the deposition of nacreous crystallites. As we will see in the next paragraph, the
transition layer marks the profound difference of the secretory regime between the
mantle cells involved in prisms formation and those involved in nacre deposition.
In Pinna nobilis, as mentioned before, the nacreous layer is restricted to the area
comprised between the two muscle scars, about one-half of the shell. In young
specimens (2–3 years old), the nacre layer is extremely thin (not more than
2–3 mm) and translucent. In old specimens (>20 years), the nacreous layer is
opaque and reaches a thickness of about 10 mm in the anterior tip. Taylor et al.
(1969) proposed a general evolutionary trend toward the reduction of the nacreous
layer in the Pinnidae family: the ancestral type, which would be nowadays
represented by Atrina, exhibits indeed a more expanded nacre layer, while this
layer is residual in the most derived Streptopinna genus, Pinna constituting an
intermediary stage. This trend toward a reduction of the nacreous layer, if it
is verified, would be in agreement with the finding of Palmer (1992), i.e.,
microstructures like nacre, which have a higher energetic cost to produce, are
consequently disadvantaged from an evolutionary viewpoint. Alternatively, it
may also correspond to different modes of life, implying different mechanical
requirements for the shell.
The fact that the nacreous layer covers only one-half of the shell has an
important implication on the geometry of the calcification process: it means that
the mineralization fronts of nacre and of prisms are distant from each other. In other
words, the mantle epithelial cells that secrete the ionic and macromolecular
components for nacre deposition are physically remote (several centimeters in a
2-year-old specimen) from those that initiate the prisms at the shell edge. It also
means that the cells involved in the prisms extension, i.e., in the increase of the
thickness of the prismatic layer, constitute an extremely large zone of the mantle
tissue.With these geometrical constraints, Pinna nobilis differs considerably from
Mytilus, Pinctada, or Unio models.
The nacreous layer of P. nobilis is rather atypical, and does not correspond to
the classical “brickwall texture” (sheet nacre) with consecutive tablets arranged
in staggered rows, found in most pterioid, mytiloid, and unionoid bivalves
(Wise 1970). The nacre of Pinna nobilis is indeed defined as “row-stack nacre”
(Wise 1970; Carter and Clark 1985) and is described as a “nacreous structure in
which mutually parallel elongate tablets show vertical stacking in vertical sections
perpendicular to their length axes, and brick wall and/or stair step stacking
in vertical sections parallel to their length axes. Row stack nacreous structure is
well developed in the bivalve Pinna, where it enhances directional flexibility of
the shell.” In cross section, as shown in Fig. 13.3c–d, nacre elements appear as
flexuous flat tablets, 0.5–0.8 mm thick and 15–20 mm long, which exhibit a pseudocolumnar disposition. This vertical disposition, which was remarkably described
a long time ago by Rose (1858), then by Schmidt (1924, Fig. 48, Tafel 4,
“Vertikalschichtung”), appears different, larger, and slightly less structurally regular than that found in haliotid gastropods e.g., but has some similarities with that of
Nucula. Vertical sections show the more or less columnar alignments of the
successive nacre laminae (Fig. 13.3c–d). When observed from above on the
366
F. Marin et al.
transition layer marks the profound difference of the secretory regime between the
mantle cells involved in prisms formation and those involved in nacre deposition.
In Pinna nobilis, as mentioned before, the nacreous layer is restricted to the area
comprised between the two muscle scars, about one-half of the shell. In young
specimens (2–3 years old), the nacre layer is extremely thin (not more than
2–3 mm) and translucent. In old specimens (>20 years), the nacreous layer is
opaque and reaches a thickness of about 10 mm in the anterior tip. Taylor et al.
(1969) proposed a general evolutionary trend toward the reduction of the nacreous
layer in the Pinnidae family: the ancestral type, which would be nowadays
represented by Atrina, exhibits indeed a more expanded nacre layer, while this
layer is residual in the most derived Streptopinna genus, Pinna constituting an
intermediary stage. This trend toward a reduction of the nacreous layer, if it
is verified, would be in agreement with the finding of Palmer (1992), i.e.,
microstructures like nacre, which have a higher energetic cost to produce, are
consequently disadvantaged from an evolutionary viewpoint. Alternatively, it
may also correspond to different modes of life, implying different mechanical
requirements for the shell.
The fact that the nacreous layer covers only one-half of the shell has an
important implication on the geometry of the calcification process: it means that
the mineralization fronts of nacre and of prisms are distant from each other. In other
words, the mantle epithelial cells that secrete the ionic and macromolecular
components for nacre deposition are physically remote (several centimeters in a
2-year-old specimen) from those that initiate the prisms at the shell edge. It also
means that the cells involved in the prisms extension, i.e., in the increase of the
thickness of the prismatic layer, constitute an extremely large zone of the mantle
tissue.With these geometrical constraints, Pinna nobilis differs considerably from
Mytilus, Pinctada, or Unio models.
The nacreous layer of P. nobilis is rather atypical, and does not correspond to
the classical “brickwall texture” (sheet nacre) with consecutive tablets arranged
in staggered rows, found in most pterioid, mytiloid, and unionoid bivalves
(Wise 1970). The nacre of Pinna nobilis is indeed defined as “row-stack nacre”
(Wise 1970; Carter and Clark 1985) and is described as a “nacreous structure in
which mutually parallel elongate tablets show vertical stacking in vertical sections
perpendicular to their length axes, and brick wall and/or stair step stacking
in vertical sections parallel to their length axes. Row stack nacreous structure is
well developed in the bivalve Pinna, where it enhances directional flexibility of
the shell.” In cross section, as shown in Fig. 13.3c–d, nacre elements appear as
flexuous flat tablets, 0.5–0.8 mm thick and 15–20 mm long, which exhibit a pseudocolumnar disposition. This vertical disposition, which was remarkably described
a long time ago by Rose (1858), then by Schmidt (1924, Fig. 48, Tafel 4,
“Vertikalschichtung”), appears different, larger, and slightly less structurally regular than that found in haliotid gastropods e.g., but has some similarities with that of
Nucula. Vertical sections show the more or less columnar alignments of the
successive nacre laminae (Fig. 13.3c–d). When observed from above on the
366
F. Marin et al.