Biology Reference
In-Depth Information
mark-recapture study in a number of streams in the Great Lakes watershed,
larval density was recognized as an important variable in predictive models
of determining when
P. marinus
will enter metamorphosis (Treble et al
.
,
2008). The effects of animal density in controlling metamorphosis requires
further study, for there has been some suggestion that some chemical or
biological agent is released into the water by the larvae that can infl uence
growth (Rodriguez-Munoz et al
.
, 2003). If such is the case, then larval density
could prove to be an important factor in regulating metamorphosis.
2.7.3 Photoperiod
As described above, there is a high degree of annual synchrony of lamprey
metamorphosis with the majority of species undergoing this phase of the
life cycle in the summer months. Although water temperature is critical at
this time, it is also the annual period when the daily period of light is at
optimum. It has long been speculated that metamorphosis in lampreys may
be triggered by a change in the length of the periods of light and darkness,
i.e., a photoperiod stimulation. Lampreys have a prominent pineal complex,
a photoreceptive organ made up of pineal and parapineal glands (Cole and
Youson, 1982), that yield melatonin in a circadian fashion. Pinealectomized
larval lampreys do not enter metamorphosis (Eddy, 1969; Cole and Youson,
1981). The study by Cole and Youson (1981) used
P. marinus
that were
immediately pre-metamorphic, based on CF and size criteria (for review see
Youson, 2003). However, since groups of sham-operated animals, control
animals, and animals kept in total darkness were not signifi cantly different
in the incidence of metamorphosis, the role of photoperiod is in question.
Similar studies over a longer period to test for the effects of accumulated
light should be undertaken.
The role of photoperiod was also tested in
P. marinus
in a short-term
experiment immediately prior to metamorphosis (Youson et al
.
, 1993). There
was no difference in the incidence of metamorphosis in groups of larvae
kept under artifi cial 15 h light : 9 h dark cycle or in constant darkness from
mid June to the end of July. A similar result was found with appropriately
sized (pre-metamorphic) animals kept under either an ambient-light regime
or a constant 15 h light : 9 h dark regime from September to the middle
of the following August (Holmes et al
.
, 1994). Although the pinealectomy
studies suggest some involvement of the pineal complex in lamprey
metamorphosis, its role may not be in response to photoperiod changes.
There is still a possibility that the pineal complex may be involved in
regulating or responding to changes in metabolism and even changes in
water temperature or responding to some endogenous factor (Youson,
1994).
