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gradually increased to high levels in spawning adults (Ficele et al
.
, 1998).
Although there is a decrease in signal density during metamorphosis,
the total number of POM-expressing cells as determined by volumetric
measurements indicates an overall increase in POM expression through
metamorphosis. The observed decrease in density is related to an increase
in the size of the PI (Ficele et al
.
, 1998). These latter fi ndings are corroborated
by northern blotting data (Heinig et al
.
, 1999; Youson et al
.
, 2006). The fact
that POM expression is maximal in immediately pre-metamorphic larvae
suggests that POM is involved in preparing larvae for metamorphosis (Ficele
et al
.
, 1998). Similarly, the elevated levels of POM late in metamorphosis
coincide with, and likely function to elicit, the changes in pigmentation
observed at stages 5 and 6 of metamorphosis.
Summary and Conclusions
Lampreys are among the few fi shes that have adopted the developmental
strategy of metamorphosis, a late stage of ontogeny that manifests itself in
a juvenile with the potential for a different life style and habitat from the
pre-metamorphic larvae (Youson, 1988; Youson, 2004). There seems to be
no phylogenetic relationship among the fi sh groups that go through this
type of indirect development but in most cases the larval interval allows
for wide dispersal of the species (Youson, 2004). It has been speculated that
this developmental strategy in all modern-day lampreys is likely derived
rather than ancient with the fi rst lamprey-like ancestors more larva-like
and perhaps paedomorphic (Youson and Sower, 2001). The speculation
was based on evidence from experiments and observations on the thyroid
and reproductive axes on present-day lampreys representing both of the
two adult life history strategies, i.e., parasitic and non-parasitic. The non-
parasitic adult life history is believed to be the more recent strategy in
lamprey evolution with extant non-parasitic species having arisen from a
parasitic ancestor (Hardisty, 2006).
Despite the differences in adult life history between species, they all
have a protracted larval life of variable duration that has both a growth
and a metamorphic phase. For the most part, the features of metamorphosis
are similar in all species permitting the usage of a universal seven stages.
Furthermore, the initiation of the process in all species is characterized by
a sharp decline is serum concentrations of the thyroid hormones (TH), 3, 5,
3'-triiodothyronine (T
3
) and 3, 5, 3', 5'- tetraiodothyronine (thyroxine; T
4
).
In fact, metamorphosis can be induced in the larva of both adult life history
types by subjecting the animals to specifi c goitrogens. This chapter provides
an extensive review of the earliest and recent studies of the role of the thyroid
axis in lamprey metamorphosis and discusses several avenues of research
that need to be explored. One of these is the question of the existence of