Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 1
velopment the initial stages of the embryonic shell are formed (Demian and
Yousif, 1973a). One hypothetical approach to explain the Pt-induced internalization
of the shell relies on a possible interaction of Pt with the Ca
metabolism and its uptake via Ca trans-membrane transport. Because Ca
supplements did not diminish Pt uptake, we conclude that Pt likely interacts
with Ca signalling pathways involved in the positional system, which may be
stabilized by increasing intracellular levels of Ca. Some heavy metals have
been shown to act as inhibitors of the ezyme CA (Christensen and Tucker,
1976; Morgan et al., 1997; Vitale et al., 1999) or to reduce shell mass in
snails (Beeby et al., 2002). We therefore tested the CA activity in Pt exposed
M. cornuarietis. The results indicated a diminuation but no significant
difference of the activity in Pt exposed snails. Hence, the mechanisms of the
internal shell production seem to be almost as effective as those involved in
the growth of the external shell, and the reduction in shell size could probably
only be attributed to a compressed shape due to the limited space inside
the gastropod’s body.
The effect of body plan changes due to Pt exposure could also be observed
in embryos of the pulmonate snail P. corneus. Also shell-less P. corneus
did not form a mantle cavity which might indicate that Pt action on the
re-direction of presumptive mantle tissue is likely universal in gastropods.
In contrast to prosobranch gastropods, however, pulmonates lack a gill but
use their mantle cavity to form a lung. A re-direction of mantle tissue here
consequently leads to a lack of any respiratory organ and, therefore, to a
short life span in P. corneus embryos.
Descriptions of various authors about the time span of torsion (from about
2 min until about 200 h) and its main cause(s) (muscular activity, differential
growth, hydraulic activity) in different groups of gastropods vary to a high
degree (for detailed informations see Wanninger et al., 2000). Therefore,
it can be assumed that ontogenetic torsion of gastropods has been highly
modified within different groups subsequent to its rise in early phylogeny.
Because no external shell is formed after exposure to Pt, torsion occurs
independently from retractor muscle action on the larval shell in M. cornuar-
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