149
structure of these and most previously reported multiple-shelled turtle eggs allows
the hypothesis that such eggs arise from oviducal retention of normally shelled eggs
of one clutch that become reshelled during shelling of eggs ovulated to form the
next clutch. This is likely only in species that lay more than one clutch per season,”
(Ewert et al. 1984 ).
Similar phenomenon occurs, probably, in dinosaurs that laid multiple-shelled
eggs during several clutches per season (Ewert et al. 1984 ).
As the eggshell is made of signifi cant amounts of calcium, what are the sources
for the mineral required for its growth? Early in the growth phase during the
second half of incubation, turtle embryos initially obtain calcium from the egg yolk.
The yolk is quickly depleted of calcium, which must then be mobilized from the
eggshell during the last trimester (Packard 1994 ; Sahoo et al. 1998 ). Calcium is the
major inorganic constituent of sea turtle eggshell (20–21 %) (Solomon and Baird
1976 ; Sahoo et al. 1998 ), and the majority required for embryogenesis is derived from
this source (60 %—Sahoo et al. 1998 ; 62 %—Bustard et al. 1969 ; 75 %—Simkiss
1962 ). Consequently, calcium depletion could be expected to infl uence not only
embryonic development (Solomon and Baird 1980 ) but also eggshell structure (Sahoo
et al. 1996a , b ). There are no signifi cant difference reported concerning calcium
content of sea turtle eggshells in loggerhead ( Caretta caretta ), fl atback ( Natator
depressus ), hawksbill ( Eretmochelys imbricata ), and green ( Chelonia mydas ) are
not signifi cantly different (Phillott et al. 2006 ).
Intriguingly, the phenomenon of calcium depletion from eggshells after fungal
invasion of sea turtle eggs is also described (see for review Phillott et al. 2006 ).
Solomon and Baird ( 1980 ) suggested hyphal penetration of the eggshell and
eggshell membranes may impair gaseous exchange, invade embryonic tissue,
and/or impede normal embryonic development by depleting the amount of calcium
in the eggshell. Phillott and Parmenter ( 2001 ) concluded that if hyphal impediment
of gas exchange occurs, the severity of its infl uence would depend upon the size and
location of the fungal growth, the sea turtle species, and the egg size. Fungi are
capable of penetrating the eggshell and invading embryonic tissue (Phillott 2002 ;
Phillott et al. 2004 ). Solomon and Baird ( 1980 ) observed fungal hyphae between the
soft shell membrane and crystalline shell layer in green sea turtle eggs. They concluded that the high calcium content of these hyphae, in conjunction with their
proximity to the calcifi ed eggshell, suggested fungi may be extracting calcium from
the eggshell, thereby causing a defi ciency in the embryo and impairing normal
development.
Calcification for the formation of the eggshell occurs during development
in reptilian embryos. In contrast to mammals and birds which show higher level
of parental care, such extant reptiles as crocodiles, turtles, squamates, and the
tuatara has developed the diverse range of strategies to prolong the egg state
(see for review Rafferty and Reina 2012 ). The biological expediency to arrest
development in reptiles enables their embryos to withstand a changing incubation
environment in a variety of ecological situations. This ability helps animals to
synchronize hatching with seasonal periods, too. As recently formulated by Rafferty
and Reina ( 2012 ):
3.4 Egg Shells of Marine Vertebrates
structure of these and most previously reported multiple-shelled turtle eggs allows
the hypothesis that such eggs arise from oviducal retention of normally shelled eggs
of one clutch that become reshelled during shelling of eggs ovulated to form the
next clutch. This is likely only in species that lay more than one clutch per season,”
(Ewert et al. 1984 ).
Similar phenomenon occurs, probably, in dinosaurs that laid multiple-shelled
eggs during several clutches per season (Ewert et al. 1984 ).
As the eggshell is made of signifi cant amounts of calcium, what are the sources
for the mineral required for its growth? Early in the growth phase during the
second half of incubation, turtle embryos initially obtain calcium from the egg yolk.
The yolk is quickly depleted of calcium, which must then be mobilized from the
eggshell during the last trimester (Packard 1994 ; Sahoo et al. 1998 ). Calcium is the
major inorganic constituent of sea turtle eggshell (20–21 %) (Solomon and Baird
1976 ; Sahoo et al. 1998 ), and the majority required for embryogenesis is derived from
this source (60 %—Sahoo et al. 1998 ; 62 %—Bustard et al. 1969 ; 75 %—Simkiss
1962 ). Consequently, calcium depletion could be expected to infl uence not only
embryonic development (Solomon and Baird 1980 ) but also eggshell structure (Sahoo
et al. 1996a , b ). There are no signifi cant difference reported concerning calcium
content of sea turtle eggshells in loggerhead ( Caretta caretta ), fl atback ( Natator
depressus ), hawksbill ( Eretmochelys imbricata ), and green ( Chelonia mydas ) are
not signifi cantly different (Phillott et al. 2006 ).
Intriguingly, the phenomenon of calcium depletion from eggshells after fungal
invasion of sea turtle eggs is also described (see for review Phillott et al. 2006 ).
Solomon and Baird ( 1980 ) suggested hyphal penetration of the eggshell and
eggshell membranes may impair gaseous exchange, invade embryonic tissue,
and/or impede normal embryonic development by depleting the amount of calcium
in the eggshell. Phillott and Parmenter ( 2001 ) concluded that if hyphal impediment
of gas exchange occurs, the severity of its infl uence would depend upon the size and
location of the fungal growth, the sea turtle species, and the egg size. Fungi are
capable of penetrating the eggshell and invading embryonic tissue (Phillott 2002 ;
Phillott et al. 2004 ). Solomon and Baird ( 1980 ) observed fungal hyphae between the
soft shell membrane and crystalline shell layer in green sea turtle eggs. They concluded that the high calcium content of these hyphae, in conjunction with their
proximity to the calcifi ed eggshell, suggested fungi may be extracting calcium from
the eggshell, thereby causing a defi ciency in the embryo and impairing normal
development.
Calcification for the formation of the eggshell occurs during development
in reptilian embryos. In contrast to mammals and birds which show higher level
of parental care, such extant reptiles as crocodiles, turtles, squamates, and the
tuatara has developed the diverse range of strategies to prolong the egg state
(see for review Rafferty and Reina 2012 ). The biological expediency to arrest
development in reptiles enables their embryos to withstand a changing incubation
environment in a variety of ecological situations. This ability helps animals to
synchronize hatching with seasonal periods, too. As recently formulated by Rafferty
and Reina ( 2012 ):
3.4 Egg Shells of Marine Vertebrates