Biology Reference
In-Depth Information
The unique life cycle of the eel, with a stage of growth in freshwater
(yellow stage) when energy reserves (fat stored in muscle tissue) and
lipophilic pollutants accumulated, followed by a long migration to the
spawning areas when pollutants are released from the fat deposits, followed
by a once-a-life spawning behaviour, makes the eel especially vulnerable to
persistent pollutants (Larsson et al., 1991). Because of their lipid contents (up
to 31%: Tesch, 1977; Boëtius and Boëtius, 1980; Geyer et al., 1994 compared
to most freshwater species: 3-9% Geyer et al., 1994), eels are the favored
targets for lipoliphilic contaminant accumulation. Contaminants, such
as lipophilic xenobiotics, accumulate in the fat tissue of eels during their
feeding stage (yellow eels). Yellow eels can stay for a prolonged period in
freshwater during which they continuously bioaccumulate xenobiotics;
they are mostly benthic and thus exposed to contaminants accumulated and
adsorbed in sediments (Van der Oost et al., 1994). Since silver eels do not
eat during their oceanic migration, contaminants previously accumulated
can be remobilized and redistributed, thus triggering potential toxic events
(Palsta et al., 2006). Moreover, as they do not reproduce during this stage,
there is no loss of contaminants due to fat metabolisation and production/
release of gametes. As long as the contaminants are stored in the fat reserves,
toxic effects are minor. But, at the start of the migration, when the lipids are
oxidized and the contaminants released, their levels in the blood plasma
may increase up to toxic levels.
Other studies investigated the toxic effects after experimental
maturation. Using an eel testicular organ culture system, Miura et al. (2005)
showed that
para
-nonylphenol, an estrogenic endocrine disruptor, induced
Sertoli cell hypertrophy and a decrease in germ cell number, in the presence
of 11-KT (which induces spermatogenesis
in vitro
). Cadmium pre-exposure
of experimentally matured silver eels strongly stimulated the pituitary-
gonad-liver axis, leading to early and enhanced vitellogenesis, followed by
oocyte atresia and death (Pierron et al., 2008). Van Ginneken et al. (2009)
also tested the effects of PCBs on morphological and blood parameters as
well as on energy metabolism in European eels that experienced a simulated
partial migration of 800 km in Blazka swim tunnels. They reported increased
spleen and liver weights and lower standard metabolic rate.
Conclusions
All the external as well as internal modifi cations occurring during eel
silvering have traditionally made scientists consider this event as a
metamorphosis. However, as reviewed in this chapter, eel silvering may
be primarily induced by the gonadotropic axis (Fig. 7). There is an overall
activation of this axis, with increases of gonadotropin and sex steroid
