Cornu aspersum (Muller, 1774)

Hausdorf, Bernhard, Bamberger, Sonja & Walther, Frank, 2021, A Sicilian-Cretan biogeographical disjunction in the land snail genus Cornu (Gastropoda: Helicidae), Zoological Journal of the Linnean Society 192, pp. 59-74 : 70-72

INTROGRESSION WITH CORNU ASPERSUM

Near Chora Sfakion, 1.5 km toward Chania, we found a peculiar population of C. aspersum ( Figs 3F, G, 4B) that differed in the smaller size and the more conical shell shape from usual C. aspersum populations ( Fig. 3H), which were also found in the surroundings of Chora Sfakion. This population resembled in size and shell shape that of C. cretense , which occurs only about 20 km away. Thus, the question arose whether the similarity can be explained by introgression with C. cretense . The ratio of flagellum length to the combined length of the penis and epiphallus of the two measured individuals from Chora Sfakion 1.5 km toward Chania (2.0–2.1; see Table 2) was smaller than the average ratio in C. aspersum [2.4; calculated based on measurement data of 2078 C. aspersum specimens from Sherpa et al. (2018); S. Sherpa, pers. comm.], but still within the range of variation of C. aspersum .

Separate ML analyses of partial COI sequences and the predominant ITS2 sequences of the population of C. aspersum from Chora Sfakion, 1.5 km toward Chania in south-western Crete, showed that their mitochondrial haplotypes and their predominant nuclear ITS2 sequences belong to the C. aspersum clade, corroborating the classification of the population as C. aspersum . However, there was a background signal in the ITS2 sequences of three of the five sequenced specimens, in which a four-base stretch, which is deleted in most specimens of C. aspersum , could be recognized. This four-base stretch was also found in three specimens of C. aspersum from Mazzaforno in Sicily ( Colomba et al., 2011; GenBank accession no. GQ402424 – GQ402426 ), which is about 5 km away from the range of C. cephalaeditanum . In a sequence from Australia ( Blacket et al., 2016; GenBank accession no. KX679295 ), these four base pairs were not deleted, but they differed in a substitution from the sequences of Erctella and Cantareus . In addition, the sequences of the three specimens from Mazzaforno agreed with the Erctella and Cantareus sequences in an A at position 235, where other C. aspersum had a T (the Australian sequence has an ambiguity, an A or a T, at that position). The presence of the ancestral four-base stretch in some copies of the rDNA cluster of three of the five sequenced C. aspersum from Chora Sfakion, 1.5 km toward Chania in Crete, as well as in the Sicilian C. aspersum , might indicate introgression with neighbouring Erctella species. However alternative explanations, namely that the Erctella -like sequence stretch represents the plesiomorphic state or an ancestral polymorphism in C. aspersum , cannot be excluded with the few data available, but have to be further tested with more molecular data.

BIOGEOGRAPHY

Although several taxa show remarkable biogeographical disjunctions in the western and the eastern parts of the Mediterranean (e.g. Zilli, 1999), cases in which a monophylum like Erctella is restricted to Sicily and Crete are rare. One such case is found in botany: the sister-species pair Zelkova abelicea (Lam.) Boiss. from Crete and Zelkova sicula Di Pasq., Garfi & Quézel from Sicily ( Ulmaceae ; Denk & Grimm, 2005). Whereas Erctella is restricted to Sicily and Crete, and Cornu s.l. is a Mediterranean genus (ignoring the world-wide distribution of Cornu aspersum by humans), the tree genus Zelkova shows a much wider Palaearctic disjunct distribution with three recent species in eastern Asia and one in western Asia, beside the two Mediterranean species ( Denk & Grimm, 2005). Since fossil Zelkova species are also known from Europe and North America ( Denk & Grimm, 2005), the occurrences of the two Zelkova species in Sicily and Crete can be interpreted as relicts of a formerly wider, probably more continuous, European distribution. Intermediate stages between a distribution restricted to Sicily and Crete and a wider European distribution are also known. For example, Ferulago nodosa (L.) Boiss. ( Apiaceae ) is distributed in Greece (southwards to Crete), Albania and, probably, North Macedonia, and is represented in Sicily by Ferulago nodosa subsp. geniculata (Guss.) Troia & Raimondo ( Troia et al., 2012; for name see: Peruzzi et al., 2015).

In the case of Erctella , no occurrences are known outside Sicily and Crete, neither recent nor fossil. Thus, there is no evidence that the current disjunct occurrences can be explained as relicts of a formerly continuous range. Other possible mechanisms are plate tectonic events and long-distance dispersal. The movement of microplates in the Mediterranean region ( Rosenbaum et al., 2002) may explain disjunct distribution patterns of land snails in the western Mediterranean ( Giusti & Manganelli, 1984; Pfenninger et al., 2010). However, west–east Mediterranean disjunctions cannot be explained by such plate tectonic events because no microplates moved from the western to the eastern Mediterranean, or vice versa ( Rosenbaum et al., 2002).

We consider it most likely that the occurrence of Cornu (Erctella) cretense in Crete is the result of a long-distance dispersal event. The snails might have been transported from Sicily to Crete with ocean currents. Actually, the Atlantic Ionian Stream passes to the north-west of Sicily and runs into the eastern Mediterranean ( Malanotte-Rizzoli, 2001). It is unlikely that this current is transporting western Mediterranean snails to Crete at present because the Levantine Surface Water current runs westwards along the south coast of Crete north of the Atlantic Ionian Stream ( Malanotte-Rizzoli, 2001). However, we cannot exclude a former west–east transport of snails by a current similar to the Atlantic Ionian Stream because the currents were certainly different in the geological past. Moreover, unusual events like a transport of large amounts of debris washed into the sea by a tsunami after an earthquake ( Lindo, 2020) might have increased the likelihood of a successful long-distance dispersal event of living land snails.

The divergence-time interval of the Erctella species includes the time of the Messinian salinity crisis when the Mediterranean Sea became disconnected from the oceans of the world and mostly desiccated by evaporation between 5.6 and 5.33 Mya (Garcia- Castellanos et al., 2009), irrespective of whether the calibration with Parachloraea ( Fig. 2) or the calibration of Teasdale (2017) (see above) is correct. We do not consider it likely that Erctella or other snails migrated through the wide, hot and salty, empty basin. However, the Zanclean flood that refilled the Mediterranean basin at the end of the Messinian ( Garcia-Castellanos et al., 2009; Micallef et al., 2018) might have been the event that washed the ancestor of C. cretense from Sicily to Crete.

A possible alternative to rafting with sea currents for long-distance transport of snails is dispersal by birds ( Rees, 1965; Gittenberger et al., 2006; Wada et al., 2012; Simonová et al., 2016). Although Cornu is large, it cannot be excluded that juveniles were transported alive in the feathers of birds from Sicily to Crete. However, the usual migration routes of birds run north–south and not east–west. Thus, we consider the rafting hypothesis more plausible than dispersal by birds.