FLYING SNAILS HOW FAR CAN TRUNCATELLINA (PULMONATA: VERTIGINIDAE) BE BLOWN OVER THE SEA?

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1 J. Moll Stud. (1997), 63, i The Malacological Society of London 1996 FLYING SNAILS HOW FAR CAN TRUNCATELLINA (PULMONATA: VERTIGINIDAE) BE BLOWN OVER THE SEA? CH. KIRCHNER, R. KRATZNER 1 & F.W. WELTER-SCHULTES 2 Max-Planck-lnstitutfiirBiophysikalische Chemie, Abteilung Molekulare Entwicklungsbiologie, Am Fassberg, D Gdttingen, Germany. 'Institut fur Molekulare Genetik der Universitat, Grisebachstr. 8, D Gdttingen, Germany. 2 II. Zoologisches Institut der Universitat, Berliner Str. 28, D-37O73 Gdttingen, Germany (Received 3 April 1996; accepted 27 January 1997) ABSTRACT With populations of land snails of very small size like Vertiginidae, questions have arisen as to whether populations of relatively distant islands in archipelagos are really isolated from each other. Apart from other flight agencies, airborne transport of loose specimens is not improbable in stormy weather conditions. Currently, mechanisms of wind-borne transport of sand particles over short and long distances have been intensively studied. The results are available in the literature on sediments, allowing the calculation of probable flight distances for particles in suspension. For living snails of the Aegean species Truncatellina rothi, an average fall velocity of m s" 1 has been determined in experiments under laboratory conditions. Applying these results, Truncatellina living on an island at 100 m altitude and close to the coast could be transported up to several kilometers in heavy storms, which are not uncommon in the Aegean archipelago (Greece). This would imply that many of the Aegean islands are not effectively isolated for minute snail species, and that genetic interchange between island populations is probably frequent. INTRODUCTION The smallest pulmonate inhabitants of the Greek islands belong to the genus Truncatellina (Vertiginidae). A widespread species in the South Aegean is Truncatellina rothi (Reinhardt) (Fig. 1). The systematics in the Southeast European Truncatellina has not been thoroughly studied yet. Maybe T. rothi is a species complex. The area of dispersal of T. rothi is not well known, presumably due to its small size. It has been found at several sites in Northern Greece and Albania (Fig. 2) (Frank, 1987; Kleram, 1962; Maassen, 1984; Dhora & Welter-Schultes, 1996). It is reported to live in the southwestern and southern parts of Turkey (Schlitt, 1993), and is also known from Attikf (Reinhardt, 1916) and several Greek islands: Lefk&da (Klemm, 1962), Thasos (Reischtltz, 1983), Lfmnos (Reischlitz, 1986), Chfos (Bar & Butot, 1986), Naxos (Mylonas, 1982), Le"ros (Reischutz, 1985), Kalimnos (Reischutz, 1986), R6dos (Maassen, 1981), Crete, Kfthira and Andikfthira (Vardinoyannis, 1994) and some surrounding islands of Crete. During investigations on the small surrounding islands of Crete by F. Welter-Schultes in , Truncatellina was found for the first time in ground-litter samples of Koufonfsi Island (South of Crete) in After 1991, Truncatellina was found in similar habitats on almost every island investigated (GaVdos, Gavdopoiila, Chrisi, Grdndes, different sites in Crete). T. rothi was found on the island of GaVdos and in Albania, in the altitude of 200 m and 400 m respectively. If there was a probability for Truncatellina to be dispersed by wind for some kilometers flying from one island to another, the probability of genetic interchange between island populations would increase. Wind-borne transport is regarded as an important factor for the dispersal of small species of land snails. Most of the information available about dispersal ability in land snails are deductions from distribution patterns (Baur & Bengtsson, 1987). The relatively quick dispersal of small species northwards and to the tops of mountains in the Late and Post Glacial Period in Europe is considered to have been wind-borne (Ant, 1963). There is also strong evidence that the land snail fauna of the Pacific islands originated primarily through aerial dispersal (Valvolgyi, 1975), at least for small species. Mechanisms of airborne dispersal of snails have never been studied under experimental conditions, as has been the wind-borne trans-

2 480 CH. KIRCHNER, R. KRATZNER & F.W. WELTER-SCHULTES Figure 1. Truncatellina rothi, with dust particles in the mouth of the empty shell. Scale bar = 0.20 mm. port of sand in desert sand storms (Anderson, S0rensen & Willets, 1990). The wind-borne translocation behaviour of particles like sand or snow has also been studied under field conditions (Jensen, Rasmussen, S0rensen & Willetts, 1984; Takeuchi, 1980). Wind blowing over a surface will, under certain conditions, impart momentum to any available loose small particles, causing them to skip along the surface. As each particle impacts the surface, yet more particles are ejected into the wind, and eventually a distinct layer forms consisting of particles in flight across the surface. This phenomenon is known as saltation. Very light particles, for which the force of gravity is small compared to that of aerodynamic origin, travel downwind at the mercy of turbulent fluctuations without undergoing impact with the surface. There is no particle-bedinteraction. These particles are in suspension. We suppose that saltation plays an important role in aerial snail dispersal on land, in the same way as it does in the case of windborne continental dispersal of sand (Bagnold, 1941; Barndorff-Nielsen, Blaesild, Jensen & S0rensen, 1983; S0rensen, 1988). Saltation on the sea surface is impossible for the snails. They have to travel in suspension. The values for sand, silt and clay when travelling long distances in suspension are known. A grain of sand (diameter 0.1 mm, fall velocity m s" 1 ) may be dispersed by wind (15 m s" 1 ) for 0.3 to 3 s reaching a distance of m. For silt grains (diameter 0.01 mm,

3 FLYING SNAILS 481 i Albania LeffuWa Greece Klthifa GAydos AttJki Lfrnoos Chfos Leros T^ Kalimnos Turfcey Figure 2. Truncatellina rothi has been found at sites in Northern Greece and Albania (dots) and on several Greek islands. fall velocity m s '), a maximum flight distance in suspension (wind 15 m s" 1 ) of km is calculated (Pettijohn, Potter & Siever, 1987). For these calculations the sand particle density is generally assumed to be 2.65 g cm" 3 (Iversen & White, 1982). For calculations of possible flight distances, one of the most important factors is the fall velocity (Bagnold, 1941). The request of the present study is to find out the fall velocity of Truncatellina rothi. MATERIALS AND METHODS Two random samples of 50 empty shells of Truncatellina rothi from two different ground litter samples which were collected on Givdos Island (UTM KU3559 and KU3460, for the lxl km UTM map of GaVdos see Welter-Schultes, 1995) have been measured (shell height and shell diameter) under microscope. The shells of the sample KU3460 were filled with paraffin jelly (Vaseline) to simulate approximately the live weight of Truncatellina. The weight of the specimens was determined using an analytical balance of g accuracy (calculated error ± 20 M-g)- Each individual of the two samples was dropped from 5.1 m and from 10.9 m altitude above base level. The time of dropping, from release to landing, was measured with a stop watch (accuracy 0.01 s, calculated error of this method ± 0.15 s). The terminal fall velocity was obtained in evaluating the results of the experiments. RESULTS Measurements and weight of the specimens The diameter of the shells (D) is between 0.70 and 0.95 mm (0 = 0.83 mm, <J X = 0.04 mm, n = 100), the height of the shells (H) varies between 1.1 and 1.8 mm (0 = 1.47 mm,ct, = 0.12 mm, n = 100) for the specimens of GaVdos (Fig. 3). There is obviously no correlation between diameter and height of the shell of Truncatellina rothi.

4 482 CH. KIRCHNER, R. KRATZNER & F.W. WELTER-SCHULTES D = shell diameter (mm) Figure 3. Size and dimensions of the specimens of Truncatellma rothi used in the experiments. Living snails of Truncatellina rothi have not been found on G&vdos. In 1995, some living individuals of Truncatellina cylindrica (Feiussac), a species which is very similar to T. rothi in size, have been collected in San Marino. The weight of these snails had an average value of jxg. The weight of empty adult shells of Truncatellina rothi was between 100 and 300 \x.g (0 = 151 jig, v x = 34 xg, n = 50). After being filled with paraffin jelly, the weight of the specimens of sample KU3460 reached values between 250 and 550 p.g (0 = 375 jxg, cr, = 69 ng, n = 50). The weight differences were principally due to the different size of the shells and to dust particles in the interior space of the empty shells and on the shell surface (Fig. 1). The density of shells filled with Vaseline varied between 0.6 and 0.9 g cm" 3 (0 = 0.72 g cm" 3, cr x = 0.14 g cm" 3, n = 50, fitting well with the few values we had for living snails), the density of empty shells g cm" 3 (0 = g cm" 3, ex, = g cm" 3, n = 50). These values have been obtained in dividing the weight of the specimens by their volume. The approximate volume of Truncatellina can be obtained by the equation V = 4 /rir- H / 2 -( D / 2 ) 2 = H-D-D V = volume (mm 3 ); H = height of the shell (mm); D = diameter of the shell (mm). which is the volume of an ellipsoid. For Truncatellina an average volume of mm 3 was calculated. The density of living Truncatellina is lower than that of water because the body of the living snail does not occupy the entire space inside the shell. The height of the ribs of Truncatellina rothi is approximately 15 urn, the mean rib distance varying between 50 and 80 n-m. Fall velocity The results of the experiments are shown in Fig. 4. The fall velocity values of a grain of sand of the same dimensions as Truncatellina and

5 FLYING SNAILS : , UU fall velocity (m/s) B k A "3 0.6 E r 0.5 '% 0.4 empty shell " living snail A sand grain empty shell E [ IB a fall velocity (m/s) a living snail ± sand grain Figure 4. Terminal fall velocity of Truncatellina rothi. A. Relation between fall velocity and the g cm" 2 values of the specimens. B. Relation between fall velocity and weight of the specimens. For living Truncatellina of an average weight of jig, fall velocity values of m s" 1 have been determined. the values of the empty shells are included in the diagram for comparison with the fall velocity values of the 'living' snails. The figures show the degree to which living Truncatellina and shells may be expected to vary as regards their wind resistance. The fall velocity depends on the g cm" 2 values of the specimens. For living Truncatellina a mean value of g cm" 2 is calculated. The more important factor influencing this value is the weight, since differences in shell size are small and can be ignored. Table 1 shows the average terminal fall velocity values for snails of approximate life weight, which were obtained in the experiments. For living Truncatellina of mean weight, regularly grown and free of any large adherent objects on their shell surface, an average fall velocity of m s" 1 was determined. Theoreticalflightdistances The simple addition of the two vectors of the terminal forward velocity, which is assumed to be close to the wind velocity, and the terminal downward velocity, which is assumed to be close to the fall velocity of the snails, results in a theoretical flight distance at laminar wind conditions (Fig. 5B). We base our calculations of flight distances on the assumption that the Table 1. Mean terminal fall velocity for Truncatellina of approximately live weight. weight 300 i g cm" 2 values (± 0.01 g cm" 2 ) 0.27 g cm"' 0.29 g cm" g cm" g cm" 2 B fall velocity (± 0.3 m s-') 2.5 ms-' 2.6 ms" ms" m s" 1 snail will start from an island from a certain altitude above sea level (100 m). Due to the slower particle response as a result of drag conditions (Anderson, 1987), particles of the size of Truncatellina will not immediately follow the trajectories of the wind turbulences on the lee side of an island. The terminal forward vector as shown in Fig. 5B principally does not describe an unreal situation. Turbulent wind conditions Particles in suspension follow two parameters. Suspension is the balance between downward advective flux as a result of the settling of grains (Table 2), and their upward flux as a

6 484 CH. KIRCHNER, R. KRATZNER & F.W. WELTER-SCHULTES island 1 terminal forward velocity terminal downward * velocity island 1 island 2 Figure 5. A. Probable natural wind conditions. The velocity of wind is approximately reflected in the length of the arrows. B. Addition of the two vectors of wind velocity and fall velocity. The dashed arrow describes the theoretic trajectory of the snail neglecting turbulent wind conditions. Table 2. Theoretical flight distances for living Truncatellina rothi, as a result of the addition of the two velocity vectors shown in Fig. 5B. Start of the snails 100 m above sea level, laminar horizontal wind 27.8 m s~ 1. weight f fall velocity (±0.03 gem-') 2.5 m s"' 2.6 ms"' 2.7 m s-' 2.8 ms" 1 theoretical flight distance (± 93 m) 1111 m 1068 m 1029 m 992 m result of turbulence (Anderson & Hallet, 1986). As shown in Anderson (1987), it is possible to incorporate turbulent wind conditions in statistical approaches on trajectories of particles in suspension, allowing calculations of probable maximum flight distances under natural conditions. Grains of sand (diameter 0.1 mm, fall velocity m s" 1 ) are able to reach a maximum flight distance of m in 15 m s" 1 wind. The maximum height reached by the particles is m respectively (Pettijohn et al., 1987). The theoretical flight distance (as applied in Table 2, taking into account the different fall velocity and wind velocity of the given example), neglecting the upward movement of the grains and setting them to start at an altitude of m, would be in the order of m. In our calculations, the upward diffusive flux as a result of turbulent wind conditions can be included as a turbulence factor, comparing the theoretical flight distances of sand grains used in the example with their actual maximum flight distances as given in Pettijohn et al. (1987). This factor 460/111 = 4.14 could be applied to approximate probable maximum flight distances under natural conditions. The point at which a particle in saltation is at the top of its trajectory can be calculated (S0rensen, 1990). Suspension is an extreme kind of modified saltation, and aeolian suspension can be modelled in the same way as modified saltation (Anderson et al., 1990). The relation between the mean rising periods and the mean falling periods of the trajectories is 24:76% (Anderson, 1987). This relation gives good fits for the suspension profiles and can be applied for the example in Pettijohn et al. (1987). When only the mean falling periods of the trajectories are considered the turbulence factor is reduced from 4.14 to The maximum flight distance for one specimen of Truncatellina rothi in a storm of a wind B

7 Table 3. Maximum flight distances for living Truncatellina rothi under turbulent wind conditions. Two different calculations are suggested, the most probable values are assumed to be close to the factor 3.15 values. Start of the snails 100 m above sea level, turbulent wind 27.8 m s\ weight 300 jig 350 ng 375 p.g 400 M-S 450 p.g flight distance applying factor 4.14 (± 385 m) 4600 m 4422 m 4342 m 4260 m 4107 m flight distance applying factor 3.15 (±293m) 3500 m 3364 m 3304 m 3241 m 3125 m velocity of 100 km h~', when starting at an altitude of 100 m above sea level, is calculated to be approximately 3300 m. If wind velocity is reduced to 50 km h" 1, the distance is halved. Setting the snails to start at an altitude of 200 m enables them to reach a distance of 6600 m in 100 km h" 1 wind. The probable maximum flight distance of snails living at 500 m altitude is 16.5 km. DISCUSSIONS AND CONCLUSIONS Outline of the study The present study has been carried out in order to ascertain whether it is theoretically possible for Truncatellina to overcome distances of several kilometers continuously over sea. The study has not been carried out in order to simulate field conditions, or to calculate exact possible flight distances of Truncatellina rothi between two Greek islands under natural circumstances. Passive dispersal by wind in general and suspension trajectories in particular are stochastic rather than deterministic. We have also avoided the question of how the snails may be dislodged and lifted into the air. Our studies are designed to give an answer to what is possible once they are airborne. Neglected influences FLYING SNAILS 485 Considering the mere fall velocity in our experiments and calculations, we neglect possible influences originating not only in variation of aerodynamic forces, but also in the existence of inter-particle forces due to moisture, electrostatic effects, and other forces of cohesion. These forces are known to be greater for small particles and relatively independent of particle density (Iversen, Pollack, Greenley & White, 1976). In comparison to the sand grains usually dealt with in the sedimentological studies, Truncatellina does not belong to the small particle fractions. We also neglect probable changes in wind velocity, and assume that the forward velocity of the snails equals the velocity of the wind. The wind is faster when striking over the top of an island (Fig. 5A), but at the same moment the snails will not yet have reached their terminal forward velocity, due to drag conditions. Biogeographical implications Direct passive dispersal by wind is not the only method of airborne translocation for Truncatellina. There are many other means by which living land snails can be transported over the sea. Dispersal of land snails by birds and insects is considered as fact (Rees, 1965; Valvolgyi, 1975). Furthermore, minute snails are able to stick on leaves, single bird feathers or other inter-island flight agencies, which can be transported by wind much more easily. In our research, direct dispersal has been studied because of the presumably increased probability for single snails to be dislodged by wind. Initiation of Truncatellina populations does not necessarily require more than one individual landing on the next island, as Vertiginidae are self-fertile in many instances (Falkner, 1990). Our results are important concerning probabilities of genetic interchange, and hence questions of systematics and taxonomy. Relatively frequent genetic interchange between island populations of minute snail species are not provided by changing sea levels combined with tectonic movements, though for explaining the distribution of the Aegean land snails, these events may be of importance (Heller, 1976). Flying to islands around Crete 100 km h" 1 is probably about the maximum wind velocity in the Aegean. At this velocity a flight from Andikfthira to Crete or back is not probable, and Crete could not be reached by Kdrpathos snails. So Crete is isolated from the rest of Greece. The island of GaVdos could be reached by snails starting their trajectory at 1500 m altitude in Western Crete. So genetic interchange is possible, but only one-way.

8 486 CH. KIRCHNER, R. KRATZNER & F.W. WELTER-SCHULTES GaVdos does not exceed 400 m altitude, so they are not able to fly back to Crete. Chrisi and Koufonfsi could be reached from various sites from Eastern Crete. In Eastern Crete the snails would also be able to be transported to Grdndes, Eldsa and the Dionis&des. The island of Dfa could probably be reached by snails starting from the top of Mount Gioiichtas ( m), and from the tops of the mountains of Rodia west of Dfa. We base our calculations on the assumption that Truncalellina lives on the highest points of the islands at the start of the trajectory. Of course, the greatest distances would not be travelled frequently, but they do appear to be theoretically possible. ACKNOWLEDGEMENTS We wish to express our gratitude to W. Zarnack (Gottingen) for helpful comments and kindly placing at our disposal the wind channels of the I. Zoological Institute of Gdttingen University, for further experiments which helped to ascertain the results of the presented study. L. Bull (Freetown) and P. Mordan (London) are acknowledged for the linguistic revision and correction of the English manuscript. REFERENCES ANDERSON, R.S Eolian sediment transport as a stochastic process: the effects of a fluctuating wind on particle trajectories. Journal of Geology, 95: ANDERSON, R.S. & HALLET, B Sediment transport by wind: toward a general model. Geological Society of America Bulletin, 97: ANDERSON, R.S. S0RENSEN, M. & WILLETTS, B.B A review of recent progress in our understanding of aeolian sediment transport. Research Reports, Departments of Theoretical Statistics, Institute of Mathematics, University of Aarhus, 213: ANT, H Faunistische, dkologische und tiergeographische Untersuchungen zur Verbreitung der Landschnecken in Nordwestdeutschland. Abhandlungen aus dem Landesmuseum fiir Naturkunde zu Miinster in Westfalen, 25: BAGNOLD, R.A The physics of blown sand and desert dunes. Methuen & Co., London. BAR, Z. & BUTOT, LJ.M The land snails of Chios. De Kreukel, 22: BARNDORFF-NIELSEN, O.E., BLJESILD, P., JENSEN, J.L. & SORENSEN, M The fascination of sand. Research Reports, Department of Theoretical Statistics, Institute of Mathematics, University of Aarhus, 93. BAUR, B. & BENGTSSON, J Colonizing ability in land snails on Baltic uplift archipelagos. Journal of Biogeography, 14: DHORA, DH. & WELTER-SCHULTES, F.W List of species and atlas of the non-marine molluscs of Albania. Schriften zur Malakozoologie, 9: FALKNER, G Binnenmollusken. In: Fechter, R. & Falkner, G. Weichtiere. Europaische Meeresund Binnenmollusken. Steinbachs Naturflihrer, 10: FRANK, CH Beitrag zur Kenntnis der Molluskenfauna der ostlichen Mittelmeerlander. Teil III (1): Zusammenfassung der Sammelergebnisse der Jahre vom kontinentalen Griechenland, dem Peloponnes, den Nordlichen Sporaden sowie einigen Inseln des Ionischen und des AgSischen Meeres. Malakologische Abhandlungen, 12: HELLER, J The biogeography of enid land snails on the Aegean Islands. Journal of Biogeography, 3: IVERSEN, J.D., POLLACK, J.B., GREENLEY, R. & WHITE, B.R Saltation threshold on Mars: the effect of interparticle force, surface roughness, and low atmospheric density. Icarus, 29: IVERSEN, J.D. & WHITE, B.R Saltation threshold on Earth, Mars and Venus. Sedimentology, 29: JENSEN, J.L., RASMUSSEN, K.R., SORENSEN, M. & WILLETTS, B.B The Hanstholm experiment 1982: sand grain saltation on a beach. Research Reports, Department of Theoretical Statistics, Institute of Mathematics, University of Aarhus KLEMM, W X. Teil. Die Gehauseschnecken. In: Beier, M. Zoologische Studien in West-Griechenland. Sitzungsberichte, Osterreichische Akademie der Wissenschaften, mathematisch-naturwissenschaftliche Klasse, Abteilung 1,171: , Tafeln \-4. MAASSEN, WJ.M De Molluskenfauna van het griekse eiland Rhodos. De Kreukel, 17: 21-32, pi MAASSEN, WJJ Enkele vindplaatsen van mollusken in Noord Griekenland. De Kreukel, 20: 23-34, pi MYLONAS, M.A Meliti pdno sti zoogeograffa ke ikologla ton chersion malaklon ton KikUdon. [The zoogeography and ecology of the terrestrial molluscs of CycladesJ. Unpublished Ph. D. Thesis, University of Athens. PETTUOHN, FJ., POTTER, P.E. & SIEVER, R Sand and sandstone. Second edition. Springer Verlag, New York. REES, WJ The aerial dispersal of Mollusca. Proceedings of the Malacological Society of London, 36: REINHARDT, O Einige Bemerkungen flber Pupa minutissima und Verwandte. Nachrichtsblatt der Deutschen Malakozoologischen Gesellschaft, 48: REISCHOTZ, P.L Ein Beitrag zur Molluskenfauna der Insel Thasos (Griechenland). Annalen des Naturhistorischen Museums in Wien, 85B: ,Tafell.

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