Biology Reference
In-Depth Information
examples such as goldfi sh);—a non-classical metamorphosis occurring
during the juvenile period (example: the secondary metamorphosis,
smoltifi cation, in salmons);—a typical indirect development involving a
larval metamorphosis (also called fi rst or “true” metamorphosis) leading
to the juvenile period (example: fl ounder and eel).
1.2.3 Two types of metamorphosis in fi shes
Pr. Youson, in Bishop et al. (2006), wrote that “Fish with a metamorphosis
are lampreys, eels, and fl atfi shes (for example fl ounder and sole). From my
point of view, the majority of fi shes do not have a true metamorphosis. I do
not consider parr-smolt transformation in salmonids as a metamorphosis”.
It is true that parr-smolt transformation (smoltifi cation) in salmonids or
silvering in eels do not correspond to a drastic change of form, as you will
see in Chapters 6 and 7. However, major physiological and behavioural
changes occurring during these transformations are necessary for the fi sh
to reach its next habitat and survive in the new environment.
Therefore, in this topic, we will consider two types of metamorphoses in
fi shes (Fig. 1 and 2). First or larval metamorphosis is a true metamorphosis
and can be observed in agnatha (lampreys; Chapter 2), elopomorphs
(eels;
Chapter 3) and pleuronectiforms (fl atfi shes; Chapter 4). Secondary
metamorphosis occurs in juveniles of some diadromic migratory teleosts
and compared to larval metamorphosis, involves less drastic morphological
changes. This is the case of smoltifi cation in salmons (Chapter 6) and
silvering in eels (Chapter 7).
1.3 Endocrine Control of Metamorphosis
1.3.1 The reference of the amphibian larval metamorphosis
In vertebrates, the most described metamorphosis is the transformation in
amphibians of the aquatic larva (tadpole) into the terrestrial juvenile. The
role of the thyroid gland in this metamorphosis was fi rst demonstrated by
Gudernatsch as soon as 1912. When feeding tadpoles with thyroid gland
extracts, he observed that their transformation to frogs was accelerated.
Then, Allen (1916) was able to completely prevent metamorphosis
by thyroidectomy. The pair of thyroid glands is fi rst detectable after
embryogenesis when the tadpole begins to feed. Under the control of
pituitary thyrotropin (TSH), these glands produce thyroid hormones (TH:
thyroxine, T4 and triidothyronine, T3), which act on target organs via
specifi c receptors. While TRH (thyrotropin-releasing hormone) is the brain
neuropeptide controlling TSH in mammals and adult amphibians, it is CRH
(corticotropin-releasing hormone), which is responsible for the activation
