Biology Reference
In-Depth Information
because of the inadequacy of collections of metamorphosing larvae
and because of the diffi culty of obtaining live leptocephali to examine
metamorphosis in an aquarium. Information about metamorphic changes
is therefore limited to a small number of species such as true eels and
conger eels.
Numerous late larval stage leptocephali of the Japanese conger eel,
Conger myriaster
, appear in spring in coastal waters around Japan. They
readily complete metamorphosis into elvers in an aquarium without
feeding. For this reason, the conger eel is one of the best examined
species in this order. As shown in Fig. 7, late stage leptocephali reached a
maximum length of about 120 mm. During metamorphosis, body length
was reduced to about 60% of its maximum, the leaf-like and transparent
body of the leptocephalus became rod-shaped and opaque, and the anus was
translocated to a position relatively much further forward. In association
with the transformation of their body shape their habitat changed from
pelagic to benthic. These metamorphic events were completed within 3
weeks (Yamano et al., 1991).
Takai (1959) divided metamorphosis into early, middle, and late stages
based on statistical analysis of measurements of various parts of the body.
During early metamorphosis, the body started to reduce in length while
marked elongation of the dorsal and anal fi ns occurred. During the middle
stage of metamorphosis, the ratios of head length, head height, eye diameter,
interorbital width, snout length (distance from the tip of snout to the anterior
edge of the eye), dorsal fi n base length (distance from the origin of the dorsal
fi n to the end of the fi n), and upper jaw length, relative to body length, all
increased. During late metamorphosis, the ratios of head length, interorbital
Figure 7.
Morphological changes in metamorphosing conger eel (
Conger myriaster
). Fish were
fi xed in 10% formalin. (a) leptocephalus at the maximum size. (b)-(e) metamorphosing larvae.
(f) fi sh at the end of metamorphosis. Arrowheads show the positions of the anus. Scale indicates
3 cm. Reproduced from Yamano et al. (1991) with permission.
