Lake Baikal is the oldest and deepest freshwater lake
of the world. The water is nearly saturated with respect to oxygen, but
contains only small amounts of electrolytes (osmolality 2 mosmol / kg).
The temperature in the lake is very low and exceeds 10 °C at the surface
only from July to September (Kozhova OM and Ismest´eva LR 1998 Backhuys
Publishers, Leiden). The fauna of Lake Baikal consists of a wide variety
of species, most of them are endemic. The extreme thinness of the shells
of the endemic gastropods has been reported as a typical feature (Kozhova
OM and Ismest´eva LR 1998 Backhuys Publishers, Leiden). This hypothesis
derived only from observations; no experimental data were available up
to now. We therefore investigated the calcium distribution in the endemic
prosobranch gastropod Benedictia baikalensis and compared the results
with the ones of Lithoglyphus naticoides a near relative of
the baikalian species.
Shell calcium: The shell of the endemic mollusc B.
baikalensis consists of 94.9 ± 26.0 µg calcium per
µl animal volume (n = 43), whereas we could measure 865.0 ±
271.5 µg calcium (n = 10) in the shell of L. naticoides. Therefore
the calcium quantity used to shelter the animal volume in the nonendemic
species is nearly tenfold compared with B. baikalensis.
Concentration of calcium and other ionic compounds in
the hemolymph: The values for hemolymph osmolality of B. baikalensis
and L. naticoides are nearly identical (84.4 ± 5.3 mosmol
/ kg, n = 40 for B. baikalensis) and does not exhibit significant
differences. Even the ionic composition of the hemolymph in both species
does not differ.
Calciumconcentration in the tissue excluding the shell:
Calcium concentrations in the tissue of B. baikalensis are 4 to
8 fold lower than in L. naticoides (lowest value 9.4 ± 5.1
µg / mg dry weight for B. baikalensis).
It can be concluded, that the closely related species
have the same ionic composition of the extracellular space but are differently
adapted to their environment by means of calcium composition in tissue
and shell. Further comparative studies with different genera have to be
performed to show, whether these adaptations are a general feature of the
baikalian molluscs.
The study was supported by the FNK of the Free University of Berlin (44/08/98).
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