Biology Reference
In-Depth Information
and lipolysis (Fig. 4) with the latter occurring later and refl ected in a decline
in mass and condition factor of late stages of metamorphosis in both
P. marinus
(Potter
et al
.
, 1978a) and
G. australis
(Potter et al
.
, 1980). Changes in
the fatty acid compositions of triacylglycerols and phospholipids have also
been examined in metamorphosis of
G. australis
(Bird and Potter, 1983).
Climax
Leptin-like ir-protein
Brain GnRH I & III
Relative
Increase
Thyroid Inhibitor
TH
lipogenesis
lipolysis
Lipid
Temperature
1
2
3
4
5
6
0
I
II
III
IV+
7
Larval Year Classes
Metamorphic Stages
Figure 4.
Diagrammatic representation of the factors involved in metamorphosis of the sea
lamprey,
P. marinus
. There is a gradual increase in the serum level of TH over the IV+ larval
year classes but metamorphosis (stages 1-7) is marked by a sharp decline in both thyroid
hormones (TH), T
4
and T
3,
at stage 1. The decline in TH occurs when stream temperatures
arise from winter lows and when body fat is highest as result of lipogenesis in older larva
and early metamorphic stages. Note the decline in body lipid is associated with lipolysis
when stored fat is used by the non-feeding animals. There is some evidence to suggest that
a leptin-like protein from the fat-storing tissues may be a trigger or be infl uenced by other
factors in metamorphosis. Concentrations of gonadotropin-releasing hormones (GnRH-I
and -III) in the brain increase at the time that serum TH levels decline. Since TH declines, a
putative inhibitor of activity in the thyroid gland is indicated in the scheme. (From Youson,
2007, modifi ed from Youson, 1994).
2.6.3.2 Endocrine
As discussed in detail below, thyroid hormones (TH) play a role in lamprey
development and metamorphosis. However, the characteristic decline
observed early in lamprey metamorphosis contrasts with the rise in TH
levels during the metamorphoses of all other vertebrates studied to date.
In addition to their potential role in the metamorphic process, it has been
suggested that elevated TH levels in larvae might function as a juvenilizing
hormone preventing metamorphosis, as in juvenile hormone in insects, and
