Biology Reference
In-Depth Information
Days post hatching
30
50
70
90
110
Forelimb development
Hindlimb development
Metamorphosis
Figure 8.2 Timeline showing timing of fore and hindlimb development and the meta-
morphic period of laboratory reared A. t. tigrinum. The dashed line indicates variation
among individuals in presenting early signatures of anatomical metamorphosis (bulg-
ing eyes, reduced tail fins). In the anuran X. laevis, hindlimb and forelimb development
initiates 2 weeks after hatching. Hindlimbs complete development prior to metamor-
phosis, while forelimbs emerge during metamorphosis.
coincide with other morphological metamorphic changes, as it does in
anurans (
Fig. 8.2
). Instead, it occurs shortly after hatching and before gills
and tailfins are resorbed at metamorphosis. This suggests either a decoupling
of forelimb development from metamorphosis or an incredibly gradual
metamorphosis in salamanders that essentially spans the entirety of the larval
period (
Duellman & Trueb, 1986
). Second, aspects of TH signaling may dif-
fer between salamanders and other amphibians. While it is clear that TH is
required for salamander metamorphosis,
Ambystoma mexicanum
tolerates
levels of TH that cause abnormal development and mortality in some
anurans (
Page et al., 2008
). Also, several orthologous genes, including
thyroid
hormone receptor beta
(
thrb
), do not show the same pattern of expression
between TH-induced
A. mexicanum
and metamorphosing anurans (
Page
et al., 2007, 2008
). Third, salamanders often utilize low temperature
habitats, either because they occur at relatively high elevations or latitudes
(
Bizer, 1978; Sexton & Bizer, 1978
). Low temperature slows growth and
development, depresses activity of neurons and glands that regulate meta-
morphosis, and reduces the responsiveness of tissues to TH (
Moriya,
1983a, 1983b
;
Uhlenhuth, 1919
). Fourth, salamander larvae present the
same body design as adults. Thus, unlike anuran tadpoles that must reorga-
nize the body cavity at metamorphosis to allow space for gonads to develop,
salamanders can potentially allocate resources to developing gonads and as-
sociated fat bodies during the larval period. Fifth, salamander larvae are car-
nivorous like adults, and thus unlike herbivorous anuran tadpoles, do not
rely upon seasonal, primary productivity of ponds for food. Many anuran
species have tadpoles that are highly specialized for rapid development, uti-
lizing seasonally abundant resources in ephemeral ponds that select for finite
