Reproductive Biology and Embryology of the ... - Seaturtle.org

374 REPROOUCTIVE BIOLOGY ANO EMBRYOLOGY OF CROCOOILIANS
THE EGGSHELL ANO SHELL MEMBRANES
375
acidic dissolution (Figs. 7, 8, and 90), and the plug becomes dislodged.
Presumably, this widening of the pore orifices facilitates respiratory and
metabolic exchanges; it also progressively weakens the eggshell, as longitudinal
cracks often pass through the pores and the erosion craters.
8. CONCLUDING COMMENTS
The complex structure of the eggshell reflects its adaptation to crocodilian
nesting biology. At laying, the eggs are strong and not very porous;
they are thus protected from physical damage at the time when they are
deposited one upon another and whenever the mother treads nesting material
on top of them, as well as from dehydration during early development.
The complex sequence of changes in the inorganic constituents
weakens the shell and makes it more porous throughout incubation, so
that the prehatchling has merely to slit the organic materials with its caruncle.
Antimicrobial properties of shell or albumen components, known for
birds (Board and Fuller, 1974) and turtles (Movchan, 1964, 1966, 1967) have
not been reported in crocodilian eggs but may be related to the development
of erosion craters and the filter bed arrangement of the porous shell
and shell membrane. Data on such properties would be valuable, not only
for their intrinsic interest, but also in view of the practical problems associated
with artificial egg incubation.
C. Chemical Composition
Relatively little is known about the chemical composition of crocodilian
eggs. The 6 g of calcium carbonate in the eggs is 99% calcite and less than
1% aragonite (Erben, 1970; Jenkins, 1975), a ratio far closer to that reported
for birds (Romanoff and Romanoff, 1949; Simkiss, 1967; Rol'nick, 1970)
than the reported predominance of aragonite in the eggshells of turtles
(Young, 1950; Erben, 1970; Packard et al., 1977; Solomon and Baird, 1979).
Eggshells of Crocodylus novaeguineae contain 82.6% calcium carbonate,
2.82% magnesium, 0.37% phosphorus, and 3.36% organic protein (Jenkins,
1975). The total protein content of the eggshell is approximately twice
that for the domestic fowl (Jenkins, 1975). The eggshell of Alligator mississippiensis
also contains calcium, magnesium, and phosphorus, as well as
traces of copper, silicon, sodium, aluminum, iron, zinc, and manganese
(Ferguson, 1982a).
The thickness of the shell membranes is reflected in the fact that they
comprise 21.39% of the total weight of dry shell and membrane (Jenkins,
1975), whereas they only comprise 0.3% in the domestic fowl (Romanoff
and Romanoff, 1949). The shell membrane acts as an intermediary calcium
store between that of the eggshell and the calcium in the blood of the
chorioallantoic vessels (Ferguson, 1982a), a factor which contributes to its
chalky white coloring beneath the opaque bands. Whether the yolk, albu-
TABLE V
Calcium Contents of the Egg and Hatchlings of Various Species"
Snake Crocodile Turtle Bird
(Vipera (Crocodylus (Dermocllelys (Gallus
berus) nouaeguineae) coriacea) domesticus)
Calcium in 25 124 34 23
egg contents, mg
Calcium in 12-16 280 138 120
hatchlings, mg
lncrease b 0.8x 2.4x 4x 5.2x
"From Jenkins, 1975.
blndex of a mount of shell calcium used by embryo.
men and extraembryonic membranes also store calcium is unknown for
crocodilians, but such storage does occur in birds (Simkiss, 1967, 1980).
The mechanisms of eggshell decalcification and calcium transport are
unknown; however, carbonic acid, formed from respiratory carbon dioxide,
apparently attacks the shell of birds (Buckner et al., 1925; Romanoff
and Romanoff, 1949; Simkiss, 1967, 1980; Rol'nick, 1970). This mechanism
is in accord with the assumed respiratory function of the opaque zone in
crocodilian eggs. The levels of calcium in egg contents and hatchlings of
Crocodylus novaeguineae given in Table V are uncertain due to poor preservation
and large numbers of infertile eggs (Jenkins, 1975). Because hatchling
crocodiles contain 2.4 times more calcium than the egg contents, the
additional calcium must derive from the eggshell. Apparently, the yolk and
albumen of crocodilians contain more calcium than those of turtles (which
obtain four times as much calcium from the shell as from the egg contents)
or birds (which obtain five times as much calcium from the shell), but much
less than those of snakes, which obtain no calcium from the eggshell (Jenkins
and Simkiss, 1968). This decreased dependence of crocodilian embryos
on eggshell calcium permits experimental embryological studies using
shell-less and semi-sheIl-less culture of alligator embryos; these survive
to later developmental stages than their avian counterparts (Ferguson,
1981a, 1982b, 1984a; Fig. 38).
Available reports on the biochemical constituents indicate general agreement
on the occurrence of various mucopolysaccharides (Neumeister,
1895; Simkiss and Tyler, 1959; Kriesten, 1975), but fibrous proteins are less
well-known. A survey of the amino-acid composition of the material of
the shell and shell membranes indicated that the former contained sequences
characteristic of collagen (in contrast to turtles), whereas the latter
contained sequences reminiscent of a keratin-like protein (Kramptiz et al.,
1974), similar to that suspected for birds and turtles (Romanoff and