Functional
Ecology 1997
11, 472–483
A biomechanical hypothesis explaining upstream
movements by the freshwater snail Elimia
A. D. HURYN*‡ and M. W. DENNY†
*Aquatic Biology Program, Department of Biological Sciences, University of Alabama, Tuscaloosa, Alabama
35487, USA, Department of Zoology, PO Box 56, University of Otago, Dunedin, New Zealand and †Hopkins
Marine Station, Department of Biological Sciences, Stanford University, Pacific Grove, CA 93950–3094, USA
Summary
1. Many taxa of freshwater invertebrates show active upstream movements, particularly the snails. Hypotheses explaining this behaviour invoke the search for food or
space, compensation for drift, avoidance of predation and hydrodynamic effects. The
pervasiveness of upstream movements among remote lineages of snails (two subclasses, three orders, 10 families), however, suggests that snails may move upstream
for mechanical rather than adaptive reasons.
2. It is proposed that upstream movements by snails are a function of torque on the
snail’s foot generated by hydrodynamic drag on the shell. When subject to high broadside drag-forces on their shells, snails are able to reduce torque and stabilize orientation only by directing their anterior aspect upstream.
3. Movements of the freshwater pleurocerid snail Elimia were studied by following
marked free-ranging individuals in six streams in Alabama, USA (four species, eight
populations).
4. Populations showed either no net movement (two streams) or significant upstream
movements ranging to a mean of ≈ 40 m over a 3-month period (four streams).
Movement patterns were stream specific rather than species or population specific.
Within populations showing significant upstream movements, snails with shell lengths
≤ 10 mm showed little net movement. Larger snails showed movements from 0 to
> 200 m upstream.
5. A torque-constrained random walk model was used to perform a post hoc test of the
hypothesis that upstream movements were a function of torque on the snail’s foot generated by hydrodynamic drag on the shell. The model predicted upstream and sizedependent movement patterns that approximated those observed for snails in the field.
Key-words: Biomechanics, hydrodynamics, snails, streams, torque
Functional Ecology (1997) 11, 472–483
Introduction
Movement patterns of invertebrates in streams have
attracted continuous interest for the better part of this
century. Although most effort has been concentrated
on downstream movements as drift (Waters 1972;
Brittain & Eikeland 1988), active upstream movements have also been documented for many taxa
(Söderström 1987; Bergey & Ward 1989), particularly
the snails (Table 1). Hypotheses explaining upstream
movements by snails have invoked the search for food
or space (Paulini 1963), compensation for downstream
drift (Carpenter 1928; Schneider & Frost 1986), avoidance of predation (Schneider & Lyons 1993) and con© 1997 British
Ecological Society
‡Current address: Department of Applied Ecology and
Environmental Sciences, University of Maine, 5722 Deering
Hall, Orono, Maine 04469–5722, USA.
straints imposed by body architecture and hydrodynamics (Haynes, Taylor & Varley 1985). Although
numerous, none of these hypotheses has been rigorously examined and all remain conjectural.
Upstream movements have been documented for a
remarkably diverse assemblage of snails in temperate
and tropical streams worldwide (Table 1). This assemblage, spanning two subclasses, three orders and 10
families is the result of repeated invasions of streams
by marine ancestors over geological time (Hutchinson
1967; Graham 1985). From an evolutionary perspective, the character ‘upstream movement’ that defines
the assemblage must logically be either a plesiomorphy derived from ancestral marine forms and not a
specific adaptation to life in freshwater streams, or
have been derived independently in different lineages.
The pervasiveness of upstream movements among
472
473
Upstream
movements by
snails
Table 1. Summary of snail families containing freshwater taxa (Davis 1982) and reports of upstream movements. Grades of
evidence are: Indirect-a = movements inferred from appearance of animals in reaches where they previously where not
observed, Indirect-b = movements of genetically distinct subpopulations; Direct-a = observation of individuals actively moving upstream, Direct-b = movements of marked individuals, Direct-c = movements of radionuclides, Direct-d = flume studies;
? = source of evidence not stated; – = not studied
Subclass
Superfamily
Family
Prosobranchia
Neritinacea
Nertinidae
Neritina latissima
N. granosa
Hydrocenidae
Viviparacea
Ampullariidae
Vivparidae
Campeloma sp.
C. decisum
Valvatacea
Valvatidae
Cerithiacea
Pleuroceridae
Elimia catenaria
E. clavaeformis
E. proxima
E. proxima
E. semicarinata
Pleurocera sp.
P. acuta
Melanopsidae
Syrnolopsidae
Thiaridae
Melanoides tuberculata
Thiara granifera
Rissoacea
Assimineidae
Baicaliidae
Bithyniidae
Hydrobiidae
Potamopyrgus antipodarum
P. jenkinsi
Cochliopina tryoniana
Lepyriidae
Pomatiopsidae
Pyrguliidae
Stenothyridae
Buccinacea
Buccinidae
Volutacea
Marginellidae
© 1997 British
Ecological Society,
Functional Ecology,
11, 472–483
Pulmonata
Acroloxidae
Ancylidae
Chilinidae
Latiidae
Lymnaeidae
Lymnaea pereger
Physidae
Physa sp.
Physella integra
Planorbidae
Biomphalaria glabrata
B. glabrata
Location
Evidence
Source
Costa Rica
Hawaii
–
Direct-a
Direct-a
–
Schneider & Lyons 1993
Ford & Kinzie 1982
–
–
–
–
Illinois
Michigan
Indirect-a
Direct-b,d
Shelford 1913
Bovjberg 1952
–
–
–
Georgia
Tennesee
North Carolina
Virginia
Kentucky
Illinois
Kentucky
–
–
Direct-d
Direct-b
Direct-b
Indirect-b
Direct-b
Indirect-a
Direct-b
–
–
Kreiger & Burbank 1976
Burris et al. 1990
Crutchfield 1966
Dillon 1988
Mancini 1978
Shelford 1913
Houp 1970
–
–
Martinique
Puerto Rico
Indirect-b
Direct-a
Pointier et al. 1993
Chaniotis et al. 1980
–
–
–
–
–
–
–
–
–
New Zealand
Britain
Costa Rica
–
SE Asia
–
SE Asia
Indirect-b
Direct-b,d
Direct-a
–
?
–
?
Wallace 1992
Haynes et al. 1985
Schneider & Lyons 1993
–
Davis 1982
–
Davis 1982
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Britain
Indirect-a
Hynes 1960
Michigan
New Mexico
Direct-c
Indirect-a
Ball, Wojtalik & Hooper 1963
Noel 1954
Brazil
Puerto Rico
Direct-b,d
Direct-d
Paulini 1963
Jobin & Ippen 1964
474
A. D. Huryn &
M. W. Denny
remote lineages of freshwater snails suggests that
this behaviour may result from an architectural constraint imposed by the general gastropod morphology rather than being a specific adaptation to existence in freshwater streams; some taxa may move
upstream for mechanical rather than adaptive reasons.
In this paper, upstream movements by the freshwater
snail Elimia (Pleuroceridae) are explained by invoking
torque generated by the interaction of hydrodynamic
drag with gastropod architecture. The utility of this
mechanical hypothesis is then demonstrated by a simulation model showing how torque may provide a simple
explanation for the widespread and well-documented
occurrence of upstream movements by snails.
STUDY SITES
Six streams were selected for study, three each from
two major physiographic provinces of Alabama
(USA), Valley & Ridge and Piedmont. All study
reaches are of moderate gradient (< 2%) and have
channels characterized by long reaches of bedrock
(> 50 m) punctuated by short riffles. Average width of
channels at base flow ranged from 3 to 9 m. Average
discharge based on semi-monthly measurements during 1990–91 ranged from 76 to 210 l s–1. Channels of
study sites in the Valley & Ridge province (Hendrick
Mill Branch – Blount County; Alligator Creek, Rocky
Branch – Bibb County) are largely carbonate bedrock.
Channels of study sites in the Piedmont province
(Choccolocco Creek, Marys Creek, South Fork
Terrapin Creek – Winston County) are primarily phyllite bedrock. The snail Elimia cahawbensis occurs at
Alligator Creek, Hendrick Mill Branch and Rocky
Branch. Elimia carinifera and E. variata occur only at
Hendrick Mill Branch and Alligator Creek, respectively. Elimia fascinans occurs at Choccolocco Creek,
Marys Creek and Terrapin Creek. Additional information about the life history and ecology of Elimia is
contained in Huryn, Koebel & Benke (1994) and
Huryn, Benke & Ward (1995).
Fig. 1. Schematic diagram showing a broadside projection of Elimia cahawbensis
where c.a. refers to the centre of projected area, c.m. refers to the perpendicular axis of
the columellar muscles and ‘lever’ is the distance from centre of projected area to the
perpendicular axis of the columellar muscles. The lever is the distance over which
torque will be generated because of drag forces on the shell when perpendicular to flow.
Methods and materials
MOVEMENTS OF SNAILS
Approximately 1000 snails (range = 747–1139) were
collected from each population and individually
marked using numbered microtags (Freilich 1989).
Once marked, snails were released midway between
the beginning and end of a 100-m study reach. All
releases were made between 18 March and 8 April
1990. Following the release, surveys for marked
snails were made at all sites at ≈ 3-month intervals
for one year (June/July 1990; October 1990;
February 1991; April 1991). Surveys began at the
downstream end of the study reach (50 m below the
release point) and continued upstream until there
appeared to be little probability of finding additional
marked snails. This procedure was repeated a minimum of three times on each date. When a marked
snail was found, its number, position and shell length
was recorded and it was released. Additional detail
concerning methods is contained in Huryn et al.
(1994, 1995).
MECHANICAL FORCES AND SNAIL SHELLS
Torque is generated when a force is exerted on a lever
arm; it tends to cause an object to rotate. Since the
columellar muscles of Elimia are offset from the centre of the shell’s lateral projected area and drag forces
act near the centre of area, drag on the shell is likely
to impose torque on the columellar muscles and foot
(Fig. 1). It is proposed that such torque will cause a
snail in flowing water to act much like a weather vane
– that is, sufficient drag forces on the snail’s shell
will cause it to rotate until its anterior end faces
upstream. Once in such a position the frontal projected area is symmetrical about the axis of the collumellar muscles, the torque is eliminated and the
orientation is stable. When exposed to appropriate
flow velocities this imposed shell orientation will
effectively steer snails in an upstream direction. If
the posterior rather than the anterior aspect of the
shell is projected into the current (Fig. 1), torque
may similarly be eliminated. This position is unstable, however.
Empty shells of the four species of Elimia were
filled with wax (to simulate the presence of the snail’s
body) and mounted on a circular plate (7-cm diameter). This plate was in turn held flush with the wall of a
wind tunnel (24-cm2 cross section) by a drag transducer (Denny 1989, 1995). The snail shell was then
subjected to a free-stream wind velocity of 28·8 m s–1,
and the resulting force on the shell and plate was
recorded. In a separate experiment, friction drag on
the bare mounting plate was measured and subtracted
from the overall force to yield the force on the shell.
Each shell was tested in two orientations, one broadside to flow and the second with the shell’s anterior
end upstream. The coefficient of drag (Cd) was calcu-
475
Upstream
movements by
snails
lated as a function of the force experienced by the
shell (N) in either frontal or broadside projection by
the following equation (Vogel 1994):
Cd = (2F)/(DV2A),
eqn 1
where F is drag force (N, measured by the drag transducer), D is the density of air (1·20 kg m–3 at 20·5 °C),
V is the velocity of the air (m s–1) and A is the projected area (m2). Projected areas were obtained by
digitizing scaled photographs of the shells in the anterior or broadside orientation. Given the Cd, the projected area of a shell, and current speed and density of
water, drag forces can be estimated by solving for F in
equation 1. Reynolds numbers (Re) of shells in the
wind tunnel (104) were higher than the maximum estimated for free-ranging snails at the study sites (103).
Measurements of Cd for marine snails show little variation as a function of Re (range = 103–105; Denny
1995), however, and it was assumed that Cds were
constant over the range of Re values experienced by
snails in the study streams.
The centre of area for the broadside projection of
shells for each species was located by hanging cutouts
of tracings of the perimeter of photographs of each shell
from a pin at three or more separate positions. Vertical
lines drawn from each pin position intersected at the
approximate centre of area. The distance between the
centre of area for the broadside projections and the axis
of the columellar muscles (approximated as the centre
of the aperture) was used to estimate the lever arm over
which the drag on the shell will act (Fig. 1). Torque on
the columellar muscles of Elimia was estimated as the
product of the drag on the shell (N) and length of the
lever arm (m). It is assumed that torque on the collumellar muscles will influence direction of movement by
rotating the snail on its foot. The mechanical effects of
hydraulic forces on snail shells should be scaled to an
index of an individual’s ability to resist torque, which
may change with size. For convenience we calculated
torque as a function of foot area (Denny, Daniel &
Koehl 1985). Foot area, measured from snails relaxed
with menthol, was regressed against shell length to provide predictive equations (P < 0·05, Table 2).
SNAIL ORIENTATION AND TORQUE
The relationship between water velocity and orientation of individual snails was measured in Hendrick
Mill Branch (E. cahawbensis, E. carinifera) and
Marys Creek (E. fascinans) using plastic glitter (silver)
as tracer particles. Glitter was released into the stream
and photographed with a 35-mm SLR camera at 1/15,
1/30 or 1/60 s as it passed over targeted snails. A ruler
placed in the photographic field provided scale needed
to estimate water velocity and shell dimensions. Water
velocity was calculated from the scaled length of
streaks caused by moving glitter and the camera’s
shutter speed. No attempt was made to assign particles
to a specific vertical position within the water column.
With some experience, however, glitter could be
released so that most particles passed within a few centimetres of targeted snails. Snails with the anterior
aspect of their shells oriented directly into the current
were assigned values of 0°. Deviation of the snail’s
longitudinal axis from 0° was measured with a protractor. For each snail, the torque that would be experienced if the shell was broadside to the current was estimated. These data were used to construct plots of
torque m–2 foot area vs snail orientation.
TORQUE AS A DETERMINANT OF SNAIL MOVEMENT
PATTERNS
Measurements of water velocity were used to construct a two-dimensional matrix of cells that represented the diversity and spatial distribution of actual
hydraulic conditions in each study reach.
Measurements were made at 1-m intervals across
transects placed perpendicular to the stream channel.
The transects were located at 2-m intervals parallel to
the study reach. Water velocity was measured ≈ 2 cm
above the substrata with an electromagnetic flow
meter (Flo-Mate 2000, Marsh-McBirney, Inc.,
Frederick, MD).
Although measurements of velocity were made
≈ 2 cm above the substrata, these data are relevant to
drag forces actually experienced by snails. The
boundary layers in the study streams are probably tur-
Table 2. Summary of parameters used to calculate torque/m2 foot area for four species of Elimia. Broadside and anterior refer
to coefficients of drag (Cd) measured for single shells at each orientation. For comparative purposes, Cds estimated for spheres
under similar hydraulic conditions (Denny 1993) are given after the diagonal. The broadside projection and foot area can be
estimated for each species by the following equation (P< 0·05): area (m2) = cLb, where L is shell length (m). Lever coefficient
is the proportion of total shell length that occurs between the main axis of the columellar muscles and the centre of the shell’s
projected area
© 1997 British
Ecological Society,
Functional Ecology,
11, 472–483
Species
Broadside/sphere
Cd
Anterior/sphere
Cd
Lateral
projection (m2)
c
b
Foot area (m2)
c
b
E. cahawbensis
E. carinifera
E. fascinans
E. variata
0·51/0·45
0·43/0·45
0·46/0·44
0·38/0·44
0·50/0·43
0·47/0·43
0·42/0·43
0·35/0·44
0·2089
0·2748
0·3990
0·3636
0·8903
0·0162
0·2914
0·8549
1·9057
2·0509
2·0479
2·0022
2·3463
1·6338
2·0144
2·2290
Lever
coefficient
0·24
0·22
0·27
0·23
476
A. D. Huryn &
M. W. Denny
© 1997 British
Ecological Society,
Functional Ecology,
11, 472–483
bulent as a result of flow separation caused by virtually infinite levels of bed roughness upstream (Hart,
Clark & Jasentuliyana 1996). Turbulent boundary layers are characterized by time-averaged velocity gradients. Energetic eddies, however, will regularly reach
almost to the substratum and bring with them velocities that are near those of the mainstream. Hart et al.
(1996), for example, measured fluctuations of water
velocity as great as 80 cm s–1 over intervals of < 0·1 s
within 2 mm of the benthos in a turbulent stream.
Except for individuals that are small enough to hide in
the viscous sublayer (in the order of 0·1 mm), snails
are likely be subject to intermittent velocities approximately equal to the mainstream. These intermittent
velocities will place the largest torque on the snails,
and are therefore the ones that are most critical in
determining hydraulic-dependent behaviour (Denny
1994). Based on this reasoning, we assume that measurements of the mainstream flow ≈ 2 cm above the
bed provide realistic information about the flow environment closer to the substratum.
Water velocity matrices constructed for each study
stream formed the basis for a computer simulation of
how torque might act to steer movements of snails in
streams. The simulation consisted of placing a virtual
snail of specified length (5, 10, 15 or 20 mm) into the
water velocity matrix at a cell equivalent to the actual
release position in each stream (‘0,0 m’) and allowing it
to step from cell to cell in a random direction unless the
snail entered a cell where torque m–2 foot area
exceeded an empirical threshold derived from plots of
torque m–2 foot area vs snail orientation (see ‘snail orientation and torque’, above). In cells where torque
exceeded the empirical threshold, the next step was to
the adjacent cell upstream. Each snail was allowed to
take 720 steps (= 720 ‘m’). This value was selected on
the basis of how far a specimen of Elimia might conservatively be expected to move over a 3-month summer
period (Beetle 1970; Burris, Bamford & Stewart 1990;
A. D Huryn, personal observation). To allow lateral
steps and longitudinal steps to be of equal distance,
water velocities were assigned to each 1-m2 cell following the assumption that flow conditions were constant
between each 2-m transect. After each series of steps
was completed, the final position of the snail was
recorded. This procedure was repeated 1000 times for
each of the four length classes. In some cases virtual
snails rapidly moved off the original matrix, and therefore matrices were extended by sequentially repeating
randomly selected transects of water velocity measurements in an upstream or downstream direction.
Results of the model were in the form of frequency
distributions of probable final positions for different
length classes and species of snails for each study site.
The success of the model was assessed by comparing
actual distributions of marked snails with simulated
distributions. Since we were originally interested in
examining only the effect of torque on movement
patterns, snails of all sizes were assumed to move at
equal velocities. However, the model was later modified to incorporate size-dependent differences in
movement speed as indicated for Elimia by Krieger &
Burbank (1976). It is emphasized that the objective of
the model was heuristic rather than predictive per se,
and is offered to show how torque may influence
movements of snails in streams.
Results
MOVEMENTS OF MARKED SNAILS
Numbers of snails recaptured during the initial survey
ranged from 4 to 13% of the total individuals marked
among the eight populations. However, the proportion
of snails captured more than once over all successive
surveys was also low, ranging from 3 to 19% of the
total recaptured (mean = 9%). The low incidence of
repeated recaptures of unique snails indicates that
marked snails were often passed over rather than having moved exceptional distances or losing tags.
After 3 months of free-ranging, significant netupstream movement was observed for every population except E. cahawbensis at Rocky Branch and E.
fascinans at Terrapin Creek (one sample t-test,
P < 0·05, Figs 2 and 3). Maximum upstream movements of individual snails ranged from 58 m for E.
fascinans at Choccolocco Creek (Fig. 3) to 200 m for
E. carinifera at Hendrick Mill Branch (Fig. 2).
Maximum downstream movements ranged from 3 m
at Choccolocco Creek to 47 m for E. fascinans at
Marys Creek. However, few snails were recovered at
positions substantially downstream of the release
point (Figs 2 and 3). Furthermore, no snails were ever
found during additional intensive field work that was
conducted several hundred metres downstream from
the release point in each stream throughout the duration of the study. Although this work was not directly
related to the snail project, field personnel were
instructed to report marked snails. Average position at
recapture for E. cahawbensis in Alligator Creek and
Hendrick Mill Branch, and for E. carinifera in
Hendrick Mill Branch was significantly further
upstream than for other populations (ANOVA, Tukey’s
HSD, P < 0·05). These differences are apparently not
species specific. For example, after 3 months, populations of E. cahawbensis showed average positions of
1, 28 and 39 m at Rocky Branch, Hendrick Mill
Branch and Alligator Creek, respectively (Figs 2 and
3). This range brackets the variation observed among
all eight populations. Snails with lengths ≤ 10 mm
showed little movement from the point of release
compared to larger snails (Figs 2 and 3).
Mean position at recapture was not significantly
influenced by date at Alligator Creek, Hendrick Mill
Branch and Terrapin Creek (ANOVA, P > 0·05).
However, this factor was significant at Rocky Branch,
Choccolocco Creek and Marys Creek (P < 0·05);
apparently because of a small but consistent down-
477
Upstream
movements by
snails
© 1997 British
Ecological Society,
Functional Ecology,
11, 472–483
Fig. 2. Left. Plots of the position and size observed for four populations of free-ranging marked snails released in Hendrick
Mill Branch and Alligator Creek. Large points indicate positions ≈ 3 months following release. Small points indicate positions
of recaptured snails ≈ 6 months to 1 year following release. Panels are arranged in order of decreasing net upstream movement.
The arrow along the y-axis indicates the length at which ash-free dry mass is ≈ 2·2 mg. This biomass threshold has been shown
to distinguish different levels of expected food satiation by Elimia (see text). Right. Plots of the frequency distributions of different positions attained by snails of different lengths (5–20 mm) predicted by a torque-constrained random walk model. Each
plot represents 1000 iterations of the model.
478
A. D. Huryn &
M. W. Denny
Fig. 3. Left. Plots of the position and size observed for four populations of free-ranging marked snails released in Choccolocco
Creek, Marys Creek, Terrapin Creek and Rocky Branch (see caption for Fig. 2 for additional explanation). Right. Plots of the
frequency distributions of different positions attained by snails of different lengths (5–20 mm) predicted by a torque-constrained random walk model. Each plot represents 1000 iterations of the model.
© 1997 British
Ecological Society,
Functional Ecology,
11, 472–483
stream movement of average position during winter.
For example, the average position observed at
Choccolocco Creek ranged from 12 m upstream of the
release point in October to 5 m in February. A similar
pattern was observed at Rocky Branch (3–0 m) and
Marys Creek (10–7 m). Snails were apparently capable of the maximum movements observed during the
1-year study within 3 months following release
(Figs 2 and 3). Compared with movement activity
during the initial 3-month period of observation
479
Upstream
movements by
snails
(spring–summer), movement activity of Elimia may
have been limited by declining water temperature (e.g.
Kreiger & Burbank 1976). Seasonal variation in movement activity may explain the apparently lower rates of
movement following spring–summer (Figs 2 and 3).
DRAG
Coefficients of drag were measured for a shell from
each of the four snail species (Table 2). Measurements
of total drag forces on each shell were repeated at least
three times. Cds calculated for each shell represent the
average of these measurements; standard errors were
always < 15% of the mean. Cds for broadside projections were similar among species. For example, the
standard error among shells from all species was ≈ 7%
of the mean (range = 0·38–0·51, mean = 0·45, standard
error = 0·03; Table 2). Cds for anterior projections
were similar to those calculated for broadside projections (range = 0·35–0·50, mean = 0·44; Table 2). For
either projection, Cds of snails were similar to those of
spheres with the same Re (0·43–0·44; Table 2).
© 1997 British
Ecological Society,
Functional Ecology,
11, 472–483
SNAIL ORIENTATION AND TORQUE
Orientation and torque m–2 foot area was measured
for E. cahawbensis (n = 55), E. carinifera (n = 58) and
E. fascinans (n = 64) in Hendrick Mill Creek and
Marys Creek. It was assumed that snails oriented into
the current with the long axis of their shells within 45°
of a vector parallel to the direction of flow were moving upstream; snails aligned within 45–135° of the
vector were moving in a direction perpendicular to
flow; snails aligned 135–180° were moving downstream. A plot of shell orientation by larger snails
(> 10 mm length) vs torque m–2 foot area (Fig. 4)
shows that E. cahawbensis and E. carinifera in
Hendrick Mill Branch orient in an upstream direction
under flow conditions that would generate a broadside
torque ≥ 0·15 N m–2 foot area. Elimia fascinans in
Marys Creek orient in an upstream direction under
conditions that would generate a broadside torque
≥ 0·25 N m–2 foot area. Threshold torque was not
measured for E. variata so a conservative torque of
0·25 N m–2 foot area was used for modelling purposes. With few exceptions, snails ≤ 10 mm in length
Fig. 4. Plot of torque m–2 foot area (N m–2) for broadside shells vs shell orientation for two length classes (≤ 10 mm, lower
panel; > 10 mm, upper panel) of Elimia cahawbensis and E. carinifera in Hendrick Mill Branch and E. fascinans in Marys
Creek. Snails are assumed to be moving upstream when oriented against and within 45° of the direction of flow.
480
A. D. Huryn &
M. W. Denny
were not observed under flow conditions that generated torques > 0·15 N m–2 foot area. When shells were
exposed to torques below the apparent threshold, orientation of E. cahawbensis and E. fascinans was not
significantly different from a random distribution
(P > 0·05, χ2 goodness of fit for circular frequency distributions; Zar 1984). However, the orientation of E.
carinifera departed significantly from a random distribution because of a bias toward individuals oriented in
neutral or upstream directions (P < 0·05, Fig. 4).
FLOW CONDITIONS AMONG STREAMS
Point measurements of water velocity among study
reaches varied from 0 to ≈ 0·7 m s–1 (Fig. 5). In general,
Valley & Ridge streams (Alligator Creek, Hendrick
Mill Branch, Rocky Branch) had a higher range of
velocities than Piedmont Streams (Choccolocco Creek,
Marys Creek, Terrapin Creek; Fig. 5). Water velocities
covering ≈ 20% of the stream bed were capable of generating torques exceeding the apparent threshold for
20-mm long specimens of E. carinifera in Hendrick
Mill Branch (e.g. 0·2 m s–1, Fig. 5). At the other
extreme, the maximum velocity in Terrapin Creek was
well below that required to produce threshold torques
for 20-mm long specimens of E. fascinans (Fig. 5).
TORQUE-CONSTRAINED RANDOM WALK MODEL
Results of the torque-constrained random walk
model predicted that stream and snail size should
differentially influence movement patterns of snails
(Figs 2 and 3). The largest net-upstream movement
was predicted for E. carinifera and E. cahawbensis
in Hendrick Mill Branch and E. cahawbensis in
Alligator Creek (Fig. 2). The Hendrick Mill Branch
model predicted complete lack of downstream
movements because of super-threshold torques
imposed on all size classes by rapid water velocities
(+ 0·7 m s–1) through a bedrock chute below the
release point. Consequently, all snails were forced to
proceed upstream. The smallest net upstream movements were predicted for E. fascinans in Terrapin
Creek, where no net movements were predicted at all
(Fig. 3).
Size-dependent movement patterns were clearly
indicated among model results for E. carinifera
(Hendrick Mill Branch), E. variata (Alligator Creek)
and E. fascinans (Choccolocco Creek) (Figs 2 and
3). Frequency distributions predicted for length
classes > 10 mm were strongly skewed in an
upstream direction, whereas frequency distributions
for snails ≤ 10 mm were, with the exception of E.
carinifera (see comments concerning Hendrick Mill
Branch above), symmetrically distributed about the
release point. Size-dependent movements were
either not apparent (E. cahawbensis, all sites; E.
fascinans, Terrapin Creek) or only vaguely apparent
(E. fascinans, Marys Creek) among the remaining
populations (Figs 2 and 3). Since size-dependent differences in snail behaviour were not incorporated
into the original model, size-dependent movement
Fig. 5. Proportion of stream bed covered by different water velocities for six streams. Black bars indicate water velocities predicted to produce torques
resulting in upstream movements for 20-mm long snails. The approximate maximum length of Elimia is ≈ 20 mm.
481
Upstream
movements by
snails
patterns are because of differences in the torque and
the ratio of torque to foot area experienced among
length classes.
COMPARISON OF MODEL RESULTS WITH FIELD
OBSERVATIONS
Average positions for snails predicted by the torqueconstrained random walk model were significantly
correlated with field observations (Fig. 6).
Discussion
The torque-constrained random walk model was successful in approximating patterns of upstream movement observed among populations of Elimia in the
field (Fig. 6). The model also showed how differences
in channel form may underlie much of the variability
observed among snail populations. For example, the
lack of movement of snails to reaches below the
release point in Hendrick Mill Branch, as both predicted by the model and observed in the field, is apparently the result of a high-velocity chute ≈ 1–2 m below
the release point. Although minor in spatial extent,
model results indicated that this channel features
greatly influenced movements of snails because it
imposed super-threshold torques on all length classes.
Consequently, random walks by all snails were forced
to progress upstream from the release point, which
resulted in substantial net movements compared with
other sites. Other effects of channel form among
streams were indicated by both field observations and
model results for different populations of E. fascinans.
For example, lack of upstream movement and the symmetrical distribution of individuals about the release
point for E. fascinans in Terrapin Creek is attributable
to lack of water velocities required to exceed torque
thresholds (Figs 3 and 5). In contrast to Terrapin
Creek, moderate levels of upstream movement by
© 1997 British
Ecological Society,
Functional Ecology,
11, 472–483
Fig. 6. Correlation of observed positions of snails following 3
months of free-ranging movements with model predictions.
The diagonal line indicates the trajectory of points expected
if observed positions are equal to predicted positions.
larger length classes of E. fascinans in Choccolocco
Creek is attributed to patches of water velocity that
exceed torque thresholds (Figs 3 and 5).
The torque-constrained random walk model predicted that snails will accumulate in reaches with currents below threshold velocities, but pass rapidly
through reaches with currents above threshold velocities. This is reflected in the complex longitudinal distributions of individuals observed for length classes
that progressed rapidly upstream (Figs 2 and 3).
Although not tested in the present study, Gore (1983)
showed that in the Buffalo River (Arkansas) large
individuals of Elimia potosiensis were significantly
more abundant in regions of low current velocity
compared with small individuals. Since the present
study indicates that large snails may at times be subject to larger effective torques than smaller snails, the
distribution reported by Gore (1983) is consistent with
our model.
Although comparison of model results generally
approximated field observations (Figs 2, 3 and 6),
size-dependence predicted by the model was usually
not as marked as that observed in the field. In particular, the extremely clumped distribution of small snails
(e.g. < 10 mm; Figs 2 and 3) was clearly not predicted
by the model. The striking contrast between movement patterns of large vs small snails may be
attributed to a number of factors. First, small snails
may simply avoid flow conditions that generate
threshold torques as suggested by Fig. 4. Second,
small snails may experience hydraulic forces different
from those experienced by large snails because they
experience lower Re values or because they are more
strongly influenced by boundary layer phenomena
(Vogel 1994). Third, small snails may not move at the
same rate as large snails. For example, Krieger &
Burbank (1976) showed that at 20 °C, specimens of
E. catenaria < 10 mm in length moved 4–12 times
slower than specimens > 10 mm in length. If snails
with lengths < 10 mm are allowed to travel 10 times
slower than snails > 10 mm, model results that closely
mimic observed patterns are obtained (cf. Figs 2, 3
and 7). Finally, since foraging-related movements by
Elimia are apparently size dependent, different levels
of food limitation among sites may be involved.
Hill, Weber & Stewart (1992) showed that under
conditions of low food availability, small Elimia
(< 2·2-mg ash-free dry mass (AFDM)) tended to be
less food limited than large Elimia (> 2·2-mg AFDM).
Food-limited snails and other grazing invertebrates
have been shown to move at higher rates and more
randomly than when food-satiated (Calow 1974; Hart
& Resh 1980). Growth rates of snails among the
Alabama study sites were strongly density dependent,
apparently owing to variation in food availability
(Huryn et al. 1995). The lowest growth rates were
observed at Alligator Creek and Hendrick Mill Branch
(‘high snail-biomass’, e.g. 3–5-g m AFDM m–2;
Huryn et al. 1995). The length threshold that distin-
Fig. 7. Plots of the frequency distributions of positions attained by E. cahawbensis of different lengths (5–20 mm) in three streams as predicted by a
torque-constrained random walk model (to be compared with Figs 2 and 3). Each plot was derived from 1000 iterations of the model. Individuals 5 and
10 mm in length were moved at a rate 10 times slower than larger individuals (e.g. 72 1-m steps vs 720 1-m steps over a 3-month period).
guishes the different movement patterns for large and
small snails at these sites compares favourably with
the biomass threshold of 2·2-mg AFDM/snail used by
Hill et al. (1992) (Fig. 2), which suggests that food
limitation may well be a factor underlying differences
in movement patterns. As would be predicted, the distinction between movement patterns of large and
small snails is less obvious at the remaining ‘low
snail-biomass’ sites (e.g. < 2-g m AFDM m–2, Huryn
et al. 1995) (Figs 2 and 3).
Size, food limitation and hydrodynamics may all
interact to influence movement patterns of snails. For
example, DeNicola & McIntire (1991) showed that
under conditions of high periphyton availability, the
pleurocerid snail Juga foraged mainly on substrata
that were sheltered from high flow velocities.
However, when periphyton biomass was reduced to
low levels, snails moved onto exposed substrata
where they would more likely be subject to hydrodynamic drag and consequently, torque. The results of
DeNicola & McIntire (1991), Hill et al. (1992) and the
torque-constrained random walk model suggest that
snails of different sizes should show different movement patterns under different levels of food limitation.
Although factors contributing to upstream movements by freshwater snails are undoubtedly complex
and multivariate, we suggest that the torque-constrained random walk model provides a testable null
hypothesis that may be used to examine other explanations for this general phenomenon.
Acknowledgements
© 1997 British
Ecological Society,
Functional Ecology,
11, 472–483
We thank Mr Charles Williams, Mr M. C. Allgood, Jr
and the Cahaba Hunting Club for allowing access to
Alligator Creek, Hendrick Mill Branch and Rocky
Branch, respectively. J. E. Freilich provided advice on
the preparation of micro-labels and F. G. Thompson
provided names for the snails. Field and laboratory
assistance was provided by J. W. Koebel, A. C.
Benke, J. W. Converse, B. Crossen, V. M. Butz
Huryn, K. Miller, K. Petris and K. Suberkropp. An
early draft of this paper was greatly improved by
advice from S. D. Gaines, B. Statzner and an anonymous reviewer. This research was supported by grants
from the National Science Foundation (BSR
88–18810 to A. K. Ward, G. M. Ward, A. C. Benke,
R. J. Donahoe and J. M. Harlin), and a grant in aid
from the University of Otago (New Zealand) to
A.D.H. This paper is contribution 240 to the Aquatic
Biology Program, University of Alabama.
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Received 22 January 1996; revised 27 November 1996;
accepted 4 December 1996