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topic, we choose to use the full defi nition of metamorphosis, based on
remarkable morphological, physiological, behavioral and habitat changes,
which accounts for its key-role in the structure of complex life cycles with
different ecophases.
1.3.3 Metamorphosis, puberty and sex change
If the hormonal control by TH is, from our point of view, not at all suffi cient
to defi ne metamorphosis in vertebrates, we believe it is a necessary criteria
to distinguish metamorphosis from other major late developmental
events such as puberty, or sex change in some teleosts (Fig. 4).
Puberty
in all vertebrate species is defi ned as the acquisition by the individual
of the capacity to reproduce for the fi rst time (Romeo, 2003).
It is often
accompanied by drastic morphological, physiological, behavioral and
even habitat changes, which represent species-specifi c secondary sexual
characters. Furthermore, in some teleost species, adults may change gender,
after a certain age or under the effect of specifi c environmental and social
cues (Frisch, 2004). A remarkable remodelling of secondary sexual characters
can be observed. All these changes at puberty or at sex change would fi t with
many criteria of metamorphosis. They are however under the major control
of sexual steroids, the production of which is induced by the gonadotropic
axis. Based on their hormonal control by sex steroids, such changes have
therefore never been considered as a metamorphosis.
Interestingly, the other example of secondary metamorphosis in teleosts
is silvering in eels (Chapter 7). Our own results revealed that during eel
silvering, gonadotropic axis is activated instead of thyrotropic axis as
commonly observed during other metamorphoses. In addition, treatments
with sex steroids and especially androgens are able to induce silvering
morphological changes. These data lead us to think of eel silvering as a
puberty rather than a metamorphosis.
1.4 First Metamorphosis in Fishes
Chapter 2 Lamprey metamorphosis. By J. Youson and R. Manzon.
Chapter 3 Metamorphosis of elopomorphs. By K. Yamano.
Chapter 4 Metamorphosis of fl atfi sh (Pleuronectiformes). By Y. Miwa and
S. Inui
Chapter 5 Gobiid metamorphosis: a true metamorphosis? By P. Keith et al.
The typical larval metamorphosis, also called true metamorphosis or
fi rst metamorphosis in fi shes (in opposition to secondary metamorphosis)
is classically restricted to lampreys (Chapter 2), and to two teleost goups,
elopomorphs (Chapter 3) and pleuronectiformes (Chapter 5).
