Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 2
Discussion
As shown by scanning electron microscopy analysis, Pt 2+ -treated and untreated
M. cornuarietis embryos developed similarly until the onset of a
torsion-like rotation of the visceral sac between Demian and Yousif ’s Stages
VI and VII (70–82 h postfertilization at 26 ◦ C) although, generally, the Pt 2+ -
treated embryos developed slower. The severity of the slowing-down effect
varied somewhat between the embryos, which accounts for observed variability
in embryonic development between different Pt 2+ -treated embryos of
the same age. This retardation of embryonic development in gastropods as
a consequence of platinum 2+ exposure has already been described by Osterauer
et al. (2009) and has also been shown for other heavy metals (e.g.
Ravera, 1991; Gomot, 1998; Coeurdassier et al., 2003; Sawasdee and Köhler,
2009). The slowing-down of embryonic development is not a unique feature
of platinum 2+ on gastropods, whereas its effect on morphogenesis definitely
is. Osterauer et al. (2010b) reported maximum susceptibility of the embryos
to platinum 2+ at days 4 and 5 of their experiment. Considering the fact that
the first day of the experiment in that study, and also in the present one,
equals the day of fertilization, exposure on days 4 and 5 means the embryos
were 3 and 4 days old, respectively (postfertilization). The most susceptible
phase identified by Osterauer et al. (2010b) therefore corresponds well to
the phase in embryonic development in which the rotation of the visceral sac
should commence, and this is exactly the phase in which the first differences
between the control and the platinum 2+ group occurred in this study.
Based on these facts, it can be concluded that platinum 2+ disrupts normal
development just before the visceral sac normally starts rotating anticlockwise.
Demian and Yousif (1973a) stated that in M. cornuarietis this rotation
was caused by differential growth of the two sides of the visceral sac. During
normal development, the mantle anlage (the tissue covered by the shell)
overgrows the visceral sac and becomes the outer epithelium of the mantle,
whereas the tissue that covers the right side of the visceral sac is enfolded
into the mantle cavity.
In embryos exposed to bivalent platinum ions, the rudimentary shell gland
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