and Giacobbe 2007a, b; Rabaoui et al. 2007, 2009). Thus, the spines constitute
an efficient adaptation to passively and actively prevent predation on juvenile
specimens, the shell of which is particularly fragile.
13.3.2 Shell Microstructures
The shell of P. nobilis exhibits the typical bilayered calcified structure (in addition
to the outer organic periostracum), found in several pteriomorphid bivalves. The
periostracal layer is visible particularly in juvenile specimens, but tends to quickly
erode in older specimens. The outer mineralized layer is calcitic and composed of
“simple” prisms, developed perpendicularly to the surface of the shell. The “simple
prisms” terminology is doubly misleading. First, “simple” would refer to an elementary structure, but we show below (Sect. 3.4) that the prisms of P. nobilis
exhibit a structural hierarchy, which is everything but simple. Second, “prism”
designates very different objects in molluscan shell microstructures, like the tiny
oblique prisms of the edible mussel, the prisms of the freshwater mussel, the
composite prisms of the clam, and those of the pearl oyster (Carter 1990). These
objects are brought together under a single term, but it seems obvious that they
are not synthesized through a unique pathway.
The internal layer is light grey and aragonitic. It exhibits the iridescence characteristic of nacreous textures. While the outer prismatic layer covers the entire shell,
the nacreous layer is restricted to the area between the adductor muscle scars, which
represents about one-half of the shell height. By analogy with more studied nacroprismatic bivalve models such as Pinctada (Sudo et al. 1997), the fact that the shell
is bilayered means that the mantle epithelium that produces the shell is not
homogeneous: the epithelial cells located at the border of the mantle are dedicated
to the synthesis of the prismatic layer while epithelial cells situated more proximally from the shell tip secrete the nacre layer. However, histological studies
should be done to confirm this finding.
From a historical perspective, it is remarkable to notice that the shell microstructure
of Pinna sp., which is a rather uncommon genus, was extensively described as early as
the beginning of the ninetieth century, with the successive works of De Bournon
(1808), Gray (1835), Bowerbank (1844), Carpenter (1844), Leydolt (1856), Rose
(1858). During the early twentieth century, other microstructural studies followed,
in particular those of Biedermann (1901), R€ omer (1903), Karny (1913), Cayeux
(1916), Schmidt (1923, 1924) and of course, the classical study of Boggild (1930).
With the development of the electron microscopy in the thirties, and the conception of the scanning electron microscope after WWII, Pinna sp. was also the subject
of different studies and monographies, including papers from Gre ´goire (1967), Wise
(1970), Mutvei (1970), Wada (1972, 1980). Let us cite the extensive work of Taylor
and coworkers (1969) on bivalves. In the eighties, the shell microstructures of
Pinna sp. were described through series of papers of Cuif and coworkers (1980,
1983a, b, 1985, 1987a, b), Cuif et Raguideau (1982), Nakahara et al. (1980), Carter
360
F. Marin et al.
an efficient adaptation to passively and actively prevent predation on juvenile
specimens, the shell of which is particularly fragile.
13.3.2 Shell Microstructures
The shell of P. nobilis exhibits the typical bilayered calcified structure (in addition
to the outer organic periostracum), found in several pteriomorphid bivalves. The
periostracal layer is visible particularly in juvenile specimens, but tends to quickly
erode in older specimens. The outer mineralized layer is calcitic and composed of
“simple” prisms, developed perpendicularly to the surface of the shell. The “simple
prisms” terminology is doubly misleading. First, “simple” would refer to an elementary structure, but we show below (Sect. 3.4) that the prisms of P. nobilis
exhibit a structural hierarchy, which is everything but simple. Second, “prism”
designates very different objects in molluscan shell microstructures, like the tiny
oblique prisms of the edible mussel, the prisms of the freshwater mussel, the
composite prisms of the clam, and those of the pearl oyster (Carter 1990). These
objects are brought together under a single term, but it seems obvious that they
are not synthesized through a unique pathway.
The internal layer is light grey and aragonitic. It exhibits the iridescence characteristic of nacreous textures. While the outer prismatic layer covers the entire shell,
the nacreous layer is restricted to the area between the adductor muscle scars, which
represents about one-half of the shell height. By analogy with more studied nacroprismatic bivalve models such as Pinctada (Sudo et al. 1997), the fact that the shell
is bilayered means that the mantle epithelium that produces the shell is not
homogeneous: the epithelial cells located at the border of the mantle are dedicated
to the synthesis of the prismatic layer while epithelial cells situated more proximally from the shell tip secrete the nacre layer. However, histological studies
should be done to confirm this finding.
From a historical perspective, it is remarkable to notice that the shell microstructure
of Pinna sp., which is a rather uncommon genus, was extensively described as early as
the beginning of the ninetieth century, with the successive works of De Bournon
(1808), Gray (1835), Bowerbank (1844), Carpenter (1844), Leydolt (1856), Rose
(1858). During the early twentieth century, other microstructural studies followed,
in particular those of Biedermann (1901), R€ omer (1903), Karny (1913), Cayeux
(1916), Schmidt (1923, 1924) and of course, the classical study of Boggild (1930).
With the development of the electron microscopy in the thirties, and the conception of the scanning electron microscope after WWII, Pinna sp. was also the subject
of different studies and monographies, including papers from Gre ´goire (1967), Wise
(1970), Mutvei (1970), Wada (1972, 1980). Let us cite the extensive work of Taylor
and coworkers (1969) on bivalves. In the eighties, the shell microstructures of
Pinna sp. were described through series of papers of Cuif and coworkers (1980,
1983a, b, 1985, 1987a, b), Cuif et Raguideau (1982), Nakahara et al. (1980), Carter
360
F. Marin et al.