Reproductive Biology and Embryology of the ... - Seaturtle.org
Reproductive Biology and Embryology of the ... - Seaturtle.org
Reproductive Biology and Embryology of the ... - Seaturtle.org
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3SE!<br />
REPRODUCTIVE alOLOGY ANO EMBRYOLOGY OF CROCODILIANS<br />
<strong>the</strong>re is no internal pipping, as seen in birds (Rahn et a1., 1979). Hence,<br />
crocodilian hatchlings are unlikely to brea<strong>the</strong> until <strong>the</strong> shell <strong>and</strong> membranes<br />
are broken <strong>and</strong> <strong>the</strong> flUids are drained from around <strong>the</strong> head. The<br />
time interval between this event <strong>and</strong> emergence from <strong>the</strong> egg is relatively<br />
short (ranging from a few minutes to 24 hours), so that <strong>the</strong> transition from<br />
chorioallantoic to pulmonary respiration must be rapid, as in mammals<br />
<strong>and</strong> megapode birds (Seymour <strong>and</strong> Ackerman, 1980), but it is contrary to<br />
<strong>the</strong> slow transition in o<strong>the</strong>r birds (Rahn et a1., 1979). Details <strong>of</strong> this transition<br />
(e.g., <strong>the</strong> extent <strong>of</strong> chorioallantoic perfusion during <strong>the</strong> time between<br />
first breathing <strong>and</strong> hatching) are unknown but <strong>of</strong> considerable interest.<br />
The problem is even more intriguing when one recalls that for many hours<br />
prior to pipping embryos vocalize in response to certain stimuli (see Section<br />
11.E); this suggests that <strong>the</strong> lungs are functional during this period. As<br />
alligator eggs progressively lose water during incubation, an air bubble<br />
forms beneath <strong>the</strong> shell membrane <strong>and</strong> this moves around in <strong>the</strong> albumen,<br />
so as to always lie beneath <strong>the</strong> uppermost surface <strong>of</strong> <strong>the</strong> egg (Ferguson,<br />
1982a): similar events OCCur in megapode eggs (Seymour <strong>and</strong> Ackerman,<br />
1980). Perhaps this air is used prior to pipping (analogous to <strong>the</strong> internal<br />
pipping <strong>of</strong> avian embryos). This area is significant because late embryonic<br />
death is common (during both natural <strong>and</strong> artificial incubation) <strong>and</strong> could<br />
be caused by improper hydric conditions. Thus, high humidity may be<br />
required during <strong>the</strong> early stages <strong>of</strong> incubation, but lower humidity may be<br />
reqUired during <strong>the</strong> later stages to facilitate water loss <strong>and</strong> <strong>the</strong> formation <strong>of</strong><br />
intra-egg air. This in turn may enable internal breathing, <strong>the</strong> maturation <strong>of</strong><br />
<strong>the</strong> respiratory system, <strong>and</strong> provide <strong>the</strong> extra oxygen required for pipping.<br />
Indeed, Vocalization may represent a signal that <strong>the</strong> embryo is mature<br />
enough to survive outside <strong>the</strong> egg, <strong>and</strong> it remains to be determined<br />
whe<strong>the</strong>r it occurs in eggs incubated under conditions <strong>of</strong> continuous high<br />
humidity. The changing hydric conditions are likely to be most critical in<br />
species with long incubation periods (e.g., Crocodylus porosus).<br />
During <strong>the</strong> hatching process, <strong>the</strong> embryo first widens <strong>the</strong> slit in <strong>the</strong> shell<br />
membrane (frequently by pushing its snout into <strong>the</strong> slit <strong>and</strong> <strong>the</strong>n opening<br />
its jaws). Second, it pUShes out <strong>the</strong> entire head <strong>and</strong> neck; after a couple <strong>of</strong><br />
minutes, one quick forceful thrust forces <strong>the</strong> whole bOdy out <strong>of</strong> <strong>the</strong> egg<br />
(Fig. 4). The claws may be used in <strong>the</strong> hatching process. The ruptured<br />
extraembryonic membranes progressively detach from <strong>the</strong> shell membrane<br />
but remain attached to <strong>the</strong> umbilical region <strong>of</strong> <strong>the</strong> hatchling for about a day;<br />
<strong>the</strong>y eventually shrivel, dry up, <strong>and</strong> break (Fig. 21). Hatchlings are COvered<br />
with slime, presumably derived from <strong>the</strong> ruptured allantois, <strong>and</strong> <strong>the</strong>y<br />
smell <strong>of</strong> ammonia. This slime dries after about 24 hours. At <strong>the</strong> time <strong>of</strong><br />
hatching, <strong>the</strong> abdomen is distended by absorbed yolk <strong>and</strong> <strong>the</strong> umbilical<br />
car is evident (Fig. 21). The yolk serves as a food supply for a few weeks<br />
ntil <strong>the</strong> hatchlings begin feeding so that <strong>the</strong> abdomen becomes less dis<br />
~ended <strong>and</strong> <strong>the</strong> umbilicus closes. In hatchlings kept below 210C, <strong>the</strong> umilicus<br />
does not heal correctly, yolk is not utilized, <strong>and</strong> <strong>the</strong> animals usually<br />
e (King <strong>and</strong> Dobbs, 1975).<br />
THE EGGSHELL AND SHELL MEMBRANES<br />
363<br />
The precise hatching trigger remains unknown. Mechanical stimuli <strong>and</strong><br />
vocalizations may playa role (see Section II.E). Growth <strong>and</strong> movement <strong>of</strong><br />
<strong>the</strong> embryo may burst <strong>the</strong> shell. Increasing levels <strong>of</strong> waste products, decreasing<br />
egg water, <strong>and</strong> <strong>the</strong> progressive inadequacy <strong>of</strong> gas exchange may<br />
also stimulate hatching. Temperature seems to be important; field reports<br />
note that an inadvertent rise in egg temperature stimulates premature<br />
hatching, <strong>and</strong>, conversely, decreases in egg temperature delay hatching<br />
(McIlhenny, 1934, 1935; Deraniyagala, 1936, 1939; Pooley, 1962, 1969a,<br />
1971). Hatchlings are sufficiently well developed to be viable if <strong>the</strong>y hatch<br />
several days prematurely (McIlhenny, 1934, 1935) so that completion <strong>of</strong><br />
development is not <strong>the</strong> final trigger for hatching. Lengths <strong>and</strong> weights <strong>of</strong><br />
<strong>the</strong> hatchlings from different species are summarized in Table III.<br />
III.<br />
THE EGGSHELL AND SHELL MEMBRANES<br />
A. General<br />
Most crocodilian eggs are elliptical with approximately equivalent ends<br />
(Figs. 5A to 1). They vary considerably in size <strong>and</strong> shape, even within<br />
species (Tables III <strong>and</strong> IV). Normally <strong>the</strong>re is little variation in size <strong>and</strong><br />
shape within an individual clutch. However, occasional large, infertile,<br />
double-yolked eggs or very small eggs are seen; <strong>the</strong>se abnormal eggs are<br />
usually laid at <strong>the</strong> beginning <strong>and</strong> end <strong>of</strong> oviposition <strong>and</strong> are common in<br />
clutches laid by young females (Ferguson <strong>and</strong> Joanen, 1983). Ferguson<br />
(1982a) described <strong>the</strong> structure <strong>and</strong> composition <strong>of</strong> <strong>the</strong> eggshell <strong>and</strong> shell<br />
membranes <strong>of</strong> Alligator mississippiensis <strong>and</strong> provided a comprehensive review<br />
<strong>of</strong> <strong>the</strong> literature.<br />
B. Egg B<strong>and</strong>ing<br />
The gradual development <strong>of</strong> a transverse white b<strong>and</strong> across fertile crocodilian<br />
eggs has been known for nearly a century (Clarke, 1888a,b, 1891),<br />
<strong>and</strong> reports are now available for several species (Ferguson, 1982a; Reese,<br />
1908, 1912, 1915a, 1931a; McIlhenny, 1935; Deraniyagala, 1936, 1939; Webb<br />
et aI., 1977, 1983a, b, e; Webb, 1977a; Beck, 1978; Hara <strong>and</strong> Kikuchi, 1978;<br />
Deitz <strong>and</strong> Hines, 1980; Tryon, 1980).<br />
Within 24 hours <strong>of</strong> egg laying, <strong>the</strong> eggs <strong>of</strong> Alligator mississippiensis show<br />
a small opaque, chalky white, oval patch on <strong>the</strong>ir top surface (Figs. 5A <strong>and</strong><br />
6). The small embryo has attached itself to <strong>the</strong> innermost surface <strong>of</strong> <strong>the</strong><br />
shell membrane immediately below this opaque patch. At this <strong>and</strong> all<br />
subsequent stages, <strong>the</strong> white color is associated with changes both <strong>of</strong> <strong>the</strong><br />
eggshell <strong>and</strong> shell membrane (which itself appears chalky white in this<br />
area). Initially <strong>the</strong> oval opaque patch exp<strong>and</strong>s in width around <strong>the</strong> shell,<br />
extending approximately half-way around <strong>the</strong> shell on days 3 to 4, threequarters<br />
way around <strong>the</strong> shell on days 5 <strong>and</strong> 6, <strong>and</strong> completely around <strong>the</strong>