A Snail's Pace in a Snail’s Space: Adult movement rates of Lavigeria gastropods in
Lake Tanganyika, East Africa
Student: Jessica Chan
Mentor: Dr. Ellinor Michel
"From a human perspective, molluscs are half plant. They move, but not much."
-- Robert T. Dillon, 2000
Introduction
Little quantitative data about the movement rates of freshwater gastropods is available. Knowledge of
these rates can lead to a better understanding of gastropods and their interactions with the environment.
Rate of movement at an individual level has several implications for survival and reproduction. In patchy
freshwater systems, movement rates are a limiting factor in the gastropod's ongoing search for suitable
habitats. These habitats must have an ample food supply, low competition (from conspecifics and other
species), sufficient protection from predators, and easy access to mates. In the adult stages of life, active
movement is a precursor to dispersal. A firm grasp on the potential for movement at a species level will
dictate how large a role dispersal plays in population structure and distribution.
Lake Tanganyika is home to the highly diverse gastropod genus Lavigeria. These species differ not only in
form (i.e. size, shape, sculpture), but also in function. They are known to have differences in trophic
ecology (Solomon 2001), life history (brood and embryo size; Kingma and Michel 2000), substrate
preference (Anderson, this volume), trematode parasite infection (Faloon, this volume) and predation
susceptibility (Socci 2001, Phifer 2000). Differences in life span and growth rates are also expected.
Differences in movement rate may be associated with these factors. The objective of this study is to
quantify the movement rates of three large-bodied species of snails in the genus Lavigeria: L. coronota, L.
grandis, and L. nassa. Furthermore I tested for differences in movement rates based on size, sex, and
presence of parasites.
Methods
Movement trials were conducted at Jakobsen's Beach (04ƒ54.64' S 29ƒ35.92' E) on the eastern shore of
Lake Tanganyika, five kilometers south of Kigoma, Tanzania. Snails were collected at a depth of 3m along
215m of the rocky southern shoreline at Jakobsen's Beach, which consists of quartzite boulders and
cobbles. Individuals from the three species were collected and marked with a species-specific nail polish
color in order to identify and track them. The net movement displaced by each gastropod was measured
every 24 hours for a total of five days. Movement trials were run in three temporal blocks from 12 July - 5
August. The movement trials were started at a depth of three meters and snails were placed on upward
facing boulders or the ledges of vertical cliff walls. Painted and numbered pebbles were used to space each
individual and to mark their position at time of measurement. A wire brush was also used to mark the exact
position of each individual. Net movement was measured as a straight line, placing a gradated string
between the current location and the position from the previous day. The first day's movement was not
included in the analysis to allow the snails to adjust to their surroundings following removal and painting.
At the conclusion of each trial, snails were collected, measured, and dissected to determine sex,
reproductive state, and presence of parasites.
To examine the differences in movement rates, measurements (cm/day) were log transformed and
compared on the basis of species, size, sex, and presence of parasites. Height was used as a proxy for size.
Day to day differences in movement rates were tested using repeated measures ANOVA in SYSTAT
version 7.0.
Results
The gastropods demonstrated a wide range of movement rates at a species and individual level. Movement
rates ranged between 2 cm/day and 267 cm/day. The three species of gastropods exhibited significant
differences in movement rates (F=6.331, df=2, p=0.002). L. coronata (n=69) was the quickest of the three
study species with an average movement rate of 63.6 cm/day. L. grandis (n=65) and L. nassa (n=52) took
second and third designations respectively. L. grandis had an average movement rate of 43.4 cm/day, L.
nassa moved 41.5 cm/day.
A positive relationship was found between size (as measured by height) and movement (F=5.997, df=1,
p=0.015). Larger gastropods tended to move faster than small gastropods. This finding agrees with the
species level analysis. L. coronata, the fastest Lavigeria gastropod, is also the largest with an average
height of 31.52 cm. L. grandis and L. nassa are slower than L. coronata with average heights of 25.82 cm
and 21.908 cm respectively. There were no significant differences in movement rates based on sex within a
species and between species. Presence of parasites did not affect movement rates.
Discussion
The positive relationship between size and movement is a parsimonious explanation for the difference in
mmovement rates among Lavigeria gastropods. L. coronata is both the largest gastropod and fastest
gastropod. The two smaller species follow suit and trail behind their larger cousin. But a closer
examination of movement by L. grandis and L. nassa reveals that the issue may not be so straightforward.
L. grandis and L. nassa have very similar average movement rates; L. grandis is 15% larger. L. coronata is
18% larger than L. grandis and is 50% faster. If the positive relationship between size and movement was
the only factor controlling movement rates, a greater disparity in movement rates would exist between L.
grandis and L. nassa.
L. grandis and L. nassa are widely distributed throughout the lake, whereas L. coronata can only be found
at two sites, Jakobsen’s Beach and Kalalangabo. Longer established populations have lower frequencies of
dispersive morphs (Krebs 2000). Perhaps L. grandis and L. nassa movement rates are low because their
need to disperse into new habitats has been satisfied; they have already managed to occupy all available
habitats in the lake. L. coronata, a relict endemic with fossils in the north end of the lake, currently has
populations existing in only two sites. It is advantageous for individuals to readily seek out a new place to
live even when the old one is still suitable.
Differential grazing efficiency may present another explanation for differences in movement rates between
Lavigeria gastropods. My results may suggest that L. grandis and L. nassa feed more efficiently than L.
coronata by selectively grazing nutritious algae and leaving behind nutrient-poor detritus. Preliminary
investigation of gastropod fecal content has been inconclusive (Rivers 2001). If grazing efficiencies are
similar, L. coronata may have higher nutrient requirements because of its size.
Food, mates, and suitable places to live are often scattered and gastropods must find their way between
them. Movement may be prompted by a gastropod’s need to seek out these things. Inter- and intra-specific
competition also puts pressure on the individual to find new microhabitats. Gastropods may also seek
cover during the night when their predators, crabs for example, may be more active. Dillon asserts that
snails crawl over surfaces, stuffing material into their mouths and there seem to be few cues which aid the
snails in finding anything nutritious (1997). In this case, snails may depend on their ability to move to meet
their metabolic requirements. If grazing rates can be linked to movement rates, the role of gastropods in the
trophic ecology of a system may be clarified. How much algal biomass is removed by gastropods on a
daily basis? How does it compare to other consumers? These kinds of questions can benefit from
information regarding the movement rates of gastropods.
Faster male movement resulting in sex-biased dispersal is common among many organisms. My study
found no sex-biased dispersal. However, female Lavigeria gastropods have the ability to store sperm,
thereby potentially lessening the importance of male mate selection. Mating events between populations are
more likely to occur now that the capacity for movement is known to add up to meters of movement per
week.
This study showed no difference in movement rates between infected and uninfected individuals. However,
the sample size for each group was unbalanced and further study may be able to approach the issue more
directly.
Comparison of movement potential among species contributes to population studies by adding a dynamic
dimension to an otherwise static measure of density and distribution. We now know that dispersal is not
limited to juveniles, but that adults have the capacity to disperse as well. The importance of dispersal at the
adult stage may reflect the ease of dispersal at a juvenile stage. L. nassa has the very small juveniles,
which may be dispersed by water currents. It may follow that dispersal by adults is not as important since
much of it is done at the juvenile stage.
Having a clearer picture of gastropod movement reveals the spatial and temporal scale at which gastropod
behavior is consequential. An organism that moves only millimeters per day may affect and/or interact with
its environment far less than an organism that moves several centimeters a day. This study shows that
Lavigeria gastropods have the capacity to move centimeters to meters per day. Lavigeria gastropods are
not dependent upon passive forces for their movement. This is important to individuals living in the patchy
environment of a freshwater system like Lake Tanganyika.
Future Studies
Now that a framework for gastropod movement trials has been set down, a number of other factors can be
examined using the same procedure. This project focused on three species of Lavigeria which prefer hard
substrate habitats. It would be interesting to test the movement ability of these species of snails on different
substrates (e.g. sand, mud) in order to determine how substrate type influences dispersal. The project can
be expanded to look into the effects of depth on movement rates by sampling at five, ten , and fifteen
meters, taking in account not only the distance moved but the change in depth as well. Direction of
movement is another interesting aspect that can be further explored. Gastropods may have a directionality
of movement dependent on water flow, tidal patterns, circadian rhythms, diurnal or nocturnal behavior, and
tropotaxis (the attraction of gastropods by chemical attractants). Movement by gastropods in different life
stages has implications for sexual recombination and population structure. It would be interesting to track
gastropods once they are released from the brood sac until they reach maturity. Even observing juvenile
movement would provide insights into the life history of gastropods. Movement data of sympatric species
paired with allozyme analysis may reveal gene flow patterns in the community of snails living in
Lake Tanganyika.
Acknowledgements
There are a several people whose guidance and support I would be lost without. I'd like to thank Dr.
Ellinor Michel for her attentive mentoring and for having enough confidence in me to let me develop my
own sampling method. I'd like to thank Pete McIntyre for his excellent seamanship and his daily taxi
service out to Jakobsen's beach. Pete's input on sample size and other statistical matters was invaluable to
the execution of this study. I'd like to thank Saskia for always being in high spirits and having such great
style. I'd like to thank the rest of the Bio team for their good-humor and their amazing work ethic. I'd like
to thank Trisha for being the best roommate a person could ask for and for putting up with my moods and
my excesses. I'd like to thank Tim Rawlings for his generous help in conducting background research. I'd
also like to thank the National Science Foundation for their support ATM9619458.
References
Anderson, L. 2002. Microhabitat and substrate preference of gastropods in Lake Tanganyika. Nyanza
Project 2002 Annual Report
Andrewartha, H.G. 1961. Introduction to the study of Animal Populations.The University of Chicago Press. Chicago, Illinois.
Begon, M., and Mortimer, M. 1981. Population Ecology. Sinauer Associates,Inc. Sunderland, Massachusetts.
Dillon, R.T. 2000. The Ecology of Freshwater Molluscs. Cambridge University Press. Cambridge, UK.
Faloon, K. E. 2002. Snail parasites: effect of sedimentation on trematode prevelance in Lavigeria. Nyanza Project 2002 Annual
Report.
Krebs, J.R., and Davies, N.B. 1997. Behavioural Ecology - An EvolutionaryApproach (4th ed.). Blackwell Sciences Ltd. London,
England.
May, R.M. (ed.). 1981. Theoretical Ecology (2nd ed.). Blackwell Sciences Ltd. London, England.
Michel, E. 1995. Evolutionary Diversification of Rift Lake Gastropods: Morphology, Anatomy, Genetics, and Biogeography of
Lavigeria (Mollusca: Thiaridae) in Lake Tanganyika. PhD, Department of Ecology & Evolutionary Biology, University of Arizona,
Tucson. 325-270.
Phifer, M. 2000. Scarring and sculptured shells: Crab impacts on morphology of Lavigeria in Lake Tanganyika. Nyanza Project 200
Annual Report.
Seddon, B., 1971. Introduction to Biogeography. Barnes and Noble Books. NewYork, New York.
Socci, A.M., 2001. Interspecific differences in snail susceptability to crab predation at Jakobsen's Beach, Lake Tanganyika. Nyanza
Project 2001 Annual Report.
Solomon, C. T., 2001. Trophic ecology of the benthic littoral of Lake Tanganyika: the role of grazers in determining algal biomass
and productivity. Nyanza Project 2001 Annual Report.
Todd, J.A., Michel, E., Cohen, A., McIntyre, P., Kingma, I., Cleary, D., & A. Grill. 2001. Point endemicity confounds diversity and
impact assessment for gastropod species flocks in Lake Tanganyika. Poster for the Natural History Museum.
Todd, J.A., Jackson, J.B.C. Johnson, K.G., Fortunato, H.M., Heitz, A., Alvarez, M., and P. Jung. 2002. The ecology of extinction:
molluscan feeding and faunal turnover in the Carribean Neogene. proc. R. Soc. Lond. B (2002) 269: 571-577.
Table 1.
Species
L. coronata
L. grandis
L. nassa
Movement Rate* Adult Size* (mm) Juvenile Size** (mm) Distribution***
63.6 cm/day
31.52 + 3.19
1
point (2sites)
43,4 cm/day
25.82 + 1.98
2.5
almost lakewide
41.5 cm/day
21.91 + 1.73
0.75
lakewide
Source data:
* this study
** Kingma & Michel 2000
*** Todd et al 2001, Michel 1995
Table 1: A Comparison of Lavigeria gastropod species
Average Movement Rate (cm/day)
80
70
60
50
40
30
20
10
0
L.coronata
n=69
L.grandis
n=65
L. nassa
n=52
Species
Average Movement Rate (cm/day)
Figure 1: Average adult movement rates (measured in cm/day) of Lavigeria gastropods in Lake
Tanganyika, Tanzania. Error bars and sample size shown.
L. coronata
250
L. grandis
200
L. nassa
150
100
50
0
15
18
21
24
27
30
33
36
39
42
Height (cm)
Figure 2: The relationship between average adult movement rates and height for three species of
Lavigeria gastropods in Lake Tanganyika, Tanzania.