Biology Reference
In-Depth Information
not to feed during at least a week after arrival in fresh water. This, along
with the energy expenditure for metamorphic changes, may contribute to
the decrease in weight and factor of condition.
The modification of the position of the mouth of
S. lagocephalus
represented the best measurable metamorphic change. The higher rate of
transformation occurred during the 10 fi rst days after capture (Fig. 4). The
most striking morphological changes coincided with the increase of thyroid
hormones (TH) levels during post-larval stages. T
4
levels increased during
post-larval stages and a peak was reached at the end of PL2 stage at around
10 days. The decrease of T
4
levels coincided with the beginning of J1 stage.
T
3
levels slightly increased during post-larval stages, then stayed stable
at the beginning of J1 stage and decreased (Fig. 4). In other teleost groups
such as Elopomorphs or Pleuronectiforms, it has been shown that TH levels
increased during larval metamorphosis (
Conger myriaster
: Yamano et al.,
1991;
Anguilla japonica
: Yamano et al., 2007;
Paralichthys olivaceous
: Miwa and
Inui, 1988; de Jesus et al., 1991;
Paralichthys dentatus
: Schreiber and Specker,
1998,
Hippoglossus hippoglossus
: Einarsdottir et al., 2006;
Verasper variegatus
:
Hotta et al., 2001;
Solea senegalensis
: Fernandez-Diaz et al., 2001; Klaren et
al., 2008; Manchado et al., 2008). The highest levels of whole body thyroid
hormones in
S. lagocephalus
were measured when morphological changes
were the deepest. This suggests an important role of the thyreotropic axis
in the control of
S. lagocephalus
larval metamorphosis.
Taillebois et al. 2011 tested this hypothesis through an experimental
approach, involving T
4
and Thiourea (TU) treatments. The hormonal
treatments lasted the 10 fi rst days of the 37 days of monitoring and where
tested for each treatment on 196 fi sh. Variations of whole-body THs levels in
S. lagocephalus
were measured by RIA at day 10: T
4
and T
3
levels were both 4
times higher in T
4
-treated fi sh than in control and 6 and 2 times, respectively,
lower in TU-treated fi sh than in controls. The variation in T
4
and T
3
levels
induced by experimental treatment in
S.
lagocephalus
are thus in the same
range of those induced by hormonal treatments or observed in natural
physiopathological conditions in other species (
Anguilla
anguilla
: Edeline
et al., 2005;
Solea senegalensis
: Manchado et al., 2008;
Paralichtys
dentatus
:
Schreiber and Specker, 1998;
Paralichtys olivaceus
: Okada et al., 2005).
The fish which THs levels had been measured had been also
morphologicaly monitored and Taillebois et al. 2011 showed that a 10-day
treatment with T
4
accelerated and amplifi ed the change in the corner of mouth
angle in
S. lagocephalus
(Fig. 5). Inversely, treatment with TU signifi cantly
delayed the mouth change of position after 5 days of treatment (Fig. 5). The
change of mouth position started again at day 18, that is 8 days after the end
of the TU treatment. The corner of mouth angle then reached a value similar
to the value of the controls. The fact that morphological transformations
