Tandonia rustica (Millet, 1843)

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 rustica (Millet, 1843)

Figs 1 , 3 , 7A-D

Limax rustica Millet, 1843, Magasin de Zoologie, ser. 2, 5: 1, plate 63, fig. 1 [nord de l’Anjou, à la Bouillant, commune de la Chapelle-Hullin, ainsi qu’à Thorigné, etc.].

Type specimens.

Probably does not exist, after Wiktor (1987: 286). See also remarks.

Differential diagnosis.

Epiphallus more than twice as long as the penis (almost the same length in T. nigra and T. budapestensis ); dark stripe above the pneumostome and on the opposite side of the mantle (missing in T. nigra and T. budapestensis ); penial papilla short, blunt, with flap-like lobes around the central porus (short and blunt but folded in T. nigra ; elongated with a stalk in T. budapestensis ).

Description.

Colouration. Many fully adult specimens (n = 56) were studied from six different Swiss Cantons (Bern, St Gallen, Grisons, Ticino, Obwalden, Nidwalden) as well as from five regions in Italy (Bolzano, Sondrio, Torino, Lecco, Trento), from the Republic of San Marino and from two Departments of France (Alpes de Haute Provence, Isère).

The general pigmentation of Swiss specimens varies from a warm reddish brown to dark reddish or chestnut brown to very dark brown without any reddish brown hue. This pigmentation normally is not fading downwards to the fringe of the sole. However, in some cases in the less dark pigmented populations in Switzerland, little fading downwards can be observed (Fig. 7A ). The mantle is of the same colour as the dorsum, except for the surrounding of the pneumostome, which usually is paler. Many specimens investigated are markedly darker when compared to Wiktor (1973, 1987) and Rowson et al. (2014b). The darkest specimens were found in southern Switzerland (Ticino, Grisons - Val Bregaglia), and in the southwest bordering areas to Italy and France (Fig. 7B ). Specimens from Central Switzerland (Lucerne, Nidwalden, Obwalden), northern and eastern Grisons, St. Gallen, and the eastern border area in Italy (Bolzano, Trento) are of a markedly lighter, pale reddish brown pigmentation (Fig. 7A, C, D ).

The whole slug is covered with small black spots on dorsum, flanks, and mantle, but not on the keel. The characteristic black streaks above the pneumostome and on the same location on the left side of the mantle can vary greatly in size and form. Even though all our investigated specimens had spots and the characteristic streaks on the mantle, in some specimens they are almost invisible because of the overall dark pigmentation (Fig. 7B ). Additionally spots in darker specimen may be much larger and sometimes irregularly jagged and elongated to streaks.

Head, neck and ommatophores are dark brown, while the tentacles are slightly paler. In many cases, the black pigmented ommatophoran retractor is clearly visible through the integument.

The keel is commonly paler in colour than the dorsum. Rarely the general pigmentation of the keel matches the dorsal colour, this occurs especially in darker specimens.

The tripartite sole in pale coloured slugs is creamy yellowish, while darker slugs have a pale yellowish brown sole. Many darker coloured specimens have a hue of greyish black at the posterior outer margins of the lateral sole fields formed by very little black dots. Isolated single greyish black spots may occur along the outer edge of the sole.

Mantle structure. The pneumostome is positioned at 2/3 of mantle length, well posterior of the centre of the mantle. The pneumostome is not surrounded by a distinct ring-like structure, like it is the case in T. nigra . The “slit” of the pneumostome in all specimens does not end in the lumen of the pneumostome but runs anteriorly to at least the dorsal edge of pneumostome. In living individuals, the mantle surface is completely smooth besides the horseshoe-shaped sinus groove. The sinus groove is completely developed in all specimens in our series, and it reaches at both sides almost the posterior end of the mantle. In the dark specimens it needs magnification to see it clearly. The posterior margin of the mantle is not tightly attached to the integument and in living and contracted animals smoothly rounded. The posterior free mantle flap covers the anterior integument-tubercles as well as the openings of the postpallial or Wiktor’s pocket organ.

Postpallial pocket organ. In all specimens examined, the posterior part of the mantle covered two slit-like openings, the postpallial pocket organ, which was first detected and described by Schneppat et al. (2011) for Tandonia totevi ( Wiktor 1975).

Integument structures. The number of tr (n = 52; tr 12/13-tr 19, ø tr 15) does not vary much in our specimens, and there is no significant variation between populations. The surface texture and the width of tubercles in live specimens vary from fully straight to torn, depending on their body position. All tr from head to the 7th-9th row posterior to the pneumostome are entirely flat, smooth, wide, and remain so from the head and flank to the peripodial tubercle. The remaining tr reaching to the keel are somewhat crenulated, but are also wide, flat, and lacking ridges. Crenulations can only be seen under magnification. The tubercle rows may be long and undivided down to the peripodial tubercle, especially the ones below the lateral mantle edge. The more dorsal rows usually are divided in several tubercle compartments.

The keel is extended from the posterior margin of the mantle to the end of the dorsum. In some specimens, the keel was observed to be even slightly extending over the fringe of the sole, like a very little terminal thorn or knob. The keel has always an entirely smooth surface structure and is therefore clearly discernible from the crenulated neighbouring dorsal tubercles. Live and preserved specimens do not differ in the extension of the keel and its structure.

Sole structure. The outermost edge or seam of the sole is separated from the dorsum by a longitudinal fold, the peripodial tubercle, which begins left and right of the mouth-flaps and runs posterior around the body. The peripodial tubercle together with the peripodial groove clearly separate the seam of the sole at its outermost posterior end, where the sole is rounded and not pointed. In the central field of the sole, many V-shaped transverse wrinkles exist, which are invisible in animal crawling on a pane of glass but are well visible in preserved specimens.

Mucus. The mucus lacks any pigmentation on body, mantle and sole and is extremely sticky. When irritated, some of the animals produced defensive mucus of white to yellowish greenish colour on dorsum, flanks, and mantle, but this could only rarely be observed.

Measurements. lw (n = 56): 1.5-6 g, ø 3 g; tl (n = 56): 52-92 mm, ø 69 mm; ml (n = 50): 18-33 mm, ø 23 mm; sw (n = 47): 5-10 mm, ø 7.3 mm. Ratio of tl/ml ranges from ca. 52/18 mm to ca. 92/33 mm; ø ml is 1/3 of tl.

Genital organs. Atrium very short and tubular; penis short, with a distal bulb harbouring the penial papilla and a second bulb consisting of the papilla basis marking the boundary to the epiphallus, interior penial walls simple; penis papilla ornamented, apex of papilla with a row of curved crests encircling the complete papilla giving it a flower like appearance (Fig. 3C, F ); penis retractor muscle inserting at the epiphallus/penis boundary; epiphallus externally with smooth surface, consistent in diameter, reaching up to 4 times the length of the penis.

Vagina twice the length of the penis, separated from the atrium by a sphincter; accessory glands entering close to the boundary of atrium and vagina; accessory glands digitiform or sac-like, either beige, brown or bright rusty red coloured; vaginal lumen with elongated, waved folds pointing towards the oviduct and the pedunculus of the bursa copulatrix; pedunculus somewhat longer than the vesicle; vesicle may be pointed or rounded.

Spermatophore. A spermatophore was found in a single specimen, broken into three parts (NMBE 571414, Ticino, Orselina, 14 February 2020). One part still stuck in the lumen of the genital pore of the slug, length 0.2 cm; it is completely covered with simple spines (Fig. 3H ). The other two parts were found embedded in the vesicle in a white, fibrous mass; digestion had already started. The smaller part is 0.35 cm long, curved and covered with undivided and, further on, bifurcated spines laterally. The largest part of the spermatophore is ~ 0.9 cm long and curled (visualized in Fig. 3I, J ). Its outer surface is covered on the entire length with bifurcated spines. This is the first record of the spermatophore of this species.

Distribution.

Our own distribution records are mainly limited to Switzerland, France, northern Italy, and the Republic of San Marino. The species itself is frequently recorded from the western alpine arc, but also from a wide range in Great Britain, Ireland, The Netherlands, Belgium, Luxemburg, France, Germany, Austria, Poland, Czech Republic, Slovakia, and Hungary ( Wiktor 1987; Gavetti et al. 2008; Rowson 2017). Reports for Romania are likely misidentifications with T. kusceri . Old reports for Bulgaria are shown to be wrong identifications of the very common T. kusceri (pers. obs.).

Habitat.

This species is confined to beech forests on limestone talus.

Remarks.

As original name of this species, Wiktor (1987) referenced a " Limax marginatus rusticus Millet, 1843", which is wrong. In his text, Millet states "Je l’ai rencontrée au nord de l’Anjou, à la Bouillant, commune de la Chapelle-Hullin, ainsi qu’à Thorigné, etc.". These localities can be found in the larger surrounding north of Angers, Region Pays de la Loire, Déptartement Maine-et-Loire, thus we consider this area the "type area" of L. rusticus . A neotype should be selected from one of the mentioned places to unambiguously fix the use of this name.

When it comes to morphology Wiktor (1987) mentioned 18-19 tr, which is higher than most of our counts, but still in the range of our data. Comparing the exterior of living and preserved specimens with the description by Wiktor (1987: 286) and Rowson et al (2014b), many differences were found. No specimens of generally "white or white creamy" pigmentation, nor those of "pinkish or violetish" hues could be recorded by us in Switzerland and bordering areas. Most specimens investigated are explicitly darker than described by Wiktor (1987) and Rowson et al (2014b) indicating a much higher intraspecific variability of the species in other areas of distribution. Colour morphs exclude each other’s following the geographical pattern as described above. In Wiktor (1987: figs 196, 197), the pneumostome is positioned on the left side of the body, which most probably is an error or a sinistral specimen.

Regarding the genital anatomy we found genetically identified T. rustica (NMBE 564708, NMBE 568978), which show a very similar anatomy to T. kusceri with a long (in one specimen) coiled epiphallus and in both a round bursa copulatrix (Fig. 3E, F ). For the coiled epiphallus it might be a sexually active specimen, very probably after the ejection of the spermatophore. Even though the characters of T. rustica and T. kusceri seem to meet, the two species can be distinguished by their habitat. Tandonia kusceri usually is found in disturbed, synanthropic habitats, whereas T. rustica is usually found in untouched more natural woodland habitats.

Wiktor (1973) and Likharev and Wiktor (1980) report a long stem of the penial papilla, an observation we could not reproduce. Later, Wiktor (1987) reported a slightly reduced length, but even this reduction does not fit to our results. In all our dissected specimen, we had a bulky papilla tightly attached to the constriction of the epiphallus and therefore without a stem.

Gerhardt (1940) observed the copulation behaviour of the species in his laboratory; however, he did not illustrate the spermatophore. He observed the first copulation on 17 November 1938 followed by two copulations on the 4 December 1938. His specimens were collected near Porto d’Ischia, Isola d’Ischia, Italy. It remains unclear whether these specimens were T. rustica ; later research on the island yielded no specimens of this species. In case Gerhard’s determination was correct, the data of the observed copulation and those from our collection with the spermatophore found on 14 February 2020 indicate that this species probably mates during the winter season, a period where malacological research activity is rather low.

Figures

Figure 1. Distribution map of collected specimens.

Figure 3. Tandonia rustica. A-C sequenced specimen NMBE 568973, Rovio, Sovaglia A preserved animal B situs of genitalia C distal genitalia showing anatomical details D penis papillae of sequenced specimen NMBE 568978, Carona E-G sequenced specimen NMBE 564708, Goldswil E situs of genitalia F distorted epiphallus G details of lumina of vagina, atrium and penis, arrows pointing at the " twin papillae " (tp) H-J spermathophore of specimen NMBE 571414, Orselina H genital pore with piece of spermatophore I, J largest part of the three pieces J detail of surface covering spines. Abbreviations: a = atrium; bc = bursa copulatrix; ep = epiphallus; fod = free oviduct; p = penis; ped = pedunculus; pp = penial papilla; spo = spermoviduct; tp = " twin papillae "; 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.