Biology Reference
In-Depth Information
at the same time prepare the larval lamprey for the metamorphic event (Fig.
4; Youson, 1997). Serum 3, 5, 3', 5'-tetraiodothyronine (thyroxine; T
4
) and
3, 5, 3'-triiodothyronine (T
3
) concentrations in larvae of pre-metamorphic
size (year class 4 [IV]), are signifi cantly greater than those of year class 2
(II) or 3 (III) larvae (Youson
et al
.
, 1994). Moreover, serum T
3
concentrations
progressively increase in each successive year class (Youson et al
.
, 1994).
The concentrations of both T
4
and T
3
also increase signifi cantly between
young and immediately pre-metamorphic larvae of
L. appendix
(Holmes et
al
.
, 1999). Thus, it is likely that the gradual rise in serum TH concentrations
and their peak in immediately pre-metamorphic larvae are in preparation
for the impending metamorphosis (Fig. 4). Consistent with this idea are
observations that serum T
4
concentrations increase seasonally between
March and June, the months prior to the initiation of metamorphosis (Wright
and Youson, 1980b). Extrapolating these data, the lower incidence of KClO
4
-
induced metamorphosis in smaller size groups (Holmes and Youson, 1993;
see below) might be due to the lack of a previous rise and peak in TH levels
that are necessary to prepare for metamorphosis. It is conceivable that
the rise in serum TH concentration prior to their rapid decline is a critical
component of normal metamorphosis. For instance R.G. Manzon et al
.
(2001)
report a correlation between the magnitude of the decline in serum TH
levels and the incidence of goitrogen-induced metamorphosis. Elevated TH
levels might function to stimulate lipogenesis and the accumulation of lipid
reserves which are characteristic of pre-metamorphic larvae. Exogenous
TH have been shown to up-regulate a variety of processes associated
with lipogenesis, and a suppression of serum TH levels following KClO
4
treatment coincides with lipolysis (Kao
et al
.
, 1999b; see below). Whether
the gradual rise and peak of TH levels in larvae is simply a consequence of
the increased capacity of the larval endostyle to produce TH and the lack
of central hypothalamic-pituitary regulation or is an actively regulated
process remains to be determined. Nonetheless, the existing data strongly
suggest that elevated TH levels are important in preparing larval lampreys
for metamorphosis.
2.6.3.3 Reproductive system
There is no clear evidence that the state of development of the ovary
or testes is an important preparatory cue for lamprey metamorphosis.
However, it should be emphasized that in the few cases where parasitic
and non-parasitic members of paired species have been compared, the
gonads of the non-parasitic member are much more advanced in their
development (Bird and Potter, 1979a). Since larvae of non-parasitic species
metamorphose at a larger length (and likely age) than larvae of parasitic
species, it is possible that gonad size and the degree of development may
