Biology Reference
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shift, major morphological change, change in adaptive landscape, rapidity,
change in feeding mode, post-embryonic event, usually pre-reproductive
to reproductive transition, hormone-regulated transition ยป, but none of
them was found to be common to all 14 authors' defi nitions. As you will
notice in this topic, change of habitat and feeding mode, in addition to acute
morphological changes and hormonally controlled processes, are common
features to all fi sh metamorphoses described here.
1.2 Metamorphosis in Fishes
1.2.1 Phylogeny
Fish is a term commonly used to name various groups of aquatic
vertebrates, which inhabit a variety of fresh and seawater environments
including extreme ones. As vertebrate species they all belong to chordate
deuterostomian phylum. However, they do not represent a monophyletic
group and are classifi ed in different major groups including agnatha
(lampreys), chondrichthyes (sharks, rays) and osteichthyes (sarcopterygii =
lungfi sh and actinopterygii = chondrostei and teleostei) (Fig. 1). Teleosts,
with more than 25,000 species, represent the largest group, accounting for
more than half of vertebrate species.
Osteichthyes
Actinopterygians
Sarcopterygians
Chondrichtyans
Petromyzontidae
Myxinoidae
Figure 1.
Craniate phylogeny. The groups presenting a larval metamorphosis are
underlined.
1.2.2 Different developmental strategies in fi shes
Four major developmental periods can be recognized in fi sh: embryonic,
larval, juvenile and adult (Youson, 1988). Youson (1988) proposed three
pathways of ontogeny between the embryo and the adult: direct development
from the embryonic period leading to the juvenile and adult period (many
