Biology Reference
In-Depth Information
the fi rst study to control for size, it was concluded that the true test for
water temperature as an important cue was to use larvae of immediately
pre-metamorphic size (≥120 mm and ≥3.0 g) exposed over a longer period
to a water temperature regime that mirrored that in a sea lamprey stream
(1 to 21°C) or to the mean temperature of that stream (21°C) at the time of
metamorphosis. Such a study was carried out from mid-September to mid-
August with ~ 1000 larvae, with the above size criteria, collected from the
same stream (Holmes et al
.
, 1994). The result was no metamorphosis at the
constant temperature and the predicted number (based on size and condition
factor [CF], see above for formula) in the ambient temperature group. The
suggestion from this study was that it is not the highest temperature
alone that is important to metamorphosis but it is likely that a cool water
temperature followed by the spring rise is a critical cue. However, since this
study was also exploring the effects of feeding-starvation, animal density,
and photoperiod there was need for further long-term investigations to
explore the temperature infl uence without as many variables.
Two investigations were undertaken to study the influence of
temperature and the CF of the animals in the fall to the incidence of
metamorphosis in
P. marinus
in the following summer (Holmes and Youson,
1994, 1997). In one case larvae with a CF of 1.50 or greater combined with
a mass of at least 3.0 g and a length of at least 120 mm were marked with
latex dye and predicted to enter metamorphosis the following summer
(Holmes and Youson, 1994). Animals of similar size (mass and length) but
with a CF less than 1.50 were deemed to be presumptively non-metamorphic
and marked with a different color of latex dye. Holding tanks contained a
mixture of both presumptively metamorphic and non-metamorphic animals
that were subjected to either an ambient (3-21°C) or a constant (21°C) water
temperature regime for 9 months. The overall incidence of metamorphosis
was 53% in the ambient group and 2% in the constant temperature. While
64% of presumptively metamorphic animals entered metamorphosis
in the ambient regime, only 10% of this group entered metamorphosis
when kept at the constant temperature. Among the presumptively non-
metamorphic group, 50% metamorphosed in the ambient temperature
and no metamorphosis occurred in animals kept in a constant temperature
regime. These results clearly showed that a more favorable environment is
created by the ambient temperature and particularly, the seasonally lower
winter temperatures, that place fewer demands on the animals and perhaps
even permit anabolic activity, such as lipogenesis. In contrast, the constant
temperature creates an over-wintering, catabolic activity that was refl ected
in a rapid negative change in animal mass. The ambient temperature regime
also created the cool winter condition followed by the spring rise in water
temperature that was suggested to be a critical cue in an earlier study
(Holmes et al
.
, 1994). It is likely that the problem lies with the absence of
