animal usually darkly pigmented, head and tentacles black to brown, pigmentation lighter on eye lobes and snout; snout about as long as wide, approximately parallel-sided, with medium distal lobation ( Fig. 10B ). Ctenidium occupying almost the total length of the pallial cavity; 20–26 gill filaments; filaments broad, triangular, fused at the base by an epithelium ( Fig. 10D ). Pallial tentacle present. Osphradium elongate, more than 3 times as long as broad, positioned opposite middle of ctenidium. Stomach almost as long as wide with two chambers almost equal in size (Supp. file 2: Table S7); style sac longer than wide, with the unpigmented intestine surrounding its distal end before continuing on as a straight rectum ( Fig. 10E ).

MALE GENITALIA. Prostate gland bean-shaped, about 2 times as long as wide (Supp. file 2: Table S9), connected by the posterior vas deferens to a convoluted seminal vesicle and the testis. Pallial vas deferens emerge near both the anterior end of the prostate gland and the external margin of the penis ( Fig. 10C ). Penis darkly pigmented, gradually tapering, attached to the neck behind the right eye; penial appendix longer than the distal end of the penis, strongly pigmented at the junction with the penis, pigmentation gradually weakens from the junction until two-thirds of the appendix where it is weak. Penial appendix base narrow, distally positioned on the inner edge of the penis ( Fig. 10B ).

FEMALE GENITALIA. Glandular oviduct 2.5 times as long as wide; albumen gland and capsule gland about the same length ( Fig. 10A ); bursa copulatrix elongate, ca 4 times as long as wide (Supp. file 2: Table S8); bursal duct shorter than bursa copulatrix; renal oviduct unpigmented, highly coiled with three loops; seminal receptacle pyriform, with a short duct, positioned on the distal part of the renal oviduct just above the junction with the bursal duct ( Fig. 10A ).

NERVOUS SYSTEM. Slightly pigmented, elongate (mean RPG ratio = 0.61; see Supp. file 2: Table S15); cerebral ganglia approximately equal in size; pleuro-supraoesophageal connective ca 7 times as long as pleuro-suboesophageal one ( Fig. 10F ).

Ecology and distribution

Mercuria tachoensis has a lower conductivity tolerance than M. similis . The analysed populations of this species from Spain, Portugal and France inhabit springs, streams and ponds with conductivities from 359 to 1805µS/cm. Water parameters of the localities in the UK and the Netherlands were not available. Specimens were usually found inside the water. Amphibious behaviour has not been commonly observed in this species, except for populations located on the tide banks of the Guadiana River (David Holyoak pers. com.) and those of the Arun River (Martin Willing pers. com.). The species occurs sympatrically with the gastropod species Theodoxus aff. fluviatilis (Linnaeus, 1758) , Belgrandia lusitanica (Paladilhe, 1867) , and Potamopyrgus antipodarum .

This species was previously recorded from springs and streams in Portugal. In this study, we report new records, extending its distribution to include northern Spain, northern France, the UK and the Netherlands ( Fig. 1 ). At these localities, the species was found in the tidal mud of small rivers near the river mouth. According to this extended distribution, M. tachoensis inhabits water bodies that have an Atlantic drainage. The type locality of the species has likely disappeared due to the extensive growth of the city of Lisbon, intensive exploitation of local aquifers and pollution. Recent surveys around Ajuda, Lisbon, did not yield any new records (Rui da Costa Mendes pers. com.).

Remarks

Based on our morphological observations and previous phylogenetic results ( Miller et al. 2022), we suggest several synonymization of several previously described species of Mercuria to M. tachoensis (see Discussion), which increases the previously known geographic range and intraspecific variation of this species.

The species’ moderate shell variation can be observed across its geographic range ( Figs 1 , 8 ). Also, the PCA ( Fig. 3 ) showed near the all populations identified by Miller et al. (2022) as M. tachoensis , (although it is closely related in shape to M. balearica ), suggesting a low level of intraspecific variation in shell shape. This variability is due to differences in overall shell shape and colour pattern. Regarding shell shape, we found populations with only elongated ovate-conic shells [e.g., Fonte dos Amores Spring (MNCN 15.05/94809), spring in Jardim da Sereia (MNCN 15.05/94810), spring in Rua da Fonte, Ereira (MNCN 15.05/94816), Arun Banks (MNCN 15.05/94828)] or only ovate-conic shells [e.g., spring in Salir de Matos (MNCN 15.05/94813), Fonte dos Tritôes Spring (MNCN 15.05/94820)]. We also observed populations with both shell morphotypes [e.g., Fonte dos Passarinhos Spring (MNCN 15.05/94807), spring in Vale de Lobos (MNCN 15.05/94823), Fonte de Oleiros Spring (MNCN 15.05/94824)]. Colour pattern does not show high intraspecific variation: only the specimens from the spring in Vale de Lobos (MNCN 15.05/94823) are remarkable for being particularly darkly coloured with a dark brown thick periostracum ( Fig. 8F ) compared with the rest of the specimens, which have yellowish-brown periostracum.

Anatomically, the bursa copulatrix is about the same size in all of the dissected specimens of the species, except for those from the spring in Jardim da Sereia (MNCN 15.05/94810) and Nascente do Senhor Jordão (MNCN 15.05/94811), which are considerably smaller (Supp. file 2: Table S8). Males from the stream near Chemin d’Elizaberry (MNCN 15.05/94826) and Nascente do Senhor Jordão, Alpedriz (MNCN 15.05/94811) have the smallest prostate glands and penises. Despite this, all males present the same penis characteristics: a rounded, pigmented penial appendix that is longer than the distal end of the penis. However, Boeters (1988) described the penis as longer than the penial appendix. Based on this discrepancy in the size and shape of the male genitalia, Holyoak et al. (2017) proposed M. edmundi as a junior synonym of M. tachoensis and described an allometric growth of the male genitalia related to the sexual maturity of the animals. Miller et al. (2022) confirmed with taxonomic multilocus data that only one species occurs in the localities where both species have been cited.

Mercuria tachoensis differs from all the Iberian congeners by its pitted protoconch microsculpture (which is granulated in the rest of the species; visible only with electronic microscopy). In addition to this character, it differs from the phylogenetically closely related species M. similis and M. egarensis sp. nov. by often having one more cusp in the central radular teeth and a longer bursa copulatrix (ca 4 times as long as wide). Although our PCA ( Fig. 3 ) was unable to differentiate M. tachoensis and M. carrillorum sp. nov. by shell shape, both species can be distinguished by differences in the radula (smaller in M. carrillorum and often with one cusp less in the lateral teeth), the bursa copulatrix (smaller in M. carrillorum ) and the male genitalia (the distal end of the penis is longer than the penial appendix, which is flattened, ovate and less pigmented in M. carrillorum ).

Figures

Fig. 8. Shells and operculum of Mercuria tachoensis (Frauenfeld, 1865). A–B. Spring at Saô Mamede, Leiria, Portugal. C. Oude Maas at Hoogvliet, Netherlands. D. Das Mouras Spring, Alpedriz, Leiria, Portugal. E.Arun Banks, Burpham, West Sussex, United Kingdom.F. Spring at Vale de Lobos, Almargem do Bispo, Lisboa, Portugal. G–I. Spring at Jardim da Sereia, Coimbra, Portugal. H. Operculum, inner side. I. Operculum, outer side.

Fig. 9. Protoconch and radula of Mercuria tachoensis (Frauenfeld, 1865). A–C. Fonte dos Amores Spring, Coimbra, Portugal. A. Protoconch. B. Details of the protoconch. C. Protoconch microsculpture. D–E. Stream near the beach in Bidart, France.D. Detailed view of the central and lateral teeth.E. Detailed view of the outer marginal teeth. F. Spring at Jardim da Sereia, Coimbra, Portugal.

Fig. 10. Anatomy of Mercuria tachoensis (Frauenfeld, 1865) from Nascente do Senhor Jordão Spring, Alpedriz, Leiria, Portugal. A. Female genitalia. B. Animal and penis. C. Prostate gland. D. Ctenidium and osphradium. E. Stomach. F. Perioesophageal ring.

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.

Fig. 3. Principal components analysis plot for species of Mercuria Boeters, 1971 based on 21 coordinates (9 landmarks and 12 semilandmarks). Each dot on the plot represents the consensus of the shape of each of the sampled populations. Warp grid deformations (TPS) show expansion (in red colour) and contraction (in blue) of the shell shape.