Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 1
dae). Their ontogeny also includes the process of torsion, which is crucial for
the anatomical configuration of these snails. Torsion is defined as a process
in gastropod ontogenesis that rotates the visceral body 180 ◦ relative to the
larval headfoot region. As a consequence the digestive tract is U-shaped and
the anus is located anteriorly, the mantle cavity is located anteriorly over the
back of the head, and the gills are located anteriorly in front of the heart.
This process is the key character defining the gastropod class.
It is generally thought that torsion involves a counterclockwise simultaneous
movement of the outgrowing mantle, the shell, and the visceral sac, and
– following Ernst Haeckel’s theory of ontogeny recapitulating phylogeny –
that this developmental process recapitulates evolutionary events at the rise
of prosobranch gastropods.
In growing snails, the mineralization of the teleoconch takes place at the
peripheral edge of the mantle fold and over the entire inner surface of the
shell in order to increase shell thickness. Usually, invagination of the dorsal
epithelium first occurs to form the shell gland, followed by shell field evagination
and the migration of shell material-secreting cells toward the mantle
edge (Kniprath, 1981; Waller, 1981). The shell gland originates at the aboral
end of the embryo, later shifts to the left and forms the mantle (Demian
and Yousif, 1973a). The helical growth of the shell occurs in parallel to the
onset of the counterclockwise movement of the outgrowing mantle due to
the process of torsion. The mantle overgrows the visceral sac in the right
dorso-lateral direction and opens to the anterior, forming the mantle cavity
sheltering the organs such as the gill(s).
The mollusc shell consists of an organic matrix and calcium carbonate
(CaCO 3 ), which is formed from calcium and bicarbonate ions present in
the extrapallidal fluid (Rousseau et al., 2003). Catalyzing the reaction CO 2
+ H 2 O ⇀↽ HCO 3− + H + and being responsible for reversible hydration of
CO 2 , the enzyme carbonic anhydrase (CA) is associated with tissues involved
in calcification and mineralization processes such as the formation of the
embryonic shell (Costlow, 1959; Maren, 1967; Wilbur and Saleuddin, 1983;
Takaichi et al., 2003).
The course of anatomical modifications during embryonic development of
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