Biology Reference
In-Depth Information
CHAPTER
3
Metamorphosis of Elopomorphs
Keisuke Yamano
3.1 Introduction
The elopomorphs, which includes tenpounders, tarpons, bonefi shes and
various eels, are representative of fi shes that undergo drastic morphological
changes during their early life stages from larvae to juveniles. The bodies
of larval elopomorphs are laterally-compressed, leaf-like, and transparent,
and the head is small in relation to the body size. These peculiar attributes
are refl ected in the use of the special name “leptocephalus” (meaning
“small head”) for this stage of development. During metamorphosis, the
transformation of the larva into an adult-like form is associated with a
reduction in both the length and the depth of the body in most species,
and the body becomes opaque by thickening and pigmentation of the
skin. The external appearance of the leptocephalus is so different from
their adult morphology that leptocephali and adults had previously been
regarded as independent species. Nowadays, however, the metamorphosis
of elopomorphs is the best-known example of metamorphosis in bony fi shes
(Youson, 1988). Grassi (1896) was the fi rst to demonstrate experimentally
that leptocephali (
Leptocephalus brevirostris
) metamorphose into young elvers
of
Anguilla vulgaris
(synonym of
A. anguilla
) in an aquarium. At present,
the occurrence of a leptocephalus stage during the larval period is the
most important taxonomic trait for characterizing a fi sh as an elopomorph
(Nelson, 2006).
Since the discovery of metamorphosis in eels, their breeding places in
the open ocean have been of great interest. In the early twentieth century,
Schmidt (1922) concluded, based on the distribution of small leptocephali,
that the European eel (
A. anguilla
) and the American eel (
A. rostrata
) spawn
National Research Institute of Aquaculture, Fisheries Research Agency, Minamiise,
Mie 516-0193, Japan.
E-mail: yamano@fra.affrc.go.jp
