Biology Reference
In-Depth Information
Figure 15.
Spawning place and larval migration of the Japanese eel. Japanese eels (
Anguilla
japonica
) spawn west of the Mariana Islands. The black line indicates the migration pathway
of
A. japonica
. Dotted areas are the places where metamorphosis into glass eel occurs. From
there, glass eels migrate toward estuaries of eastern Asian countries.
glass eel in about one year whereas
A. anguilla
takes longer than two years.
Subsequently, based on the patterns in the appearance of larvae and glass
eels, the timing of the metamorphosis after hatching has been estimated
to be 8-12 months (Kleckner and McCleave, 1985) for
A. rostrata
and 12-15
months (BoeÈtius and Harding, 1985) for
A. anguilla
.
Analysis of otolith microstructure is another method for estimating
the timing of life history stages during larval development. The daily
otolith increment is marked as a ring pattern and the increment width
refl ects the growth rate (Fig. 16). In addition, acute physiological events
such as hatching and metamorphosis are recorded as a discontinuous
zone (Fig. 16). Sr : Ca ratios in the otolith, which change in association with
environmental and physiological conditions, decrease sharply at the onset
of eel metamorphosis (Otake et al., 1994). Thus, the larval life history can
be reconstructed from the otoliths of glass eels.
From otolith analysis, the age at metamorphosis was estimated to be
about 200 days (Lecomte-Finiger, 1992; Arai et al., 2000) and 350 days (Wang
