148
Here, we must note that reptilian eggs have much higher effective pore areas than
those of birds. The functional signifi cance of this structural difference is well
explained in the following way:
“The relatively high porosities of these reptilian eggs presumably facilitate the
transport of oxygen and carbon dioxide into eggshells in later stages of incubation
when air trapped inside nest chambers may become hypoxic and hypercapnic,
yet does not seem to lead to excessive transpiration of water vapor owing to the
high humidities in nests where incubation occurs,” (Packard et al. 1979 ; see also
Doody 2011 ).
No difference concerning biomineralization of the eggshells from wild and
captive animals have been reported (Solomon and Baird 1976 ; Baird and Solomon
1979 ). However, species related specialization occur. For example, aragonite is the
only calcium carbonate phase that was found within Olive ridley’s ( Lepidochelys
olivacea ) eggshell. However, other species possess eggshells that consist of calcite,
vaterite and aragonite. Intriguingly, the eggshells of the leatherback ( Dermochelys
coriacea ), has all three phases together (Sahoo et al. 1996a , b ). The analysis of the
ultrastructure and elements of three marine turtle eggshells ( Chelonia mydas ,
D. coriacea , and Eretmochelys imbricata ) showed that the eggshell had three layers:
the outer cuticle layer or the crystalline layer, the middle layer, and the inner fi brous
layer (Nuamsukon et al. 2009 ). The outer layer was thick and had porosity appearing
like the clusters of branching needle-like crystals, the middle layer was compact
thick, while the inner layer was compact thin. The eggshell thicknesses of C. mydas ,
D. coriacea and E. imbricata were 108.66 ± 1.74 μm, 114.86 ± 0.37 μm, and
98.73 ± 3.56 μm, respectively (Nuamsukon et al. 2009 ). In the loggerhead marine
turtle ( Caretta caretta ) three eggshell layers were also recognized (Al-Bahry et al.
2011 ). Here, the detailed description:
“The outer calcareous layer consists of loose nodular units of different shapes
and sizes with loose attachment between the units, resulting in numerous spaces and
openings. Each unit consists of CaCO 3 crystals in aragonite (99 %) and calcite
(1 %). The middle layer has several strata with numerous openings connecting the
calcareous and the inner shell membrane. Crystallites of the middle layer are a mix
of amorphous material with aragonite (62 %) and calcite (38 %). The inner shell
membrane has numerous reticular fi bers mixed predominantly with halite (NaCl)
and small amounts of sylvite,” (Al-Bahry et al. 2011 ).
It is suggested (Hirsch 2001 ; Jackson and Varricchio 2003 ) that prolonged egg
retention, often resulting from physiological or environmental stress, can be the
reason for development of heavy mineralized multilayered eggshells of some extant
amniotes. However, as reported in at least nine extant species (see for review
Jackson et al. 2004 ) this category of eggs is relatively common in some hard-shelled
turtle eggs. Very special feature have been reported for eggshells of the turtles
Rhinoclemmys areolata. These possess complex pores with basal openings partially
occluded with crystallites. According to (Ewert et al. 1984 ):
“Each of the abnormally thick eggshells has the usual membrane and mineral
layer encased within a second membrane and mineral layer and, in one case, within
yet a third membrane and mineral layer. Information on oviducal function and the
3 Biocomposites and Mineralized Tissues
Here, we must note that reptilian eggs have much higher effective pore areas than
those of birds. The functional signifi cance of this structural difference is well
explained in the following way:
“The relatively high porosities of these reptilian eggs presumably facilitate the
transport of oxygen and carbon dioxide into eggshells in later stages of incubation
when air trapped inside nest chambers may become hypoxic and hypercapnic,
yet does not seem to lead to excessive transpiration of water vapor owing to the
high humidities in nests where incubation occurs,” (Packard et al. 1979 ; see also
Doody 2011 ).
No difference concerning biomineralization of the eggshells from wild and
captive animals have been reported (Solomon and Baird 1976 ; Baird and Solomon
1979 ). However, species related specialization occur. For example, aragonite is the
only calcium carbonate phase that was found within Olive ridley’s ( Lepidochelys
olivacea ) eggshell. However, other species possess eggshells that consist of calcite,
vaterite and aragonite. Intriguingly, the eggshells of the leatherback ( Dermochelys
coriacea ), has all three phases together (Sahoo et al. 1996a , b ). The analysis of the
ultrastructure and elements of three marine turtle eggshells ( Chelonia mydas ,
D. coriacea , and Eretmochelys imbricata ) showed that the eggshell had three layers:
the outer cuticle layer or the crystalline layer, the middle layer, and the inner fi brous
layer (Nuamsukon et al. 2009 ). The outer layer was thick and had porosity appearing
like the clusters of branching needle-like crystals, the middle layer was compact
thick, while the inner layer was compact thin. The eggshell thicknesses of C. mydas ,
D. coriacea and E. imbricata were 108.66 ± 1.74 μm, 114.86 ± 0.37 μm, and
98.73 ± 3.56 μm, respectively (Nuamsukon et al. 2009 ). In the loggerhead marine
turtle ( Caretta caretta ) three eggshell layers were also recognized (Al-Bahry et al.
2011 ). Here, the detailed description:
“The outer calcareous layer consists of loose nodular units of different shapes
and sizes with loose attachment between the units, resulting in numerous spaces and
openings. Each unit consists of CaCO 3 crystals in aragonite (99 %) and calcite
(1 %). The middle layer has several strata with numerous openings connecting the
calcareous and the inner shell membrane. Crystallites of the middle layer are a mix
of amorphous material with aragonite (62 %) and calcite (38 %). The inner shell
membrane has numerous reticular fi bers mixed predominantly with halite (NaCl)
and small amounts of sylvite,” (Al-Bahry et al. 2011 ).
It is suggested (Hirsch 2001 ; Jackson and Varricchio 2003 ) that prolonged egg
retention, often resulting from physiological or environmental stress, can be the
reason for development of heavy mineralized multilayered eggshells of some extant
amniotes. However, as reported in at least nine extant species (see for review
Jackson et al. 2004 ) this category of eggs is relatively common in some hard-shelled
turtle eggs. Very special feature have been reported for eggshells of the turtles
Rhinoclemmys areolata. These possess complex pores with basal openings partially
occluded with crystallites. According to (Ewert et al. 1984 ):
“Each of the abnormally thick eggshells has the usual membrane and mineral
layer encased within a second membrane and mineral layer and, in one case, within
yet a third membrane and mineral layer. Information on oviducal function and the
3 Biocomposites and Mineralized Tissues