Biology Reference
In-Depth Information
Yellow eel
Growth
Feeding
Secondary metamorphosis
= silvering
River
(freshwater)
Silver eel
Glass eel
Feeding
migration
Reproductive
migration
Ocean
(seawater)
Larval metamorphosis
= fi rst metamorphosis
Fasting
Leptocephalus larva
Reproduction
Figure 1
. Biological life cycle of catadromous European eel,
Anguilla anguilla
. The secondary
metamorphosis, silvering, occurs after a long period of growth and feeding in freswater. It
prepares the eel for the long reproductive migration in seawater, during which the eel will
not feed anymore.
oceanic migration towards the Sargasso sea (Tesch, 1982, 1989; Dufour and
Fontaine, 1985; Fontaine, 1985; Dufour, 1994). At the silver stage, eels are
blocked at a prepubertal stage (Dufour et al., 2003).
7.2 Morphological, Behavioural and Physiological Changes
Occurring During Silvering
During the silvering process, in addition to important behavioral changes
such as transition from a sedentary to a migrating behavior and start
of starvation, drastic morphological changes are observed. The most
spectacular modifi cations concern the tegument and the eyes but various
other organs are also remodeled.
7.2.1
Changes linked to adaptation to seawater (Osmoregulation)
The primary gill epithelium of silver eels is thicker and contain larger and
more numerous chloride cells with enlarged mitochondria, as compared
to yellow European eels (Fontaine et al., 1995). More recently, Kaneko et
al. (2003) demonstrated in Japanese eels that fi lament chloride cells were
activated in saltwater-adapted or silver eels, and lamella chloride cells were
mainly observed in freshwater-adapted eels or yellow eels, presumably
acting as sites responsible for salt secretion in seawater and ion uptake
in freshwater respectively. Moreover, toward the time of downstream
migration of the eel to the sea, the ratio of fi lament chloride cells increased,
while that of lamella chloride cells decreased. These observations indicate
