Biology Reference
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that the silvering process in eels corresponds to a preparation to seawater
life.
And, in constrast to parr, eels at yellow stage have already the capacity
to survive if transfered to seawater. In addition, during their biological
cycle, eels are commonly going from freshwater to brackish water at yellow
stage and they are living in coastal waters throughout their sedentary life
(otolith microchemistry with wavelength dispersive X-ray spectrometry:
Tzeng et al., 1997) . However, Tsukamoto et al. (1998), as well as Tsukamoto
and Arai (2001) reported, using Synchroton Radiation Induced X-ray
Fluorescence analysis for quantifying strontium and calcium deposits
in otoliths, populations of yellow and silver eels of both Japanese and
European eels in marine areas adjacent to their typical freshwater habitats
which have never migrated into freshwater and have spent their entire life
history in the ocean.
Experiments of transfer to seawater of yellow or silver eels showed
changes in the levels of chloride cells and (Na+, K+)-dependent ATPase
in the gills (Thomson and Sargent, 1977; Cutler et al., 1995a and b), in the
expression of guanylin (peptide which regulates electrolyte and water
transport) in the intestine (Comrie et al., 1999) and aquaporins (water
transporters) in the kidney (Martinez et al., 2005).
When yellow eels were transferred from FW to SW, percentage of
chloride cells increased from about 2 to 5% after 6 days and to 6% from 13
to 21 days, and Na+, K+-ATPase activity increased 2.5 times after 2 weeks
(Thomson and Sargent, 1977). In contrast, for silver eels, chloride cells
formed about 6% in FW and SW and only a slight increase of Na+, K+-
ATPase activity was observed (Thomson and Sargent, 1977).
In the Japanese eel, the ratio lamellar to fi lamentary chloride cells is
much lower in yellow eels than in silver eels; silver eels caught in brackish
water during downstream migration had very few chloride cells on the
lamellae (Kaneko et al., 2003).
In summary, the silver eel does show pre-adaptations to marine life
as far as gill chloride cells are concerned. There may be an advantage in
having the machinery for marine osmoregulation in place before feeding
stops preparatory to the spawning migration, during which all the energy
reserves of the body will be required for swimming and maturation in
addition to maintaining homeostasis. This energy required for the latter
includes the cost of osmoregulation in a hyperosmotic environment, which
the eel continues to carry out effi ciently up to the time of maturation. The
situation resembles that in pressure acclimation where the yellow eel can
acclimatize to high pressure but the silver eel is already prepared.
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