Biology Reference
In-Depth Information
7.3.1.5 Experimental induction of silvering changes
All these results clearly indicate that the gonadotropic axis is activated
during silvering. The interesting question is to understand if this activation
of the gonadotropic axis is related to the “metamorphic” changes observed
during the transition from yellow to silver eel.
In 2008, the fi rst discovery of mature freshwater eels in the open ocean,
at the southern part of the West Mariana Ridge, was reported by Tsukamoto
and collaborators (Chow et al., 2009). The three male eels, one giant mottled
eel (
A. marmorata
) and two Japanese eels, presented dark brown or blackish
gray body color, larger eye index than silver eels and bigger GSI compared
to silver eels or even compared to fully matured male eels induced by
hormonal injection (Chow et al., 2009).
Previous studies on the experimental induction of eel sexual maturation
showed an amplifi cation of anatomic changes observed during silvering
(Pankhurst, 1982 a, b, c). Thus, morphological changes observed during silvering
such as increase of eye diameter (Boëtius and Larsen, 1991), enhancement of
silver-colour-body and decrease of gut weight (Pankhurst and Sorensen, 1984)
could be further induced by gonadotropic treatments (with human chorionic
gonadotropin in males or carp pituitary extracts in females).
Experimental data using exogenous sex steroid treatments
are in
agreement with this involvement of gonadotropic axis in the induction of
silvering. Early study showed that injections of male silver European eels
with 17α-methyltestosterone resulted in enlarged eye diameter, increased
skin thickness and darkened head and fi ns (
A. anguilla
: Olivereau and
Olivereau 1985). Similarly, implants of testosterone induced an increase of
eye size in male silver eels (
A. anguilla
: Boëtius and Larsen, 1991). Moreover,
immature female
Anguilla australis
which received implants of 11-KT, a non-
aromatizable androgen, for 6 weeks presented the external morphological
changes observed during silvering: increased eye diameter, larger gonads
and thicker dermis, compared to controls (Rohr et al., 2001). Finally, recent
studies in
A. anguilla
showed that treatment with testosterone induced a
decrease in the digestive tract-somatic index (Vidal et al., 2004; Aroua et
al., 2005) and an increase in ocular index (Aroua et al., 2005), while E2 has
no effect (Aroua et al., 2005) (Fig. 5).
All these data suggest that
the silvering changes observed during our
experiments are androgen-dependent.
In contrast,
a 3-month-treatment
of yellow eels with thyroid hormone (T3) did not induce any changes in
ocular index and digestive tract-somatic index (Aroua et al., 2005) (Fig. 6).
However, cortisol may have a synergistic action with steroids in this complex
process of eel silvering, as we demonstrated that concomitant administration
of E2, T and cortisol was most effi cient in inducing the silvering of the skin
in eels (Sbaihi, 2001).
