Biology Reference
In-Depth Information
study indicating that sexually mature males can stimulate spermatogenesis
of their neighbors (Huertas et al., 2006).
In summary, during silvering, there is a switch from feeding related-
olfaction to migration/reproduction related olfaction.
7.2.3.2 Metabolism
At the silver stage, eels starve (Fontaine and Olivereau, 1962) and this
starvation is accompanied by a regression of the digestive tract (Han et al.,
2003; Aroua et al., 2005; Durif et al., 2005). Eels need important metabolic
changes to be able to accomplish their long oceanic migration, which is
comprised between 4,000 km for Japanese eels (
A. japonica
: Tsukamoto,
1992) and American eel (
A. rostrata
: Tucker, 1959; Mc Cleave et al., 1987)
and ≥ 6,000 km for European eel (
A. anguilla
: Schmidt, 1923). To permit
this long oceanic migration, animals have to optimize the use of energy
stores, for both swimming and gonadal maturation. Van Ginneken and
van den Thillart (2000) demonstrated, using large swim-tunnels, that for
their swim effort of 6,000 km, 40% of the European eels' energy reserves
are needed while the remaining 60% of their energy stores can be used for
gonad development. During silvering, changes in intermediary metabolism
were observed (Boström and Johansson, 1972; Lewander et al., 1974; Dave
et al., 1974; Barni et al., 1985; Eggington 1986; Van Ginneken et al., 2007a).
Dave et al. (1974) reported a slightly higher amount of unsaturated and
longer fatty acids in muscle and a signifi cantly lower level of 18:0 and
20:4n-6 in the liver of silver compared to yellow European eels. Comparable
trends were more recently observed in the shortfi n eel
Anguilla australis
(De Silva et al., 2002). An increase in lipids (phospholipids, free fatty acids
and cholesterol) content has also been reported recently at silvering in the
blood of European eels (van Ginneken et al., 2007a). According to Lewander
et al. (1974), a redistribution of cholesterol occurs from other tissues to
the gonads in silver eel. These metabolic variations would be amplifi ed
by environmental conditions, encountered during the migration, such as
depth and water temperature (Théron et al., 2000), as well as by locomotor
activity (van Ginneken et al., 2007b).
During the silvering process, red muscle volume increases (Pankhurst,
1982c), probably due more to an increase in fat and mitochondria than to
an increase in number of muscle fi bers (Lewander et al., 1974; Pankhurst,
1982c). Activities of enzymes involved in the aerobic pathway are higher
in silver than yellow eels (Boström and Johansson, 1972; Egginton, 1986)
together with higher slow muscle output and power (Ellerby et al., 2001)
and also a change in the main energy stores from glycogen in the yellow
stage to fat in migrating fi sh (Lewander et al., 1974; Barni et al., 1985; Zara
et al., 2000). Concomitantly, an increase in myoglobin content is observed
