When European silver eels (Anguilla anguilla) venture in the ocean for their 5,500-km semelparous spawning run to the Sargasso Sea ,
they are still in a prepubertal stage. Sexual maturation has thus to
occur during or after this long distance journey. Maturation in eels,
as in other vertebrates, is regulated by the gonadotropic
follicle-stimulating hormone (FSH) and luteinising hormone (LH) that
are produced by the pituitary. If prevented from undertaking their
oceanic migration, gonad development remains blocked by dopaminergic
inhibition of pituitary activity as well as the absence of stimulation
by Gonadotropin-Releasing Hormone (GnRH) .
Information about natural maturation is lacking, because migrating and
spawning eels have never been caught near the spawning grounds. There
is however an urgent need for an understanding of eel reproduction,
because populations are collapsing on a global scale .
Investigations on eel reproduction have been mainly focussed on
females. They stay 7 – 30 years in the freshwater before migration, in
contrast to 4–9 years for males. As a consequence, females reach a
ten-fold larger size than males at the onset of migration. The long
female growth stage is likely required for a successful production of
more than one million eggs, which at spawning time accounts for 40–60%
of the body weight .
As the energy requirements for males are far less than those for
females, it is possible that the observed dopaminergic inhibition is
sex-specific. We have tested this hypothesis by subjecting male and
female eels to a GnRH-agonist (GnRHa), specifically the commercial
product Gonazon For Fish (Intervet), as well as to stimulation by
long-term swimming in seawater (SW) that is supposed to stimulate GnRH
excretion by the hypothalamus. Recently we found that swimming in
freshwater (FW) triggers the enlargement of the eyes and development of
oocytes in female eels [5,6];
all signs of early maturation, suggesting that swimming is a natural
trigger for sexual maturation. However, further maturation (e.g.
vitellogenesis) was not stimulated by FW-swimming, and may only be
stimulated by SW-swimming during natural migration.