Biology Reference
In-Depth Information
be a factor in determining the time of metamorphosis. On the other hand,
the larger gonad could be a consequence of the protraction of larval life in
non-parasitic species and the longer period of potential for both somatic
and gonadal growth. As will be discussed later, the hypothalamic hormone,
gonadotropin-releasing hormones (GnRH-I and -III), become elevated in
immediately pre-metamorphic larvae and early stages of metamorphosis
of
P. marinus
(Youson and Sower, 2001) and the non-parasitic,
L. appendix
(Youson et al., 2006), but there is no evidence that the target is the gonad
(Fig. 4). For instance, there is speculation, based on existing data from extant
lampreys, that the interaction between the reproductive and thyroid axes
has been a key factor in the development of metamorphosis as a life cycle
strategy in lampreys (Youson and Sower, 2001).
2.7
Cues for Metamorphosis
There are two primary and essential cues for lamprey metamorphosis that
have been recognized to date, and they are not mutually exclusive. These
cues involve TH and water temperature. Other critical factors may be a
signal from the fat stores (leptin-like molecule) and from the reproductive
system, as illustrated through GnRH. In
P. marinus
, there is undoubtedly
some involvement of fat within the adipose tissue but whether activity
within this tissue is a consequence of a cascade of events or an essential cue
is uncertain. These defi nitive cues and potential cues are summarized in
Fig. 4. Other secondary factors may be photoperiod and animal density.
2.7.1
Water temperature
That metamorphosis is a seasonal event and restricted to summer and
early autumn of the majority of lamprey species in both the Northern and
Southern Hemispheres is not just a coincidence. Water temperature is not
only a cue for the event but also is likely a critical feature to permit survival
of the animals during a time when they are most vulnerable due to the
immense changes that occur during their metamorphosis (Potter, 1970).
Recently, Hardisty (2006) summarized the exceptions to the summer-fall
incidence scenario described above around the viewpoint of altitude and
latitude, and hence, water temperature. He described a metamorphosis
occurring in early April in a Mexican freshwater lampreys (
Lampetra
[Tetrapluerodon] spadicea
and
geminis
) living at high altitude at latitude
20°N, that is, far below the southerly limit of any other holarctic species
(Lyons et al., 1994). Other exceptions were found in most northerly (61°N
latitude) Arctic lampreys (
Lethenteron camtschaticum
) with June-March
metamorphosis and variations (mid July to October) within populations
of the Ukranian lamprey (
Eudontomyzon mariae
) that were related to their
