Life history strategies in the gastropod genus Lavigeria – Differences in brood and
embryo sizes
Student: Aptone Phiri
Mentor: Dr. Ellinor Michel
Introduction
“An organism’s life history is its lifetime pattern of growth, differentiation, storage and, especially,
reproduction.” (Begon and Harper, 1996) It reflects the genotype, the environment and the interaction
between the two. Life history theory views the number and size of the offspring produced in a single
reproductive event as having been adjusted by evolution to optimize individual fitness, including evolved
reactions to environmental conditions. The aim of this study was to see if there was any life history
variation in the gastropod genus Lavigeria. This endemic gastropod includes a diverse radiation, wholly
endemic to Lake Tanganyika. Closely related species live in similar habitats and often sympatrically, at the
same sites. I found clear differences in brood and embryo sizes
Methods
Specimens were collected from three sites around Kigoma (Hilltop, Kalalangabo, Jakobsen’s Beach, see
reports from Menone and Mulongaibalu for site descriptions), measured for shell variables (see Phifer, this
report for specifics), and dissected to sex the specimen, check for parasites and remove the full contents of
the mature female’s brood pouches. Specimens were used also for predation scarring studies (Phifer) and
chromosome studies (Hora). Shell and tissue vouchers were saved in 95% EtOH alcohol for potential DNA
sequence work. Brood contents were separated according to stage (Kingma & Michel 1999) under a
binocular (dissecting) microscope and the maximum diameter of individuals at each stage was measured
with a plastic millimeter grid sheet directly under the specimens. Species names refer to open
nomenclature of Michel & Todd (revision in prep) and are detailed in West et. al. (in prep.).
Results
Figures 1 and 2 show that there are variations in the life history strategies of the genus Lavigeria. There are
significant differences (estimated by eye). L. coronata had the most embryos in brood, followed by L.
nassa. L. sp. X has the fewest number embryos, and they were the smallest in size at final release (stage 5)
as well. Not all stages had the same number of embryos even within the same species. However variation
was greater between species than within them (Figs. 1 & 2). In some cases though, the number remained
almost constant between stages I and V.
Discussion
Since the gastropod species studied here live sympatrically, they experience similar, large-scale ecological
pressures, thus the variation we found must have a basis in finer scale interactions, or else are non-adaptive.
I suggest two possibilities: 1) direct competition between Lavigeria species may select alternatively for
producing many small young or few large young; or 2) predation may produce different benefits for
different reproductive strategies. For example, L. nassa and L. coronata have relatively large broods, but
the maximum size of their offspring is small. The other species generally showed a smaller brood but had
larger maximum sizes. (except L. sp. X which has small broods and small embryos). This variation comes
about due to differences in the energy investment, and must ultimately be controlled by natural selection, as
it appears to be a species level trait. Variation in the number of embryos at different stages indicates that
there may be temporal changes in reproduction for the mother. Life history components include size,
growth and development, clutch size, clutch number, offspring size, parental care and reproductive
allocation, thus there is a complex of interactions at work determining the patterns we saw.
Table 1: Collections information for brood study
Max adult
Sample
length
Species
Site
Size
(mm)
L. sp. X
HTP
51
11
L. sp. L
KLG
51
16
L. sp. W
KLG
48
24
L. coronata
JKB
26
36
L. nassa
JKB
10
24
L. grandis
JKB
24
34
Figure 1 - Lavigeria Total Brood Sizes with std. errors
Embryos in brood (all stages)
120
100
80
60
40
20
0
coronata
grandis
sp. W
nassa
sp. L
sp. X
Species
Figure 2 - Brooding differences among Lavigeria species by stage
100
90
80
sp. X
sp. L
nassa
sp. W
grandis
coronata
70
Number of
Individuals
60
50
40
30
20
coronata
grandis
sp. W
nassa Species
10
0
I
sp. L
II
III
IV
Developmental Stage
sp. X
V
Total
(size arrayed)
Conclusion
Lavigeria species show interspecific variation in their life histories, specifically in the number and size of
their broods. Further work should address whether large babies, such as those produced by L. grandis,
survive predation better than smaller babies, such as those produced by L. coronata or L. nassa.
Furthermore, growth rates and survival studies may be able to determine under what conditions competitive
advantages go to few large or many small offspring in different habitats in Lake Tanganyika.
Acknowledgements
I would like to thank Dr. Ellinor Michel for guiding me in this project. Also I would like to thank Pete
McIntyre, Megan Phifer, Chris Menone, Katja Hora, and Ellinor Michel for assisting with collecting and
processing snails.
References
Begon, M. et al, (1996). Ecology. Blackwell Science, Ltd.
Michel, E. & Todd, J. (in prep) A revision of the gastropods in the genus Lavigeria from Lake Tanganyika (Thiaridae: Cerithioidaea).
Kingma, I. & Michel, E. (2000). Variation in voortplantingsstategien in het ovovivipare genus Lavigeria (Gastropoda:Thiaridae) uit
het Tanga nyikameer, Oost Africa. Corresp.-blad Ned. Malac. Ver. 312:15-24. And: Life History variation in the gastropod genus
Lavigeria (Thiaridae) of Lake Tanganyika (manuscript).
Stearns, S. C. (1997), The Evolution of Life Histories. Oxford University Press 259pp.
West, K., Michel, E., Todd, J. Brown, D., Kiza, J.P, Clabough, J. (in prep) A key to the gastropods of Lake Tanganyika, East Africa.