Snail diversity and abundance at sediment disturbed and undisturbed sites around
Kigoma, Tanzania
Student: Christopher Menone
Mentor: Dr. Ellinor Michel
Introduction
Previous studies of biological communities in Lake Tanganyika have revealed that species richness of some
groups of aquatic organisms is lower in areas that are highly impacted by sediment pollution than in
unimpacted areas (Cohen et al. 1993 [ostracodes and fish], Alin et al. 1999 [molluscs and ostracodes]).
France and McIntyre (1998) found that gastropod (snail) species richness is, on average, lower at disturbed
sites. My goal was to continue the collection of gastropod community data at disturbed and undisturbed
sites and to compare the diversity of these communities across sites.
I surveyed the snail communities at three sites on the eastern shore of Lake Tanganyika around Kigoma:
Hilltop Hotel, Jakobsen’s Beach, and Kalalangabo, the first village north of Kigoma Bay. Hilltop and
Kalalangabo are impacted by sediment pollution caused by watershed and shoreline deforestation, while
Jakobsen’s is considered undisturbed. Within the sampling depths of this project, the substrate at
Jakobsen’s is mostly granitic bedrock and large boulders, while stromatolites are the dominant substrate at
the other two sites.
Methods
At each of the three sampling sites, I sampled five transects at four depths (one, two, five, and ten meters).
I attempted to make the transects perpendicular to the shoreline, though this was not always possible in
areas where suitable rocky substrate was scarce. The quadrats were delineated by rope squares whose
corners were weighted with sandbags. Quadrats were situated so that surface area was relatively constant;
when possible, boulders or bedrock were chosen over cobbles. When cobbles were present, I did not turn
them over to sample underneath, in keeping with the protocol of France and McIntyre (1998). At
Jakobsen’s and Kalalangabo, I sampled quadrats of four square meters; at Hilltop, because of the scarcity
of rocky substrate at depths of five and ten meters, the quadrats were one square meter. When analyzing
these data, I extrapolated the Hilltop numbers to four square meters by multiplying by four.
I collected all the snails within each quadrat. It is important to note that many small species of snails (e.g.
Anceya spp. and Martelia spp.) were probably not faithfully surveyed, and thus, although I have included
them in my data sheets, they were not considered in the analysis of the community data. Also, because of
the nature of the substrate in some of the quadrats, it is not unlikely that even some individuals of the larger
species were left behind. It is particularly horrifying to consider the number of algae-coated Lavigeria
species J that undoubtedly escaped collection by hiding in the myriad minute craters of the Hilltop
stromatolites. I later identified each snail to species, counting the number of individuals of each species at
each depth.
I calculated Simpson’s index (1/D), a measure of evenness, and Fisher’s alpha, which is predominantly a
measure of species richness, for each depth at each site.
Results and Discussion
I recorded fourteen species of snails: three species (Lavigeria nassa, Reymondia horei, and Nov. gen. nov.
sp.1 ) were found at all three sites, five were found at two sites (Lavigeria grandis and Spekia zonata at
Jakobsen’s and Kalalangabo, L. sp. J at Jakobsen’s and Hilltop, and L. sp. L and Anceya giraudi at Hilltop
1
Names of Lavigeria spp. and Nov. gen. spp. under revision by Michel and Todd (in prep.), referring to
names in West et al. (in prep.).
Firgure 1: Snail Densities
1600
Individuals/4 sq.m.
1400
1200
1000
1m
2m
800
5m
10m
600
400
200
0
Jakobsen's
Hilltop
Kalalangabo
Figure 2: Species evenness and richness
8
Simpson's index (1/D) [bars]
# Species [lines]
7
6
1m
2m
5m
10m
5
4
3
2
1
0
Jakobsen's
Hilltop
Kalalangabo
Figure 3: Species richness
1.4
Fisher's alpha
1.2
1
1m
2m
0.8
5m
0.6
10m
0.4
0.2
0
Jakobsen's
Hilltop
Kalalangabo
and Kalalangabo), and six were found at only one site (Lavigeria coronata and Paramelania minor at
Jakobsen’s; Martelia tanganyicensis, Lavigeria sp. X, and Bridouxia ponsonbyi at Hilltop; and L.sp. W at
Kalalangabo).
Snail density is greatest at Hilltop, with the greatest density occurring at a depth of two meters (Figure 1).
At both disturbed sites, density drops sharply at ten meters. At Jakobsen’s, the undisturbed site, density is
relatively constant across all depths. The decline of snail density at deeper areas of disturbed sites may be a
result of uneven sediment distribution across depth caused by wave action near the lake surface.
Species richness as measured by total number of species is greatest at the two shallow depths at
Kalalangabo, where species richness decreases with depth (Figure 2). At both Jakobsen’s and Hilltop,
species richness, as measured by both total species number and Fisher’s alpha, is lower at the shallower
depths (Figures 2 and 3). In general, evenness decreases with depth at Jakobsen’s and Kalalangabo, while
at Hilltop it is relatively constantly and low at all depths (Figure 2). This is due to the very strong
numerical dominance of L. sp. J at this site.
The various ecological factors that must be taken into account in a cogent interpretation of any study of
gastropod diversity and community structure in this lake are indeed rather daunting. Care should be taken
not to allow instances of point endemism (e.g. L. coronata, which is restricted to a small stretch of coastline
to the south of Jakobsen’s Beach) to confound interpretations of the effects of local environmental
conditions. The possible metapopulation structures of the members of gastropod communities, with the
resulting high degree of spatial and temporal variability among snail populations, can only be resolved
through long-term monitoring over large areas. The founding and extinction of local populations can be
truly understood in the context of sediment pollution only when we have an idea of natural turnover rates.
A study of microhabitat preference among members of the gastropod communities of Lake Tanganyika,
such as the one envisioned but not realized for Nyanza 2000, would doubtless provide valuable baseline
data for further studies of gastropod diversity and community structure. Phenotypic variation within
species between sites should also be looked into.
Acknowledgements
I would like to thank Dr. Ellinor Michel for being a fabulous mentor. She is both incredibly knowledgeable
about the snails and wonderfully amusing to work with. I would also like to thank Mr. Pete McIntyre, Miss
Katja Hora, Miss Megan Phifer, Dr. Andy Cohen, and Mr. Robert Wakafumbe for playing the role of dive
buddies and collaborators. I would be remiss to neglect acknowledging Issa for his boating ability and
general coolness. I’d also like to make a shout out to Heather, who is very nice. Robia too. Of course,
where would I be without the snails themselves? I’d like to thank the gastropod community of Lake
Tanganyika, especially those individuals who will never again mow the sweet algae of their mother lake.
References
Alin, S. R., et al. 1999. Effects of landscape disturbance on animal communities in Lake Tanganyika, East Africa. Conservation
Biology 13: 1017-1033.
Cohen, A. S., et al. 1993. The impact of sediment pollution on biodiversity in Lake Tanganyika.
Conservation Biology 7: 667-677.
France , K., and P. McIntyre. 1998. Gastropod community ecology on the rocky eastern shore of Lake
Tanganyika. The Nyanza Project 1998 Annual Report: 62-70.