Tandonia nigra (C. Pfeiffer, 1894)

Schallenberg, Vivianne M., Heim, Rene, Schneppat, Ulrich E., Mueller, Peter, Rueetschi, Joerg & Neubert, Eike, 2022, Revision of the family Milacidae from Switzerland (Mollusca, Eupulmonata, Parmacelloidea), ZooKeys 1116, pp. 149-179 : 149

Tandonia nigra (C. Pfeiffer, 1894)

Figs 4 , 5 , 7C, D , 8

Tandonia nigra Amalia Tandonia nigra C. Pfeiffer, 1894, Nachrichtsblatt der Deutschen Malakozoologischen Gesellschaft, 26: 68 [Gipfel des Mte. Generoso (1695 m.)].

Type specimen.

Holotype SMF 107558.

Differential diagnosis.

Torus with spikes inside the vagina, epiphallus with a field of nodes on the surface; for other character states, refer to the paragraph under T. rustica .

Description.

Colouration. The animals at the type locality are dark blackish brown coloured with the dorsum almost black (Figs 4 , 8B, C ). In lower altitudes of the mountain, animals were dark grey (Fig. 8A ) to almost white in colour, with the mantle and dorsum finely blotched (Fig. 8D, E ). Two animals also had a light greyish cream ground colouration, one with darker small grey blotches all over the body (Fig. 8F ).

The mantle generally matches the dorsal colour. For the topotypic specimen, dots at the edges of the mantle are lacking. However, in light colour morphs, the mantle appears slightly darker than the dorsum because of the accumulated dots and blotches. In many of these specimens the highest density is along the sinus groove.

For the topotypical colour morph the flanks are pigmented as the dorsum, but with a narrow, paler greyish stripe just above the edge of the sole with little blackish grey dots and stripes. This also refers to the flanks below the edge of mantle. In the light colour morphs, the dorsum and flanks are covered by very fine dark dots on the top of the tubercles (not along the grove lines; not a reticulation). This pigmentation pattern is found from the dorsum down to the edge of the sole. Some larger, irregularly scattered black spots more posterior on dorsum and flanks add to this remarkable colouration. The flanks below the mantle lack this dark pigmentation and are of a paler colour than the rest.

Neck and head in the dark colour morphs are black as are the ommatophores. The ommatophores are somewhat translucent, and the black ommatophoran retractor can be seen through the integument. No black dotting on the ommatophores is visible. The tentacles are black. In the light colour morphs, the head, neck and the ommatophores are always darker if compared to the body colour.

The keel is of the same colour as the body and thus difficult to see in a crawling animal.

The sole is uniformly creamy yellowish grey in all specimens. In one topotypic specimen, the posterior ends of the lateral sole fields are pigmented with dark grey dots. In two other specimens, the seam of the sole is pale grey with irregularly dispersed, small black dots.

Mantle structures. The pneumostome is positioned on the right side, at ca. ¾ of mantle length, well posteriorly of the middle of the mantle, surrounded by a narrow and almost invisible ring-like structure (Fig. 4F ). The visibility of surface structures in living animals strongly depends on the age of the specimen, its condition, and the general air humidity when it is observed. The surface of mantle, besides the sinus groove, can be totally smooth (high temperature/humidity and high production of mucus) or very finely crenulated, or a stage between; the sinus groove is completely visible, often accentuated by a dark pigmentation, reaching almost the end of the posterior mantle edge. The slit of the pneumostome runs anteriorly to at least the dorsal edge of pneumostome. The posterior margin of the mantle is not tightly attached to the integument. The posterior free mantle flap covers the anterior integument-tubercles (Fig. 4A ) as well as the slit-like transverse openings of the postpallial pocket organ (Fig. 4E ). The posterior mantle edge is markedly indented (curved).

Postpallial pocket organ. As in Tandonia rustica .

Integument structures. The number of tr of integument, from the slit of pneumostome to the keel does not vary much (n = 3; the topotypes only); from tr 15-17). The surface texture and the width of tubercles in live specimens vary depending on their position on the body. The tr do not appear to be strongly divided in several compartments, only few compartments exist. The tr are all finely crenulated, but also can appear to be totally smooth. In all specimens, the keel extends from the posterior margin of the mantle to the end of the dorsum and is entirely smooth. It is not erected, but evenly rounded, sometimes almost flat and not exposed over the dorsal tr.

Sole structure. As in Tandonia rustica .

Measurements. lw (n = 3): 1.35-2.5 g, ø 1.85 g; tl (n = 2): 46 mm and 70 mm; ml (n = 2): 17 mm and 22 mm; sw (n = 2): 4.5 mm and 5 mm.

Ratio of tl/ml ranges from ca. 46/17 mm to 70/22 mm; ø ml being a little more than 1/3 of tl.

Mucus. The mucus is transparent on body, mantle and sole, and sticky. So far, no coloured defensive mucus was observed.

Genital organs. Atrium short; penis tubular, constricted in the middle; interior penial wall with a prominent transversely oriented fold; distal to the fold a simple penis papilla (Fig. 5B, C ); papilla base slightly swollen; penis and epiphallus equal in length and diameter, divided by a constriction, where penis retractor muscle inserts; epiphallus surface with nodular structures primarily on its proximal end.

Vagina shorter than penis; accessory glands entering distally on vagina, close to pedunculus and oviduct, formed by a broad truncus with bundles of tubuli attached; the vaginal walls richly covered with folds forming a zig zag pattern; a prominent torus with a row of acute conical spikes running through the vagina from atrium almost reaching the branching point of the pedunculus of the bursa copulatrix (Fig. 5D, E ); pedunculus shorter than vesicle; vesicle elongated, rounded at the tip; oviduct slim and long.

Spermatophore. No spermatophore found in our specimens. It was described and figured by Regteren Altena (1953: fig. 7).

Distribution.

Tandonia nigra is a rarely found species in Switzerland, and all locally constricted to the Sottoceneri, which is the southern part of the Canton of Ticino, including the districts Lugano and Mendrisio. Our series is very small (n = 9). All specimens are from Canton Ticino, from a very restricted area around the Monte Generoso only, including the type locality (n = 3). For more information refer to chapters Remarks and Discussion.

Habitat.

In the summit region of Monte Generoso, T. nigra lives in crevice-rich limestone rocks, which are sparsely vegetated. Under dry weather conditions T. nigra is rarely found active on the surface, it is hidden in rock crevices, at least during the day. In the summit region, only very dark-coloured animals have been observed so far. In lower altitudes, T. nigra is so far recorded in semi-natural deciduous forests. These are various mixed deciduous forests with beech, hop hornbeam, lime, field maple, ash, hazel, common whitebeam, and manna ash (i.e., southern alpine toothwort beech forest, Cardamino-Fagetum cyclametosum, hop hornbeam forest, Orno-Ostryon). All habitats are relatively shady. At higher altitudes, these forests are relatively dry, at lower altitudes they tend to be fresh to moist. For the snails, retreat sites with a certain soil cover and air humidity are important. In autumn 2021, however, two animals could be observed crawling during the day in relatively high humidity on a crack-rich, largely dry retaining wall. The first wall was on a mortared garden retaining wall on a natural road, the other on a dry stone wall along a road. Both walls are situated in semi-shade and are only exposed to sunlight for a few hours a day. In the forest, T. nigra is found under stones, amongst fallen leaves, in rock crevices and on dead wood. During several hikes in the summit region, only single animals were observed in humid conditions and during sunrise.

So far, all records in Switzerland originate from areas with a good calcium supply. The altitudinal range of its habitats spans from 435 m near Rovio to the highest point at 1700 m.

Life history.

There are almost no observations on the biology of the species published. Tandonia nigra feeds on fallen leaves, but possibly also on lichens and detritus. Mating behaviour and reproduction mode are not yet described. A specimen with spermatophore was collected on 30 July 1948 ( Regteren Altena 1953). We conclude that the copulation season for T. nigra is in July.

Remarks.

The author of this species is Karl Ludwig Pfeiffer (1874-1952), who usually published under the name Karl L. Pfeiffer = K. L. Pfeiffer ( Zilch 1972).

The most remarkable new finding is the presence of several/additional colour morphs in T. nigra . Pfeiffer’s (1894) sketchy description of the "small and slim" single specimen (most probably a juvenile) he had collected in autumn 1893, is rather uninformative. Nonetheless, he described a few interesting details concerning the colouration: "almost entirely deep black, only towards the sole somewhat lighter, with two longitudinal black lines leaving a very fine yellow line between". The "very fine yellow line" describes the peripodial groove, the first black line a stripe (peripodial fold) just above the yellow line, the second black line must be the edge of the sole. In our topotypes (n = 3) such a fine yellow line was missing in two specimens, while in the other one the line was creamy white. The extremely fine black lines are in fact not visible by the bare eye only. Pfeiffer found "the keel as black as the dorsum and mantle". Wiktor (1987) mentioned that "Pfeiffer thought it can be a melanistic form of T. rustica ", but Pfeiffer (1894) did not mention T. rustica at all. Wiktor described the colouration based on a single topotypic specimen (Rijksmuseum Leiden no. 988), which also had been examined by Regteren Altena in 1953. This specimen was differing in some respects from the original description by Pfeiffer: "Mantle at first sight uniformly black, in fact at its edges very dense black dots on dirty creamy ground, visible however only in magnification". The differences in these descriptions already indicated that for this species some intraspecific variation can be expected.

Our recent research on the new additional colour morphs of T. nigra at the lower altitudes of Mt. Generoso at Mendrisio, Val Cürta (PM), and from the surrounding of Rovio (pers. comm. K. Lassauer 2020) has confirmed this impression. These pale creamy specimens were first thought to belong to a different, potentially new species, but our genetic assessment evidenced that these are just colour morphs of T. nigra . The penis papilla in some pale colour morphs was less fleshy and bulgy than the topotypic adult specimens. The difference in the penis papilla of some lighter colour morphs might be due to their subadult stage. Concerning the anatomy, the nodular structure on the epiphallus has been visible in fully adult but also in subadult specimens. In some cases, the lumen of the epiphallus was filled with transparent, tiny globuli (Fig. 5C ) of unknown function.

In each of the Swiss specimens examined, the posterior end of mantle is markedly indented, as it was already described by Pfeiffer (1894). Pfeiffer (1894) found nine tr on both sides of the body. Most probably he counted the number of tr between keel and the edge of the sole, i.e., at mid of the dorsum downwards to the edge of the sole. Wiktor (1987) reported 13 tr from the single specimen examined by him. This result also does not compare to our counts in the topotypes. The tr can also be totally smooth as already reported by Pfeiffer (1894). The high number of tr will always allow differentiating T. nigra from T. budapestensis , but not necessarily from juvenile T. rustica if not taking pigmentation into account. Concluding, the main intraspecific variability can be found in the body pigmentation as well as in the number of tr on the body. Given the few specimens investigated so far, essentially more research is required to cover all variants in this species.

For a rather long period, T. nigra , was thought to constitute an endemic species for Switzerland, or it was omitted as a member of the Swiss Fauna ( Forcart 1942). Quite recently, Rähle (1997) identified a population of a small black Tandonia species from Valle Brembilla, Bergamo, Italy (coll. SMNS) as T. nigra . Forcart (1959) identified a sample (NMB 06166a) from Austria, north Tirol as Milax simrothi Hesse 1923, which subsequently was corrected by Wiktor (1987) to T. nigra . We received a specimen identified by G. Nardi as T. nigra from Italy, Laxolo, Valle Brembilla, Bergamo (NMBE 561307), which is almost identical with Rähle’s locality. As a result, Nardi (2017) shaped the distribution area of T. nigra over a rather wide range in northern Italy. We tried to re-assess the identity of these specimens.

Unfortunately, it was impossible to extract DNA from the specimen from Laxolo (NMBE 561307), and the preservation state of the animal was poor. As a result, there is no genetic evidence that this is the same species as T. nigra . It shares some details with the topotypic Swiss specimens, particularly the presence of the penial papilla, the vaginal torus with the spiked papillae, and the epiphallial node field. However, Rähle (1997) found major differences in the genital organs: He did not find a penial papilla (large and well developed in our specimens), and the accessory glands in his specimen were restricted to some unbranched tubes situated around the distal end of the vagina (large, branched bundles of tubuli in our specimens). This is astonishing as both specimens originate from the same locality. Revision of the Austrian sample (NMB 06166a) failed because of its bad preservation status.

Since its description, eight malacologists (CSCF mapserver, May 2022) have recorded this species from the summit of Mt. Generoso. New sites were found by two malacologists from 2005-2021. Given the doubts in identification of the non-Swiss populations, the question whether this is an endemic species for Switzerland or not is not finally answered.

Figures

Figure 4. Tandonia nigra. A-F sequenced topotype NMBE 569047, Monte Generoso, Rovio, Calvagione, 1.650 m alt A lateral view right B dorsal view C ventral view D lateral view left E dorsal view showing the postpallial pocket organ (indicated with arrows) F lateral right view of mantel with pneumostome. Abbreviation: ppo = postpallial pocket organ.

Figure 5. Tandonia nigra. A-D sequenced topotype NMBE 569047, genital organs A situs of genitalia B penis with epiphallus showing the granulation on the epiphallial surface and the penis papilla C epiphallus opened to show the transparent globuli D vagina opened showing the torus with the row of spikes E sequenced specimen NMBE 568980, situs of genitalia. Abbreviations: a = atrium; bc = bursa copulatrix; ep = epiphallus; fod = free oviduct; p = penis; ped = pedunculus; pp = penial papilla; spo = spermoviduct; v = vagina; vd = vas deferens; vg = vaginal gland.

Figure 7. Living specimens of Tandonia. A, B Tandonia rustica A sequenced specimen NMBE 568974, Wartau, Truebbach B sequenced specimen NMBE 568973, Rovio, Sovaglia C, D Tandonia rustica (two mottled specimens) meeting a pale colour morph specimen of Tandonia nigra, Val Cuerta, Mt. Generoso E, F Tandonia budapestensis E NMBE 571881, Roggwil, F NMBE 571882, Meggen.

Figure 8. Colour variations of Tandonia nigra. A greyish morph from Rovio, waterfall at Botto, NMBE 568980 B, C topotype from summit area; NMBE 569047 D Mendrisio, Val Cuerta, NMBE 570972 E Capolago, Val Cuerta, NMBE 570971 F Rovio, Sasso Piatto, NMBE 570974.