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This cranium metamorphosis is correlated to changes in diet and spatial
distribution between post-larvae and juveniles (Keith, 2003). The digestive
system is also modifi ed (Schoenfuss et al., 1997). The juveniles must switch
from a planktonic feeding mode (copepods) to a benthic feeding mode
(periphyton), when they graze on the rocky substrate (Bielsa et al. , 2003).
Metamorphosis of feeding structures from the offshore larval phase is vital
for the survival of the fi sh once it enters freshwater. The appearance of
premaxillary teeth adapted to scraping diatoms off the substrate, typical of the
Sicyopterus genus, is gradual throughout the metamorphic process. Indeed,
the change in the position of the mouth and the cranium reorganisation
start just before the arrival in estuaries and ends between 14 to 21 days after
entrance in river. These changes are essential to allow new recruits to begin
feeding on benthic algae (Schoenfuss et al., 1997).
Pectoral fi n transformation was complete after only 2-3 days spent in
fresh water but the caudal fi n started to shift from forked to straight after c.
21 days; it was then straight after c. 28 days. The cerebral spot on the head
was practically completely pigmented (> 85%) after c. 14 days following
the arrival of individuals in freshwater. Finally, ctenoid scales spread to the
anterior part of the body from c. 14 days after arrival.
When the transformation is complete, Sicyopterus is able to climb over
waterfalls by using alternatively its pelvic suction cup and its lips (Blob et
al., 2006; Schoenfuss and Blob, 2007). Non-metamorphosed Sicyopterus are
unable to reach the upper part of the streams, because of high waterfalls and
riffl e areas that commonly block their migration (Keith, 2003).
According to these morphological observations, Keith et al. (2008)
defi ned three post-larval stages (PL0 to PL2) and two juvenile stages (J1,
J2) for S. lagocephalus . The true metamorphosis process takes place between
stages PL0 and PL2.
5.2.1 Post-larval stages
The Post-Larval Stage 0 (PL0) is characterised by the beginning of the
morphological transformation needed to recruit: the sucker is already
formed and lateral scales start to appear. This stage takes place in salt water
and individuals are then competent (i.e ., physiological and morphological
features required for colonisation of the adult environment are present)
(Murphy and Cowan, 2007). The duration of this stage is unknown, and it
probably varies according to hydrological and climatic conditions, the latter
having to be favourable for migration into the river. This stage is complete
as larvae enter the estuary.
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