Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 1
treatment 29.9 ± 32.78% of surviving M. cornuarietis juveniles continuously
exposed to 74.2 µg/L Pt (85.1 ± 9.41% survival) did not form an external
shell, whereas at exposure to 163.4 µg/L Pt (84.4 ± 6.23% survival), all
surviving animals were ‘shell-less’ (100 ± 0%). To detect the most sensitive
stage in embryogenesis in which Pt interferes with the formation of the shell,
we conducted two separate runs of a pulse-exposure experiment. These experiments
included pulses of 200 µg/L PtCl 2 with a duration of either 1 day
or 2 days. Data varied between the two experimental runs but indicated the
embryonic stages at days 4 and 5 postfertilization to be most susceptible to
Pt action on shell formation (Fig. 1).
We repeatedly raised large numbers of M. cornuarietis embryos without
external shell by pulse exposure to nominal concentrations of 200 µg/L PtCl 2
at days 4 and 5 postfertilization. Subsequently, the embryos in their egg capsules
were transferred to uncontaminated water and cultivated at 26 ◦ C. We
never obtained reversal of the Pt pulse-induced body plan changes under
these conditions and, thus, none of these individuals ever developed an external
shell. Without exception, all juveniles shared the following alterations
relative to the nontreated snails (Fig. 2a): no mantle cavity was formed at all
and hence the gill, which remained posterior to the heart at the hind part of
the visceral sac, protruded from the visceral sac into the surrounding water
(Fig. 2b). Nevertheless, as indicated by the deflected intestine, the position
of the anus at the lower right side (Fig. 2b), and by a slight movement of
the gill from hind left to hind right, part of developmental processes associated
with torsion was accomplished in all individuals. The hepatopancreas,
as usual, surrounds the mid part of the intestine. Dorsal of the hepatopancreas,
the hindgut bends to the front. In addition, the visceral sac surface of
these snails bloats and blisters to varying degree, forming hemocoel cavities
of different sizes (Fig. 2, b, d, and f). An operculum was present (Fig. 2c)
and the morphology of the head and ventral part of the body, including the
operculum, did not differ from the archetypical body plan (Fig. 2, a, b, and
e).
Up to now, M. cornuarietis individuals with induced body plan changes
due to Pt exposure reached a maximum age of 7 months and a maximum
41