Lamprey Metamorphosis - Metamorphosis in Fish - page 12

CHAPTER 2
Lamprey Metamorphosis
John H. Youson 1, * and Richard G. Manzon 2
2.1 Introduction
Lampreys are one of two extant members of the ancient group of jawless
vertebrates, the Agnatha, whose ancestry can be traced back to at least 550
million years ago to the armoured ostracoderms. The other extant agnathan
is the strictly marine group, the hagfi shes, and there is recent fossil evidence
from southern China that lamprey- and hagfi sh-like agnathans coexisted
(Shu et al., 1999) in the lower Cambrian period. Identifi able lamprey
fossils appear in mostly marine deposits from the upper Carboniferous
period and morphological evidence suggests that lamprey evolution has
been relatively conservative for the past 300 million years (Potter and Gill,
2003). The taxanomic relationship between the agnathans, the lampreys
and hagfi shes, has been an on-going controversy. Although extensive
morphological and physiological, and some molecular, data suggest that
lampreys are more similar to the jawed fi shes than they are to hagfi shes,
other molecular data imply a close relationship, in fact monophyly, of the
two extant agnathan groups (see Potter and Gill, 2003; Hardisty, 2006). One
way in which hagfi shes and lampreys differ is in their development.
In particular, the two groups of extant agnathostomes, the lampreys
and hagfi shes, differ markedly in their mode of post-hatched development.
Whereas immediately post-hatched hagfi shes closely resemble the adult
and a have direct development into sexually mature adults all in a marine
environment, all lamprey species have phenotypically distinct larvae
1 Department of Biological Sciences, University of Toronto Scarborough, Toronto, Ontario
MICIA4 Canada.
E-mail: [e-mail] or [e-mail]
2 Department of Biology, University of Regina, Regina, Saskatchewan S4S0A2 Canada.
E-mail: [e-mail]
*Corresponding author