Snail Distribution and Diversity South of
Kigoma Bay, Lake Tanganyika, East
Africa
Nathan Zorich
Northern Arizona University, Flagstaff,
AZ, USA
Mentor: Ellinor Michel
Introduction
At 12 million years old and 1470m deep Lake
Tanganyika is one of the most unique places on
earth. It is the birthplace of many species flocks
including those found in cichlid fish and algaegrazing gastropods. The highly sculptured endemic snails of Lake Tanganyika have been a
source of amazement and questions since they
were first brought to the attention of the European scientific community. Due to their striking
similarity to marine gastropods they were a key
element in the formation of “The Tanganyikan
Problem” (Moore 1903). This hypothesis attempted to explain the many apparently marine
elements of the L. Tanganyika fauna (large crabs,
freshwater sardines, and jellyfish) by claiming it
was, at one time, connected to the Indian Ocean.
While this hypothesis finds little support today,
the unique fauna of L. Tanganyika continues to
draw the attention of evolutionary biologists. Due
to its remote location, the logistical difficulties of
performing research here has slowed our understanding of gastropod phylogeny and ecological
relationships. The goal of my Nyanza Project
was to develop a distribution and diversity map
that can serve as a base of knowledge to help
forward our ecological and evolutionary understanding of L. Tanganyika’s gastropod community.
Methods
I sampled the eastern shore of L. Tanganyika,
from Kigoma Bay south to Kitwe Point every
kilometer at seven sites (Figure 1). I also
sampled two sites at Gombe Stream National
Park which had been sampled by last year’s
team as well, however these sites are not included
on the map. Sites were reached by boat as most
were unreachable by road. Although this project
was my own, I sampled cooperatively with K.
Hinkely and B. At each site we used snorkel
gear or SCUBA to collect all macroscopic snails
from a 2m x 2m quadrat delineated by a rope
with sandbag corners. We collected gastropods
only from the tops or sides of rock. Cobbles
were not overturned, following last years protocol (France and McIntyre 1998) allowing for a
temporal comparison. This technique, while time
saving, excluded many juvenile gastropods and
as many as six individual Reymondia horei per
cobble. We did three replicates at each of 1m
and 2m water depth at each site. We chose
representative substrate types for the shoreline
at each site and did three replicates, with consistent substrates, at approximately 20m distance
from each other. All collected gastropods were
counted, determined to species and then returned. Each site was categorized as boulders,
cobbles, sand or mixed, and sedimented or
unsedimented. If substrate was mixed types, we
estimated the relative percentages of each type.
Snail densities were calculated for each site/
depth, as was Simpson’s D and Fisher’s Alpha
and the Jaccard Index of faunal similarity.
Results
During this survey I found eight species of snails
falling into three genera: Lavigeria grandis, L.
“nassa”, L. coronata, L. paucicostata, L. sp.”F”,
Lavigeria ”spiny tanzanian”, Reymondia horei,
and Spekia zonata. Gastropod species richness
changed from site to site. Sites 2, 4, and 5 had
the maximum species number with 5 and site 6
had the minimum of two species (Fig. 1). The
2m depth at site 3 was the most even with 41%
R. horei, 38% L. grandis, and 20% L. coronata.
At the other sites species abundance was
skewed. L. nassa had the widest distribution,
Site 1
Site 2
Site 3
Site 4
Site 5
Site 6
Site 7
Site 1
Site 2
Jaccard Index
Site 3
Site 4
Site 5
Site 6
0.33
0.20
0.33
0.33
0.67
0.40
0.33
1.00
0.67
0.40
0.80
0.29
0.13
0.20
0.14
0.40
0.80
0.50
0.67
0.40
0.80
being found at all sites except 3 (Jacobsen’s
Beach) while L. paucicostata and L. coronata
were only found at one place each (sites 1 and 3
respectively). Gastropod density and diversity
also changed between and within sites with site
5 (2m) showing highest density (125/m2) and site
2 (1m) the lowest (6/m2) (Fig.2). L. grandis
was always found at bouldered sites and was
frequently the dominant species. Here it was
joined by S. zonata and R. horei. This situation
shifted as the coast turned to sedimented cobbles
where L. nassa took the dominant role supported
by L. sp. “F” and L. “spiny tanzanian” both uncommon in the low sedimented, bouldered habitat type. Species richness between sites was
measured using the Jaccard Index and varied
from most similar J = 1 (sites 2 & 4) to J = .13
(sites 3 & 5). Site 3 (Jacobsen’s Beach) was
the least similar to all sites using the same index
(Table 1).
Conclusion
Due to the time crunch involved with this type of
program no statistical analysis have been performed. This is my obvious next step as it will
allow for comparison between and within sites
and hypothesis testing and further interpretation
of results. R. horei was found at all sites and
was frequently observed under cobbles, though
these individuals were not included in the sample,
sticking with last year’s protocol. This led to an
under reporting of the real density of this species
and for actual snail density overall. An excellent
project for next year would be to do a more
complete sampling, counting all snails with a higher
Site 7
number of replicates, which could be extrapolated to gastropod standing biomass of Lake Tanganyika. Gastropods concentrate not only energy and protein, but also calcium carbonate. If
they are removed from the ecosystem there might
be potential for large-scale water chemistry
change. Therefore I also call for an extension of
this distribution and diversity map that could be
used to learn more about the ecology and evolution of gastropods, as well as monitor their
health. One element I would have liked to add
was a field measure of sediment and wave energy that would allow for between-site comparisons. These parameters are difficult to quantify,
and surely change temporally (from wet to dry
season). I will also compare the re-sampled site
with last year’s results lending an important temporal aspect to this report that is so often missing in science today.
Finally, I wish to stress that from a gastropod
and substrate view Jacobsen’s Beach is the least
similar to all sites. This is reflected in its consistently low Jaccard Indices and low species richness (3) and in being the only site were L.
coronata was found. It is also reflected in the
geology of Jacobsen’s Beach. Of the seven kilometer of coast surveyed it was the only area
with a pure sand substrate – not mixed with any
cobbles. Because so many Nyanza Projects
take place at Jacobsen’s Beach it is important to
recognize the uniqueness of that site and care
must be taken not to over-extrapolate data collected there. This site is commonly used because it is easily accessible by car or boat but
we must not let access affect our conclusions.
Acknowledgements
My warmest thanks to Dr. Andy Cohen for organizing the Nyanza Project, Dr. Ellinor Michel
for mentoring and support, and Karen Hinkley
for all her help and humor.
Figure 2. Gastropod Den
Diversity
Indiv. /m sq.
50
40
30
20
10
0
1
2
3
4
Site
Density 1m
S's D 1m
5
6
De
S'