1. Alopiinae, Medora group:
In Nordsieck (1984: 194) a form intermediate between Isabellaria = Carinigera chelidromia and C. praestans from Piperi island, northern Sporades, an assumed hybrid subspecies, has been described as C. c. piperica.
In Nordsieck (1974: 132) a form intermediate between Isabellaria thermopylarum and I. almae from Amfissa, Phocis, an assumed hybrid subspecies, has been described as I. t. faueri.
In Nordsieck (1974: 137) I communicated on a copulation of Albinaria scopulosa and A. senilis from Plakoti, Epirus.
Kemperman & Degenaars in Kemperman (1992) stated that the subspecies of Albinaria jonica and A. adrianae from Kefalonia were in allozyme frequencies less similar to the other subspecies than to neighbouring subspecies of A. contaminata and A. senilis, respectively (: 142-143, dendrograms figs. 7-8). This might be caused by introgression from hybridization. Introgression was also assumed because of the occurrence of rare alleles in neighbouring populations of different species on the island (: 151-152).
Kemperman & Gittenberger in Kemperman (1992) found that in contact zones of all species taxa from Kefalonia intermediates which have originated by interspecies hybridization occur:
A. c.contaminata and A. s. senilis (: 185, fig. 9);
A. c. contaminata and A. maculosa samiensis = m. liebetruti (: 191);
A. c. contaminata and A. jonica assicola (: 207, fig. 17);
A. c. periporon and A. a. adrianae (: 190, 201, 203, figs. 15A-C);
A. c. periporon and A. adrianae dubia (: 190, 204, figs. 15D-F);
A. s. senilis and A. j.jonica (196, 205);
A. s. senilis and A. adrianae dubia (: 197, fig. 14).
Gittenberger (1987: 84) described Isabellaria = Albinaria vrondamasa from Vrontamas, Laconia, as having a closing apparatus = CA intermediate between N- and G-type (with weak spiral lamella and very weak parallel lamella). In the collection of Fauer (Nordsieck 1997: 59) I found a sample from Vrontamas, in which most specimens had a G-type CA, but about a fourth of the specimens a N-type CA or an intermediate CA like the types of A. vrondamasa. Obviously this population is the product of a hybridization of I. = A. butoti (G-type CA) and A. grisea (N-type CA).
Gittenberger (1994: 58) described a form intermediate between A. discolor (N-type CA) and A. haessleini (G-type CA) from Molai and Elea, Laconia, as A. d. eureka. In the coursa of a collecting trip in the eastern Peloponnese I could confirm the occurrence of this and further taxa of A. haessleini with CAs intermediate between N-type and G-type (Nordsieck 2007: 114, 166).
Neubert (1998: 153) communicated on interspecies hybrids between A. krueperi and A. grisea in the northwestern Peloponnese.
In the course of the already mentioned collection trip (Nordsieck 2007: chapter VII, new species taxa 163-168), I discovered in northeasternmost Arcadia three assumed hybrid subspecies as follows:
A. petrosa tanocola, intermediate between A. petrosa and A. mixta (: 164);
A. litoraria profugella, intermediate between A. litoraria and A. profuga (: 164);
A. mixta interposita, intermediate between A. mixta and A. petrosa (: 165).
Later on (Nordsieck 2015: 6), I described an assumed hybrid subspecies fom Ithome, Messinia: A. arcadica occulta, intermediate between A. arcadica and A. maculosa (= former A. schuchii).
Wiese (1989: 41-42) communicated on hybrids Albinaria retusa x A. torticollis on Dia island near Crete.
Schultes & Wiese (1990: 32-33) gave the same information, additionally on hybrids A. jaeckeli x A. torticollis, observed in a terrarium.
Welter-Schultes (1992) gave more detailed informations on hybrids between A. species from Dia island. He found nearly no hybrids (< 0.1%) of A. teres and A. torticollis. As concerns A. retusa and A. torticollis, 4 of 10 specimens between Ankinara peninsula and mainland Dia were hybrids; other syntopic occurrences were without hybrids.
Schilthuizen et al. (1993), in their revision of Albinaria hippolyti, mentioned hybrids A. h. hippolyti x A. spratti (: 143) and A. h. holtzi x A. spratti as well as A. h. holtzi x A. idaea (: 146). Near Milatos cave a broad area with extensive hybridization between A. h. arthuriana and A. maltzani was said to exist (: 153).
Welter-Schultes (1998) discovered hybrid zones at the boundary of the ranges of A. leonisorum and A. corrugata (: 276, 278) and of A. fulvula and A. corrugata (: 278). He found intermediates between A. moreletiana and A. corrugata (: 278). Also at the contact of the ranges of A. sturanyi and A. teres a hybrid zone was stated (: 278). The same information was given in Welter-Schultes (2000), and once more that on the hybrid forms A. arthuriana x A. maltzani and A. retusa x A. torticollis.
Gittenberger et al. (2001) found assumed hybrids A. cretensis cf. vexans = A. troglodytes niproensis x A. sphakiota in the Imbros gorge, Sfakia (: 79).
Nordsieck (2004) mentioned intermediate forms between A. troglodytes and A. virginea in the region west of Rethimno (: 61), between A. tenuicostata and A. virginea in westernmost Crete (: 64), and between A. sublamellosa and A. virginea in Agia Roumeli, Sfakia (: 61).
Nordsieck (2017) described the hybrid form A. arthuriana x A. maltzani from Milatos cave, which was already mentioned by Schilthuizen et al. (1993), as A. a. xenogena (: 14). Furthermore, a hybrid population A. tenuicostata x A. loosjesi from Tigani-Balos, Gramvousa peninsula, was communicated (: 13). The sample of A. troglodytes from Agios Nektarios, Sfakia (: 15), was assumed to contain hybrids of A. troglodytes and A. sublamellosa. The subspecies described as A.maltzani ecristata (:19-20) is intermediate between A. maltzani and A. corrugata.
Most recently I found hybrids of A. rebeli and A. teres from Kavousi, in a sample together with pure A. rebeli, designated by A. J. Wagner on the label as syntypes of A. rebeli (SMF).
A. Schmidt (1868: 102-103) was the first who described hybrids between Cristataria fauciata (= C. delesserti) and C. strangulata (Clausilia fauciatae similis, C. strangulatae similis; Nordsieck 1971b: 241). Another hybrid form of both species is C. sancta Bourguignat (: 243).
C. albersi from Nahr el-Kelb near Beyrouth is in several characters intermediate between C. boissieri and C. strangulata, which occur syntopically in this valley. It could be a species which has originated by interspecies hybridization (Nordsieck: 243).
In the course of the revision of the genus (Nordsieck 1970) I could trace intermediates in shell morphology between several species from central Dalmatia:
M. dalmatina from Živogošće, with characters of M. contracta (: 32);
M. dalmatina josephinae from Staza pass above Podgora, with characters of M. macascarensis (: 34);
M. stenostoma klemmi fromTurija cave, transitional to M. dalmatina (: 41);
M. almissana mariae from Dubci, transitional to M. macascarensis (: 60).
In the supplement of the revision (Nordsieck 2009) a form intermediate between M. macascarensis and M. stenostoma from Biokovo, between Vošac and Sveti Jure (: 6, 10), has been described.
Fehér et al. (2013) described and figured interspecies hybrids of A. biloba and A. neutra from the Drin valley in northern Albania (Shkodër and Tropojë districts) (: 8, figs. 4 E-J). In the mtDNA tree one of them (from Koman) is basal within the A. biloba clade. Specimens from the localities concerned could be confirmed by me as intermediates.
Lampedusa, Muticaria (with figures 1-6):
Giusti et al. (1995: 370-374) communicated on hybrids Lampedusa imitatrix x Muticaria macrostoma from Malta.
Giusti et al. (: 364-365) informed on the examination of the relationships of the known Muticaria species from Malta and Sicily by an allozyme study. As result some M. syracusana were indistinguishable from M. neuteboomi, while others were different. Therefore, they united M. syracusana and M. neuteboomi to one species. The lunellar, which was figured as belonging to M. syracusana (from Fontane Bianche near Siracusa, fig. 404), exhibits an elongation of principalis rudiment like in M. neuteboomi (fig. 407), unlike the lunellar of M. syracusana from Siracusa (type of syracusana Rossmässler, SMF). Both results make probable that the syracusana specimens, which were indistinguishable from M. neuteboomi in the allozyme study, and the syracusana specimen from Fontane Bianche were hybrids M. syracusana x M. neuteboomi.
Colomba et al. (2010) found that M. syracusana and M. neuteboomi are separated as two clades in their COI tree; therefore, they regarded them as different species. On the other hand, they figured forms of M. neuteboomi from Pantalica with palatal plicae more like those of M. syracusana (Colomba et al.: figs. 13-14). In contrast to them, the specimens of M. neuteboomi from Pantalica examined by me(SMNS) are more coarsely ribbed and have a lunellar like the type form of the species.
Colomba et al. (2012) showed that a form of M. syracusana from Cugno Lungo, M. brancatoi, comes out in their COI tree outside of the clade M. syracusana + M. neuteboomi; therefore, it was treated as a different species. The paratype at my disposal (SMF) differs from M. syracusana only by the sculpture, not in the lunellar.
Another form from Epipoli, which is also not much different from the type form of M. syracusana, M. cyclopica, shall be a different species, too (Liberto et al. 2016), because in COI analysis it has a position outside of the clade M. syracusana + M. neuteboomi like M. brancatoi (Colomba et al. 2019). Two samples at my disposal (SMNS, SMF) are intermediate between M. syracusana and M. neuteboomi, because the principalis rudiment is not connected with the anterior upper palatal plica. In a form of M. cyclopica from Magnisi (SMF), unknown until now, the principalis rudiment is even somewhat elongated in front.
These results can only be explained by the fact that the M. syracusana specimens genetically close to M. neuteboomi have suffered introgression from that species (like a part of the syracusana specimens examined by Giusti et al., see above), while M. brancatoi and M. cyclopica are subspecies of M. syracusana, which are genetically more distant from M. neuteboomi.
Figs. 1-6. Muticaria species from Sicily.
Frontal and body whorl dorsal. Shell height = H (mm).
1. M. s. syracusana, I, Sicily, Syrakus = Siracusa, holotype of syracusana Rossmässler, SMF 67317; H 14.4.
2. M. s. cyclopica, I, Sicily, Castello di Eurialo near Siracusa, ex N 1831 = SMNS ZI0119827; H 14.5.
3. M. s. cyclopica, I, Sicily, Magnisi, SMF 349576; H 13.4.
4. M. s. brancatoi, I, Sicily, Cugno Lungo near Siracusa, paratype, ex SMF 341353; H 11.7.
5. M. neuteboomi, I, Sicily, Cava d'Ispica, paratype, ex SMF 329825; H 13.4.
6. M. neuteboomi, I, Sicily, Pantalica, ex N 1828 = SMNS ZI0119828; H 15.45.
2. Alopiinae, other Alopiini:
By a mtDNA study of the genus Montenegrina Mason et al. (2020) could find two cases of interspecies hybridization of Albanian species:
M. rugicollis desaretica x M. dofleini prespaensis in Sveti Marena at Lake Prespa, and
M. n. nana x M. perstriata callistoma near Lunik.
For other taxa interspecies hybridization can be assumed because of their aberrant position in the mtDNA tree: Specimens of M. drimmeri, M. t.tomorosi and M. perstriata callistoma each in another clade with another species than the rest of the samples.
Some species, much differing in shell characters but occurring in close neighbourhood, are combined within the mtDNA tree. This leads to the suspicion that the genetical similarity is caused by interspecies hybridization. The following taxa are concerned:
M. dofleini pinteri in the same subclade as M. stankovici (shore of Lake Ohrid);
M. dofleini plenostoma and M. d. occidentalis in the same subclade as M. perstriata drimica (region of Struga);
M. f. fuchsi in the same subclade as M. rugilabris golikutensis (region of Tepelenë);
M. f. pallida in the same subclade as M. r.lambdaformis (northern Epirus near Albanian frontier);
M. f. muranyii in the same clade as M. tomorosi (Tomorr Mountains).
In the course of the revision of the genus Herilla (Nordsieck 1971a) I found intermediates in shell morphology between some species from Serbia and Montenegro:
H. bosniensis from Javorovo near Rožaj, with a clausilium plate like H. ziegleri (: 69-70), later on (1974: 124) described as H. z. edlaueri;
H. bosniensis from upper Tara valley and adjacent Durmitor (: 71), with lunellar and clausilium plate like H. ziegleri, later on (1974: 125) described as H. b. dux;
H. bosniensis from lower Piva valley (: 72-73), with a white surface layer like H. illyrica, described as H. b. hannae.
Interspecies hybridization of enantiomorphic or closely related Alopia species has been described in two of my publications (Nordsieck 2007: chapter VI, 102-106 and 2016).
Hybridization between left- and right-coiled Alopia species can be demonstrated phenotypically (by shell morphology), if the CA of one species concerned is more strongly developed than in the other. This is the case in the following species pairs which I have examined (Nordsieck 2016):
A. straminicollis subspecies (left-coiled, CA fully developed) and A. livida subspecies (right-coiled, CA strongly reduced), with syntopic occurrences in several places in the Bucegi Mountains (well-examined: Valea Velicanului, Cheile Tătarului, Valea Suchelniţei with adjacent Bătrâna);
A. glorifica galbina (left-coiled, CA fully developed) and A. livida deaniana (right-coiled, CA strongly reduced), with syntopic occurrences in a part of the Piatra Craiului Mountains (Măgura Mică, Măgura Mare);
A. nixa (left-coiled, CA strongly reduced) and A. livida bipalatalis (right-coiled, CA less strongly reduced), with syntopic occurrences in two places in the Bucegi Mountains (Valea Doamnelor with adjacent Bătrâna , Valea Obârşia Ialomiţei).
The hybridization between A. nixa and A. l. bipalatalis in Valea Obârşia Ialomiţei could also be proved by examination of the genitalia of the latter (end genitalia tending to those of A. nixa, own investigations).
In a former paper (Nordsieck 1984: 191-192) a hybrid form A. mauritii (left-coiled, CA strongly reduced) x A. h. helenae (right-coiled, CA fully developed) from Ciucaş Mountains (Gropşoare) has been described as A. h. interjecta. In a recent paper (Nordsieck 2019) I communicated on hybrids of A. h. helenae and A. h. zagani from the valley separating Gropşoare and Zăganu, demonstrated by their reduced CA, which they could have got by hybridization with A. mauritii or A. nefasta.
Hybridization between left- and right-coiled Alopia species cannot be demonstrated phenotypically, if the CAs of both species are equally developed, though it is certainly present. This is, e. g., the case in the following species pairs:
A. glorifica (left-coiled) and A. lischkeana (right-coiled), both with fully developed CA, with syntopic occurrences in the main part of Piatra Craiului Mountains (e. g., A. g. boettgeri and A. l. obesa in Valea Râului, A. g. glorifica and A. l. lischkeana in Valea Brusturetului);
A. nixa (left-coiled) and A. fussi (right-coiled), both with strongly reduced CA, with syntopic occurrences in the higher parts of the Bucegi Mountains (e. g., Obârşia-S).
The hybridization of A. glorifica and A. lischkeana is made probable by the subspecies which cluster in COI analysis (Fehér et al. 2013) with the other enantiomorphic species (A. g. boettgeri from Valea Râului with A. lischkeana subspecies, A. l. lischkeana (deceptans) from Valea Brusturetului with A. glorifica subspecies).
The hybridization of A. nixa and A. fussi is made probable by their clustering in one clade of COI analysis (Fehér et al.), though A.fussi is in shell morphology nearly indistinguishable from A. livida.
In my paper on interspecies hybridization of Alopia (Nordsieck 2016: 7) I communicated on a probable case of interspecies hybridization of A. pomatias and A. fussi.
One case of interspecies hybridization of Alopia, the best known one (A. straminicollis x A. livida), has been analysed more exactly by a DNA study of Koch et al. (2017). In a further paper (Koch et al. 2020) the authors stated hybridization between all species taxa of the A. livida group from Bucegi Mountains (A. straminicollis, A. livida, A. nixa, A. fussi) and united them to the (mega)species A. livida, in spite of morphological differences, especially of A. nixa and A. fussi to the two other species (own investigations).
3. Alopiinae, Delimini:
Some forms of Charpentieria from the southern limestone Alps between Como Lake and Garda Lake are similar to C. stenzii by some shell characters and their habitat, but agree with C. itala in essential characters of shell and genitalia (Nordsieck 1963). Therefore, in that paper, they were treated as so-called stenzioid subspecies of C. itala: C. i. clavata, C. i. variscoi, C. i. balsamoi and C. i. lorinae (C. i. trepida included).
Their relationships to C. itala are different from normal subspecies of that species, but also from C. stenzii (1963: 171, 173).
The normal subspecies of C. itala are connected by intermediate forms and do not occur syntopically with other subspecies.
Forms transitional between C. stenzii and C. itala have never been found; C.stenzii occurs syntopically with C. itala in several localities.
C. i. lorinae does not occur syntopically with C. itala, but is connected with it by transitional forms (allatollae, 1963: 186-187), distributed next to the range of C. i. lorinae.
C. i. clavata occurs syntopically with C. itala at some localities; a form transitional to C. itala is known from Lecco (leccoensis, 1963: 182-183).
The same is true for C. i. variscoi; at one locality (mouth of Val Taleggio) C. i. variscoi has been found together with C. itala and transitional specimens.
C. i. balsamoi occurs syntopically with C. itala; transitional forms have not been found.
In Nordsieck (1984: 171) I proposed to regard the stenzioids as separate species, C. clavata. The transitional forms were interpreted as interspecies hybrids.
Scheel & Hausdorf (2012), in their DNA study, treated the stenzioids as subspecies of C. itala. In their AFLP network the stenzioids form five clusters, because C. i.trepida forms a cluster of its own (though this taxon does not differ from C. i. lorinae by shell characters), and the transitional forms (C. i. allatollae) an additional cluster.
The assumed interspecies hybrid forms of Siciliaria from Sicily are the following (material in SMF):
S. grohmanniana from Priola, with characters of S. septemplicata;
S. septemplicata from Parco, with characters of S. calcarae;
S. septemplicata near Cinisi, with characters of S. tiberii, described as S. s. alcamoensis and S. s. hemmeni;.
S. calcarae from Monte Belliemi, with the ribbing of S. tiberii, described as S. c. belliemii;
S. tiberii from Monte della Scala, with characters of S. calcarae;
S. leucophryna from Sferracavallo, with characters of S. calcarae;
S. crassicostata from Scurati near Custonaci, transitional to S. eminens.
As result of the taxonomic revision of the genus Delima (Nordsieck 1969: 274-275) I found several intermediate forms, especially in central Dalmatia, which have characters of two neighbouring species, but none of their own. These assumed interspecies hybrid forms occur at the boundaries of the ranges of the neighbouring species. They consist of uniform populations and have a more or less extended range, or of less uniform populations between populations of the neighbouring species, or are individuals in syntopic populations of these species. Only the first-mentioned forms are regarded as subspecies of the species, with which they share most characters.
The assumed hybrid forms are the following:
Central Dalmatia (see Nordsieck 2007: chapter VI, map 1):
D. blanda conspurcata and D. latilabris = D. b. blanda, from Makarska coastland;
D. b. conspurcata and D. pachystoma = D. b. schmidti, in Vrlika;
D. b. conspurcata and D. pachystoma, in Drniš and Petrovo Polje;
D. pachystoma and D. albocincta, in Drniš;
D. pachystoma and D. latilabris = D. p. satricensis, between Hrvace and Maljkovo;
D. pachystoma and D. b. conspurcata = D. p. vicariella, near Muć;
D. latilabris and D. pachystoma = D. l. angusticollis, between Sinj and Muć;
D. latilabris and D. b. conspurcata, in Dicmo Polje;
D. latilabris and D. b. conspurcata, = D. l. duarensis, inland of Omiš;
D. albocincta and D. b. conspurcata, in Čikola valley near Drniš;
D. albocincta and D. b. conspurcata = D. a. sororia, in middle Krka valley;
D. semirugata and D. bilabiata, in western part of Hvar island.
D. latilabris and D. binotata, in Gračac.
It is striking that most assumed interspecies hybrid forms originate from species of the D. blanda group. Because of shell and genital differences there is no doubt about their species rank. In the majority of cases D. b. conspurcata is involved, a species widespread in central Dalmatia, at least in part distributed by man. The meeting of this species with D. pachystoma in the villages of Drniš and Vrlika (in Drniš also with D. albocincta), which I investigated in 1965 and 1966, is especially interesting. In these villages I found in several populations both stem species and hybrid forms. In Vrlika D. b. conspurcata is modified by introgression from D. pachystoma to a subspecies intermediate between both species (described as D. b. schmidti H. Nordsieck). I assume that D. b. conspurcata was introduced by man and hybridized there with the autochthonous D. pachystoma (in Drniš also with D. albocincta from Čikola valley, via D. pachystoma which is intermediary in size).
Brandt (1956, 1958), in his papers on the clausiliids of Cyrenaica (Barcania), described some subspecies which are intermediate between B. kaltenbachi and other B. species:
B. kaltenbachi albaensis (1956: 131) from the eastern part of the species range, intermediate between B. kaltenbachi and B. apolloniana;
B. chaligi tigaensis (1958: 5) from the western part of the species range, intermediate between B. kaltenbachi and B. chaligi;
B. k. susaensis (1958: 6), restricted to Wadi Haulla, intermediate between B. kaltenbachi and B. klugi.
B. klugi from Wadi en Nsuria had been described as separate species (1956: 133), because it differs in shell characters from the surrounding B. kaltenbachi by being similar to B. bengasiana. After the discovery of the ribbed subspecies B. klugi streyi (1958: 6) from Wadi Sneides, it was downgraded to a subspecies of B. kaltenbachi, because west of Wadi Sneides B. kaltenbachi with transitions to B. kl. streyi had been found. The examination of the material available to me (SMF) showed that in Wadi en Nsuria B. kaltenbachi and B. kl. streyi occur syntopically, without transitions, while in other valleys (Wadi Sneides, Wadi Haulla) transitional populations exist, in Wadi Haulla even regarded as subspecies B. k. susaensis (see above). Obviously there is interspecies hybridization between B. kaltenbachi and B. klugi.
B. sasaensis from Wadi Zaza, which has a smooth shell like B. kaltenbachi, is regarded as separate species (1956: 126). In both wadis neighbouring Wadi Zaza the ribbed subspecies B. s. costellata has been found. In one of these wadis it is more coarsely ribbed, like B. bengasiana, with which it occurs syntopically (Nordsieck 2019). Obviously there is interspecies hybridization between B. sasaensis and B. bengasiana .
In Nordsieck (1966: 30, 2013: 52) I described a population of C. exoptata from Bracca Serina, Val Serina, Bergamasque Alps, which occurred together with C. whateliana (20% of the specimens) and a small percentage of transitional specimens (8%). Both taxa are clearly different in shell and genital morphology, so that an introgression at secondary contact was concluded. Therefore, until now I treated C. exoptata as a separate species and its hybridization with C. whateliana as interspecies hybridization.
Because of the gene flow stated in a DNA study, Hausdorf & Nägele (2016) classified C. exoptata in spite of the morphological differences as subspecies of C. whateliana.
The subgenus includes the species M. circassica with N-type CA and M. caucasica with an incipient G-type CA. A form of M. caucasica, with a transitional CA, has been described as M. c. interjecta . This name is used by Koch et al. (2016) in a DNA study for the M. (Baleopsina) populations from the Lagonaki Mountains , northwestern Caucasus, which have characters intermediate between both species and form a genetic unit of their own, which shall have originated by interspecies hybridization. This unit, however, includes specimens with very different morphological characters (e. g., M. annae with strongly developed N-type CA and pure M. caucasica).
Baidashnikov (1990: 29-30) stated that M. velutina from S. W. Crimea, separated by him as independent species from M. gracilicosta because of differences in shell and genitalia, is connected with that species by transitional forms at the boundary of the range in Ai-Petri and the high plateau of Čatir Dag. He therefore (: 31) considered M. velutina as a „semispecies“. Neiber et al. (2019), after a DNA study of Mentissa, treated M. velutina in spite of the differences as subspecies of M. gracilicosta, because in their tree representing a subclade of the M. gracilicosta clade. On the other hand, they found gene flow between the species M. gracilicosta and M. canalifera.
In the course of a revision of the genus Bulgarica (Nordsieck 1973: 193-195), now named Strigillaria, I stated that there are two pairs of closely related taxa, which on the one hand are connected by intermediate forms and on the other occur syntopically without transitions. Until now (Nordsieck 2007) they have been united in one species each.
One of these taxa pairs, S. rugicollis – S. pagana, is distributed in S. W. Romania and adjacent northernmost Serbia. S. rugicollis (shell densely ribbed) and S. pagana (shell widely ribbed) occur sympatrically e. g. in the Cerna valley, in some localities even syntopically without transitions, but are connected by forms with intermediary rib densities in other localities.
The other taxa pair, S. bulgariensis – S. osmanica, is distributed mainly in northern Bulgaria. S. bulgariensis (shell widely ribbed) and S. osmanica (shell densely ribbed) occur sympatrically in the region of Veliko Târnovo, in some localities even syntopically without transitions, while in other localities of the same region populations with intermediary rib densities have been found.
Therefore, until now (Nordsieck 1973, 2007), the taxa of these pairs have been treated as subspecies.
Between the taxa of both pairs there may be gene flow, limited by reproductive barriers. Because of the syntopies and differences of the genitalia (only S. rugicollis – S. pagana examined) they are now treated as species.
Interspecies hybridization and ecology
In all described examples of interspecies hybridization rock-dwelling clausiliids, mostly from southern Europe, are concerned. Within the genera which are widespread in the woodlands of remaining Europe, Cochlodina (Alopiinae), Macrogastra and Clausilia (C.) (Clausiliinae), no case of interspecies hybridization is known, though a great amount of material of the species belonging to has been examined. An exception could be the assumed hybrids of Cochlodina laminata and C. dubiosa, two closely related species, which I have found in the southern Alps.
While the species of rock-dwelling clausiliids have in general similar copulatory organs, which make possible interspecies hybridization, in the forest-dwelling species these organs are more or less different. Contrary to the rock-dwelling clausiliids, which as a rule occur allopatrically, for forest-dwelling species, which often occur syntopically, interspecies hybridization might be disadvantageous, because they have different ecological niches.
III. Conclusions for the species concept
For future research it is predicted that there will arise severe problems, when authors will try to reconstruct the phylogeny within clausiliid genera from southern Europe, in which interspecies hybridization is common. It will certainly not be sufficient to propose phylogenies and new systems without considering this phenomenon.
On the other hand, uniting species which demonstrably hybridize to megaspecies will also not be an satisfactory solution. The arguments are as follows:
1. Imbalance within a genus: Investigations which prove interspecies hybridization are as a rule focussed within a genus to certain cases with two or few species concerned; in uniting these species the other cases with similar preconditions in the same genus are not considered. If one unites certain species with interspecies hybridization within a genus to megaspecies, one must do this also with the other species with such hybridization in order to avoid an inappropriate imbalance. A good example is the Alopia livida group from Bucegi Mountains, Romania (Koch et al. 2020, see part II).
2. Imbalance in comparison with other genera: To treat species of a genus (e. g., Albinaria, Alopia, Delima) which hybridize, as subspecies, which are on the other hand morphologically and from their distributional relations as well characterized as other species of another genus (e. g., Montenegrina), in which species because of isolation mostly cannot hybridize, or other species of another genus (e. g., Cochlodina), in which species do not hybridize, leads to another inappropriate imbalance. The ability to hybridize is a character of the species of certain groups; species of these groups must remain comparable with species of groups without that ability.
3. Obscuring of diversity: A further consequence of these unions is a loss of information by obscuring the diversity. If within these megaspecies subspecies of the former species and the former species, which now are subspecies, are recorded, these have different rank. This leads inevitably to a neglecting of the subspecies of lower rank, with consequences for their perception as taxa and for their eventual protection, the same problem as discussed for neglected subspecies of polytypic Albinaria species.
Therefore, species of groups which hybridize, but are on the other hand comparable with species of other groups which cannot or do not hybridize, should not be united to megaspecies, but remain to be treated as species.
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