Biology Reference
In-Depth Information
The Post-Larval Stage 1 (PL1) starts as soon as larvae arrive in the
estuary. It is characterised by the beginning of river colonisation and
therefore by changes in osmoregulation. The individual is practically
translucent, indeed the pigmentation of the spot on the head is ≤ 60%;
the mouth is terminal. Metamorphosis of the head has started: the mouth
corner angle is < 67°. The caudal fi n is forked (%C < 63%). The length of
the pectoral fi n is less than 20% that of the standard length (SL). The lateral
ctenoid scales generally reach, at most, half the insertion of the fi rst dorsal
fi n. Cycloid scales are already present on the belly and in the predorsal
zone. Fish in this stage remain in the river mouth less than 48 hours. The
notches on the lips are light but visible. The rake-like teeth used by adults
for grazing algae on river stones are not or just slightly visible. This stage
is complete when the pectoral fi n has fi nished transforming and when the
teeth start to appear clearly.
During the
Post-Larval Stage 2 (PL2), individuals show vertical
coloured stripes on their sides and the pigmentation of the cerebral spot
on the head now covers 60 to 90% of the observed skin surface. The mouth
is sub-inferior, the head metamorphosis is ongoing: the corner of mouth
angle is 67 to 80°. The caudal fi n is still forked (% C < 63%). The pectoral
fi n has fi nished transforming and its length is more than 20% of that of the
SL. The lateral ctenoid scales generally reach over half of the insertion place
of the fi rst dorsal fi n, but do not overtake the anterior part of the insertion.
PL2 individuals are found in the lower course of the river and have been
in fresh water for 2 to 14 days. The notches on the lips are larger and now
clearly visible. The rake-like teeth used for grazing are in place. This stage
is complete when the metamorphosis of the cranium is completed.
During the two following juveniles stages (J1, J2), individuals have
complete cerebral pigmentation, similar to that of non-mature adults: the
spot on the head pigmentation is 95 to 100%. The caudal fi n border is nearly
to completely straight (%C between 63 and 100%). Lateral ctenoid scales
and belly cycloid scales are in place, similar to those of adults.
5.3 Involvement of Thyroid Hormones in the Control of Larval
Metamorphosis in
S. lagocephalus
To study the infl uence of thyroid hormones in the metamorphic changes,
Taillebois et al. 2011 performed an analytical study on a cohort of fi sh caught
in estuary in Reunion Island (Indian Ocean) at PL1 stage when recruiting
back to river and maintained in fl ume tanks (called fl uvarium) which mimics
as close as possible natural conditions. Biometrical parameters (total and
standard length, corner of mouth angle, weight) and whole body L-tyroxine
(T
4
) and triiodothyronine (T
3
) contents were measured on fi sh regularly
sampled from the tank during 37 days.
