formed “prism” (at that stage, it looks like a tile), the existence of concentric
patterns when the edge of the shell is observed from the outer surface, “through”
the thin periostracal layer (Marin, unpublished data). This clearly suggests that
nodules (or spherulites) are the first event of mineralization, similarly to what
happens during the abiotic competition process. Third, Checa and coworkers
observed, in cross sections of P. nobilis shells, competition for space, in a very
narrow zone, comprised between 50 and 100 mm below the periostracal surface
(Checa et al. 2005). However, we cannot subscribe to the view of an analogy
between the abiotic competition process and the growth of P. nobilis prisms for
the following reason: as underlined before, the initial “prisms” – or tiles, to be more
exact – develop in a preformed honeycomb network, which consequently, antedates
the early mineralization. At that stage, no competition for space occurs, since the
polygons are already constrained by the periprismatic framework. As underlined by
Checa and coworkers (2005) referring to Cuif et al. (1983a, b) the “unmineralized
stage extends for at least some 100 mm backward from the margin and that empty
organic cells (polygons) may attain some 5 mm in height at the onset of mineralization.” Clearly, the competition for space observed previously seems to intervene at
a later stage, during the early elongation phase of the prisms between 50 and
100 mm. How this competition occurs and why it occurs at that stage is still unclear,
but would indicate that the growth of the mineral phase (prisms) catches up that of
the organic periprismatic membranes.
In parallel to these observations, a remarkable idea has been developed by Checa
and coworkers (2005) for explaining the reason of the polygonal shape of the
prisms. These authors observed that the honeycomb-like organic framework has
the structure of foam. Foams are gas-in-liquid dispersion mainly stabilized by
interfacial tensions, in which the liquid phase is the thin film that separates gas
bubbles. In ideal foams, the thin liquid films join at angles of about 120
to form
triple junctions. In the present case, Checa et al. suggested that, rather than foam,
the initial stage of the precursors of the mineralization, including the prisms matrix
and the extrapallial fluid, may be a liquid–liquid emulsion, i.e., a mixture of two
non-miscible liquids. Similarly to foam, an emulsion is patterned by interfacial
tension. In this case, the matrix represents the viscoelastic hydrophobic fluid that
makes a continuous mesh at the contact of the inner surface of the periostracum.
The aqueous discontinuous solution – the extrapallial fluid – would be entrapped in
the polygonal cells.
We can try to relate what is described here to the putative functions of calprismin
and caspartin, according to their biochemical properties and to their location in the
prismatic layer. In our previous study, we have shown that caspartin is intracrystalline
and intercrystalline as well (Marin et al. 2007). A similar location seems to occur with
calprismin (Narayanappa, in preparation). Because they are extremely acidic, both
proteins may act as nucleators of nanocrystals. We can imagine that clusters – rather
than films – of caspartin–calprismin complexes may locally catalyze the formation of
nanocrystals that self-orient in the forming mesocrystal. The intercrystalline caspartin
(and calprismin as well) may play an active role in stabilizing the emulsion, being at
the surface of extrapallial fluid droplets, while the more hydrophobic organic network
384
F. Marin et al.
patterns when the edge of the shell is observed from the outer surface, “through”
the thin periostracal layer (Marin, unpublished data). This clearly suggests that
nodules (or spherulites) are the first event of mineralization, similarly to what
happens during the abiotic competition process. Third, Checa and coworkers
observed, in cross sections of P. nobilis shells, competition for space, in a very
narrow zone, comprised between 50 and 100 mm below the periostracal surface
(Checa et al. 2005). However, we cannot subscribe to the view of an analogy
between the abiotic competition process and the growth of P. nobilis prisms for
the following reason: as underlined before, the initial “prisms” – or tiles, to be more
exact – develop in a preformed honeycomb network, which consequently, antedates
the early mineralization. At that stage, no competition for space occurs, since the
polygons are already constrained by the periprismatic framework. As underlined by
Checa and coworkers (2005) referring to Cuif et al. (1983a, b) the “unmineralized
stage extends for at least some 100 mm backward from the margin and that empty
organic cells (polygons) may attain some 5 mm in height at the onset of mineralization.” Clearly, the competition for space observed previously seems to intervene at
a later stage, during the early elongation phase of the prisms between 50 and
100 mm. How this competition occurs and why it occurs at that stage is still unclear,
but would indicate that the growth of the mineral phase (prisms) catches up that of
the organic periprismatic membranes.
In parallel to these observations, a remarkable idea has been developed by Checa
and coworkers (2005) for explaining the reason of the polygonal shape of the
prisms. These authors observed that the honeycomb-like organic framework has
the structure of foam. Foams are gas-in-liquid dispersion mainly stabilized by
interfacial tensions, in which the liquid phase is the thin film that separates gas
bubbles. In ideal foams, the thin liquid films join at angles of about 120
to form
triple junctions. In the present case, Checa et al. suggested that, rather than foam,
the initial stage of the precursors of the mineralization, including the prisms matrix
and the extrapallial fluid, may be a liquid–liquid emulsion, i.e., a mixture of two
non-miscible liquids. Similarly to foam, an emulsion is patterned by interfacial
tension. In this case, the matrix represents the viscoelastic hydrophobic fluid that
makes a continuous mesh at the contact of the inner surface of the periostracum.
The aqueous discontinuous solution – the extrapallial fluid – would be entrapped in
the polygonal cells.
We can try to relate what is described here to the putative functions of calprismin
and caspartin, according to their biochemical properties and to their location in the
prismatic layer. In our previous study, we have shown that caspartin is intracrystalline
and intercrystalline as well (Marin et al. 2007). A similar location seems to occur with
calprismin (Narayanappa, in preparation). Because they are extremely acidic, both
proteins may act as nucleators of nanocrystals. We can imagine that clusters – rather
than films – of caspartin–calprismin complexes may locally catalyze the formation of
nanocrystals that self-orient in the forming mesocrystal. The intercrystalline caspartin
(and calprismin as well) may play an active role in stabilizing the emulsion, being at
the surface of extrapallial fluid droplets, while the more hydrophobic organic network
384
F. Marin et al.