J. Moll Stud. (1997), 63,479-487
i The Malacological Society of London 1996
FLYING SNAILS—HOW FAR CAN TRUNCATELLINA
(PULMONATA: VERTIGINIDAE) BE BLOWN OVER THE
SEA?
CH. KIRCHNER, R. KRATZNER1 & F.W. WELTER-SCHULTES2
Max-Planck-lnstitutfiirBiophysikalische Chemie, Abteilung Molekulare Entwicklungsbiologie, Am Fassberg,
D-37077 Gdttingen, Germany. 'Institut fUr Molekulare Genetik der Universitat, Grisebachstr. 8, D-37075
Gdttingen, Germany. 2II. Zoologisches Institut der Universitat, Berliner Str. 28, D-37O73 Gdttingen, Germany
(Received 3 April 1996; accepted 27 January 1997)
and southern parts of Turkey (Schlitt, 1993),
and is also known from Attikf (Reinhardt, 1916)
With populations of land snails of very small size like
and several Greek islands: Lefk&da (Klemm,
Vertiginidae, questions have arisen as to whether
1962), Thasos (Reischtltz, 1983), Lfmnos (Reispopulations of relatively distant islands in archichlitz, 1986), Chfos (Bar & Butot, 1986), Naxos
pelagos are really isolated from each other. Apart
(Mylonas, 1982), Le"ros (Reischutz, 1985),
from other flight agencies, airborne transport of
Kalimnos (Reischutz, 1986), R6dos (Maassen,
loose specimens is not improbable in stormy weather
1981), Crete, Kfthira and Andikfthira (Vardiconditions. Currently, mechanisms of wind-borne
noyannis, 1994) and some surrounding islands
transport of sand particles over short and long disof Crete.
tances have been intensively studied. The results are
available in the literature on sediments, allowing the
During investigations on the small surroundcalculation of probable flight distances for particles
ing islands of Crete by F. Welter-Schultes in
in suspension.
1987-1994, Truncatellina was found for the
For living snails of the Aegean species Truncatel- first time in ground-litter samples of Koufonfsi
1
lina rothi, an average fall velocity of 2.6-2.7 m s" has Island (South of Crete) in 1991. After 1991,
been determined in experiments under laboratory
Truncatellina was found in similar habitats
conditions. Applying these results, Truncatellina
living on an island at 100 m altitude and close to the on almost every island investigated (GaVdos,
Gavdopoiila, Chrisi, Grdndes, different sites in
coast could be transported up to several kilometers
Crete). T. rothi was found on the island of
in heavy storms, which are not uncommon in the
GaVdos and in Albania, in the altitude of 200 m
Aegean archipelago (Greece). This would imply
and 400 m respectively.
that many of the Aegean islands are not effectively
isolated for minute snail species, and that genetic
If there was a probability for Truncatellina
interchange between island populations is probably
to be dispersed by wind for some kilometers flyfrequent.
ing 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 disperINTRODUCTION
sal of small species of land snails. Most of the
The smallest pulmonate inhabitants of the information available about dispersal ability in
Greek islands belong to the genus Truncatel- land snails are deductions from distribution
lina (Vertiginidae). A widespread species in patterns (Baur & Bengtsson, 1987). The relathe South Aegean is Truncatellina rothi (Rein- tively quick dispersal of small species northhardt) (Fig. 1). The systematics in the South- wards and to the tops of mountains in the Late
and Post Glacial Period in Europe is considered
east European Truncatellina has not been
thoroughly studied yet. Maybe T. rothi is a to have been wind-borne (Ant, 1963). There is
also strong evidence that the land snail fauna of
species complex.
The area of dispersal of T. rothi is not well the Pacific islands originated primarily through
known, presumably due to its small size. It has aerial dispersal (Valvolgyi, 1975), at least for
been found at several sites in Northern Greece small species.
and Albania (Fig. 2) (Frank, 1987; Kleram,
Mechanisms of airborne dispersal of snails
1962; Maassen, 1984; Dhora & Welter-Schultes, have never been studied under experimental
1996). It is reported to live in the southwestern conditions, as has been the wind-borne transABSTRACT
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 0.824 m s"1) may be dispersed by wind
(15 m s"1) for 0.3 to 3 s reaching a distance of
46-460 m. For silt grains (diameter 0.01 mm,
FLYING SNAILS
i
481
Albania
Lfrnoos
Greece
Turfcey
LeffuWa
Chfos
AttJki
Leros
T^ Kalimnos
Klthifa
GAydos
Figure 2. Truncatellina rothi has been found at sites in Northern Greece and Albania (dots) and on several
Greek islands.
fall velocity 0.00824 m s '), a maximum flight
distance in suspension (wind 15 m s"1) of
400-4000 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.
the specimens was determined using an analytical
balance of 0.00001 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
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
Measurements and weight of the specimens
The diameter of the shells (D) is between
0.70 and 0.95 mm (0 = 0.83 mm, <JX = 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.
482
CH. KIRCHNER, R. KRATZNER & F.W. WELTER-SCHULTES
1.8-1
1.1
0.7
0.75
0.8
0.85
0.9
D = shell diameter (mm)
0.95
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 350-400
jxg. The weight of empty adult shells of Truncatellina rothi was between 100 and 300 \x.g
(0 = 151 jig, vx = 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, crx = 0.14 g cm"3, n = 50, fitting well with
the few values we had for living snails), the
density of empty shells 0.2-0.4 g cm"3 (0 =
0.306 g cm"3, ex, = 0.054 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 = 0.5236-H-D-D
V = volume (mm3); 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 0.525 mm3 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
FLYING SNAILS
0.8
483
1
k
0.9
0.7
0.8
: 0.6
0.7
"3 0.6
E
r 0.5
B
0.4
'%
, 0.3
IB
[
•a
0.4
03
0.2
E
0.2
0.1
0.1
UU 5
0
0
1
1.5
empty shell
2
2.5
3
3.5
fall velocity (m/s)
" living snail
4
4.5
15
A
A sand grain
|
2
2.5
3
3.5
fall velocity (m/s)
empty shell
a
living snail
4
4.5
B
± 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 350-400 jig, fall velocity values of 2.4-2.9 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 0.28-0.32 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
2.6-2.7 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
g cm"2 values
(± 0.01 g cm"2)
fall velocity
(± 0.3 m s-')
300 i
350
400
450
0.27 g cm"'
0.29 g cm"3
0.31 g cm"2
0.33 g cm"2
2.5 m s - '
2.6 ms" 1
2.7 ms" 1
2.8 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
484
CH. KIRCHNER, R. KRATZNER & F.W. WELTER-SCHULTES
island 1
terminal forward velocity
terminal downward
* velocity
island 1
island 2
B
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
300
350
400
450 f
fall velocity
(±0.03 gem-')
theoretical
flight distance
(± 93 m)
2.5 m s"'
2.6 m s " '
2.7 m s-'
2.8 ms" 1
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 0.824 m s"1) are able to
reach a maximum flight distance of 46-460 m in
15 m s"1 wind. The maximum height reached
by the particles is 0.61-6.1 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 0.61-6.1 m, would
be in the order of 11.10-111.0 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 3.15.
The maximum flight distance for one specimen of Truncatellina rothi in a storm of a wind
FLYING SNAILS
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
375
400
450
ng
p.g
M-S
p.g
flight distance
applying
factor 4.14
(± 385 m)
flight distance
applying
factor 3.15
4600
4422
4342
4260
4107
3500 m
3364 m
3304 m
3241 m
3125 m
m
m
m
m
m
(±293m)
485
static 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
velocity of 100 km h~', when starting at an alti- only method of airborne translocation for
tude of 100 m above sea level, is calculated to Truncatellina. There are many other means by
be approximately 3300 m. If wind velocity is which living land snails can be transported over
reduced to 50 km h"1, the distance is halved. the sea. Dispersal of land snails by birds and
Setting the snails to start at an altitude of 200 m insects is considered as fact (Rees, 1965;
enables them to reach a distance of 6600 m in Valvolgyi, 1975). Furthermore, minute snails
100 km h"1 wind. The probable maximum flight are able to stick on leaves, single bird feathers
distance of snails living at 500 m altitude is or other inter-island flight agencies, which can
be transported by wind much more easily. In
16.5 km.
our research, direct dispersal has been studied
because of the presumably increased probability for single snails to be dislodged by wind.
DISCUSSIONS AND CONCLUSIONS
Initiation of Truncatellina populations does
not necessarily require more than one individOutline of the study
ual landing on the next island, as Vertiginidae
The present study has been carried out in order are self-fertile in many instances (Falkner,
to ascertain whether it is theoretically possible 1990). Our results are important concerning
probabilities of genetic interchange, and hence
for Truncatellina to overcome distances of
several kilometers continuously over sea. The questions of systematics and taxonomy. Relastudy has not been carried out in order to tively frequent genetic interchange between
island populations of minute snail species are
simulate field conditions, or to calculate exact
possible flight distances of Truncatellina rothi not provided by changing sea levels combined
between two Greek islands under natural with tectonic movements, though for explaincircumstances. Passive dispersal by wind in ing the distribution of the Aegean land snails,
general and suspension trajectories in par- these events may be of importance (Heller,
ticular are stochastic rather than deterministic. 1976).
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 Flying to islands around Crete
what is possible once they are airborne.
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
Neglected influences
probable, and Crete could not be reached by
Considering the mere fall velocity in our experi- Kdrpathos snails. So Crete is isolated from the
ments and calculations, we neglect possible rest of Greece. The island of GaVdos could be
reached by snails starting their trajectory at
influences originating not only in variation of
1500 m altitude in Western Crete. So genetic
aerodynamic forces, but also in the existence of
inter-particle forces due to moisture, electro- interchange is possible, but only one-way.
486
CH. KIRCHNER, R. KRATZNER & F.W. WELTER-SCHULTES
land snails on Baltic uplift archipelagos. Journal of
GaVdos does not exceed 400 m altitude, so they
Biogeography, 14: 329-341.
are not able to fly back to Crete. Chrisi and
Koufonfsi could be reached from various sites DHORA, DH. & WELTER-SCHULTES, F.W. 1996. List
of species and atlas of the non-marine molluscs of
from Eastern Crete. In Eastern Crete the snails
Albania. Schriften zur Malakozoologie, 9: 90-197.
would also be able to be transported to
FALKNER, G. 1990. Binnenmollusken. In: Fechter, R.
Grdndes, Eldsa and the Dionis&des. The island
& Falkner, G. Weichtiere. Europaische Meeresof Dfa could probably be reached by snails
und Binnenmollusken. Steinbachs Naturflihrer, 10:
starting from the top of Mount Gioiichtas
112-280.
(700-800 m), and from the tops of the moun- FRANK, CH. 1987. Beitrag zur Kenntnis der Moltains of Rodia west of Dfa.
luskenfauna der ostlichen Mittelmeerlander. Teil
III (1): Zusammenfassung der Sammelergebnisse
We base our calculations on the assumption
der Jahre 1982-1985 vom kontinentalen Griechenthat Truncalellina lives on the highest points of
land, dem Peloponnes, den Nordlichen Sporaden
the islands at the start of the trajectory. Of
sowie einigen Inseln des Ionischen und des AgSiscourse, the greatest distances would not be
chen Meeres. Malakologische Abhandlungen, 12:
travelled frequently, but they do appear to be
101-124.
theoretically possible.
HELLER, J. 1976. The biogeography of enid land
snails on the Aegean Islands. Journal of Biogeography, 3: 281 -292.
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.
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