The thickness of this sheath is less than 1 mm (we estimate that it varies between 0.8
and 1 mm, except at the triple junctions where it is thicker). This sheath is structurally
coherent and flexible: when the prisms are partly or totally dissolved with EDTA or
with weak acid, the sheath remains in place. Because the sheath represents a high
proportion of the shell (about 4% wt/wt) and because of its honeycomb structure, it
confers to the shell a surprising bending capability, in particular, in juvenile
specimens. On the other hand, the prismatic layer of P. nobilis can be entirely
dissociated in single prism units, by selectively degrading the organic sheath. The
most classical chemical reagent used for that purpose is dilute sodium hypochlorite.
In our lab, we currently treat fragments of prismatic layers for few days with a 0.26%
of active chlorine solution (dilution ten times from a commercial solution). The
single prisms can be subsequently collected and extensively washed with water on a
5-mm filter. Removal of the organic sheath has also been successfully performed by
enzymatic proteolysis with an enzyme – like pronase B – that exhibits a broad
spectrum of proteolytic activity (Keller 1981; Keller and Dauphin 1983).
13.3.4 Ultrastructure of the Prisms of P. nobilis and Complexity
of the Organo-mineral Interactions
By many aspects, the calcitic prisms of Pinna nobilis appear as paradoxical objects,
because they conciliate an apparent crystallographic simplicity and an ultrastructural complexity. The nature of the calcitic prisms of P. nobilis is revealed
by investigating the crystallographic properties on single prisms, or on prisms
preparations observed in cross sections, in combination with analysis of their
substructures. As mentioned by Carter (1990, p 213), “the calcitic, regular simple
prisms in pinnid shells are commonly optically homogeneous with perfect, monocrystalline, non-grainy extinction under crossed polarized light.” This means that
each prism behaves like a single crystal, with a single extinction (Wada 1961; Cuif
and Raguideau 1982). This optical property is confirmed by X-ray diffraction on
single prism (F. Marin, unpublished data, 2003). This technique demonstrates that
one prism diffracts like a single crystal.
Although the prisms of P. nobilis are optically homogeneous, they are neither
“single” nor “simple” crystals, and many studies have underlined their complex
substructures that superimpose at different scales. What makes their peculiar
ultrastructural complexity is the presence of an intraprismatic organic matrix, the
characteristics of which are detailed in Sect. 4.
Decalcification experiments aiming at dissolving the prisms and keeping the
organic sheaths show that these sheaths exhibit a transverse striation (Gre ´goire
1967). To this striation, perpendicular to the long axis of the prisms, corresponds a
striation on the prisms themselves (Fig. 13.2), visible in longitudinal view. This
striation marks the successive growth plans. In spite of the presence of the sheath
that separates each prism from its neighbors, the growth lines can be followed for
millimiters throughout the length of the shell section (Fig. 13.2).
13 Acidic Shell Proteins of the Mediterranean Fan Mussel Pinna nobilis
363
and 1 mm, except at the triple junctions where it is thicker). This sheath is structurally
coherent and flexible: when the prisms are partly or totally dissolved with EDTA or
with weak acid, the sheath remains in place. Because the sheath represents a high
proportion of the shell (about 4% wt/wt) and because of its honeycomb structure, it
confers to the shell a surprising bending capability, in particular, in juvenile
specimens. On the other hand, the prismatic layer of P. nobilis can be entirely
dissociated in single prism units, by selectively degrading the organic sheath. The
most classical chemical reagent used for that purpose is dilute sodium hypochlorite.
In our lab, we currently treat fragments of prismatic layers for few days with a 0.26%
of active chlorine solution (dilution ten times from a commercial solution). The
single prisms can be subsequently collected and extensively washed with water on a
5-mm filter. Removal of the organic sheath has also been successfully performed by
enzymatic proteolysis with an enzyme – like pronase B – that exhibits a broad
spectrum of proteolytic activity (Keller 1981; Keller and Dauphin 1983).
13.3.4 Ultrastructure of the Prisms of P. nobilis and Complexity
of the Organo-mineral Interactions
By many aspects, the calcitic prisms of Pinna nobilis appear as paradoxical objects,
because they conciliate an apparent crystallographic simplicity and an ultrastructural complexity. The nature of the calcitic prisms of P. nobilis is revealed
by investigating the crystallographic properties on single prisms, or on prisms
preparations observed in cross sections, in combination with analysis of their
substructures. As mentioned by Carter (1990, p 213), “the calcitic, regular simple
prisms in pinnid shells are commonly optically homogeneous with perfect, monocrystalline, non-grainy extinction under crossed polarized light.” This means that
each prism behaves like a single crystal, with a single extinction (Wada 1961; Cuif
and Raguideau 1982). This optical property is confirmed by X-ray diffraction on
single prism (F. Marin, unpublished data, 2003). This technique demonstrates that
one prism diffracts like a single crystal.
Although the prisms of P. nobilis are optically homogeneous, they are neither
“single” nor “simple” crystals, and many studies have underlined their complex
substructures that superimpose at different scales. What makes their peculiar
ultrastructural complexity is the presence of an intraprismatic organic matrix, the
characteristics of which are detailed in Sect. 4.
Decalcification experiments aiming at dissolving the prisms and keeping the
organic sheaths show that these sheaths exhibit a transverse striation (Gre ´goire
1967). To this striation, perpendicular to the long axis of the prisms, corresponds a
striation on the prisms themselves (Fig. 13.2), visible in longitudinal view. This
striation marks the successive growth plans. In spite of the presence of the sheath
that separates each prism from its neighbors, the growth lines can be followed for
millimiters throughout the length of the shell section (Fig. 13.2).
13 Acidic Shell Proteins of the Mediterranean Fan Mussel Pinna nobilis
363