Lamprey Metamorphosis - Metamorphosis in Fish - page 29

a change in the temperature, rather than that the constant temperature of
21°C is too high, for even a constant temperature of 15-16°C for 9 months
on animals of >130 mm failed to produce metamorphosis (Holmes and
Youson, 1994).
The study by Holmes and Youson (1994) showed a high incidence
of metamorphosis in animals in the ambient temperature regime that
were predicted in the fall to be non-metamorphic based on a CF of <1.50.
O'Boyle and Beamish (1977) had suggested that lipogenesis occurs in the
fall and winter months and it seems that the ambient temperature regime
of the study by Holmes and Youson (1994) created conditions for fall, non-
metamorphic animals of appropriate mass and length to prepare themselves
for the non-trophic phase of metamorphosis in the following summer. Thus,
a lower CF than 1.50 may be more appropriate for suitably sized animals
in the fall prior to metamorphosis. A CF of 1.45 in animals of minimum 120
mm and 3.0 g in the fall and the spring rise in temperature resulted in a 56%
incidence of metamorphosis (Holmes and Youson, 1997). It is noteworthy
that groups of animals subjected to a spring rise in water temperature from
the winter low of ~ 4°C to either ~ 13° or ~ 21°C did not vary signifi cantly
in the incidence of metamorphosis and accounted for 92% of the observed
metamorphosis (Fig. 3). In contrast, animals kept between 8-9°C over the
fall (November) to the summer period of metamorphosis (July-August)
showed a signifi cantly lower incidence of metamorphosis compared to
what was predicted based on their size and CF.
The above studies provide support for the hypothesis (Holmes et al . ,
1994) that rising water temperature in the spring is an important cue for
metamorphosis in P. marinus (Fig. 4). Furthermore, the study by Holmes
and Youson (1997) showed that the magnitude of the increase in water
temperature does not affect the incidence of metamorphosis. However, a
subsequent study showed that there is an optimal upper temperature of 21ºC
and a lower thermal limit of between 9 and 13ºC in the spring immediately
prior to metamorphosis that are important factors infl uencing the incidence
of metamorphosis (Fig. 5). In a study that compared the incidence of
metamorphosis in appropriately sized and CF P. marinus kept at water
temperatures of either 9, 13, 17, 21, and 25ºC starting at the beginning of
June, 0% metamorphosis occurred at 9ºC, 80% at 21ºC, and 58% at 25ºC.
The incidence of metamorphosis only met expectations, based on size and
CF, at 21ºC (Holmes and Youson, 1998). Overall, the above results may
explain why P. marinus is distributed over such a wide thermosphere in both
North America (Beamish, 1980) and Europe. All the regions of distribution
of this species are subjected to seasonal variation in water temperature of
the streams but they do not reach comparable highs nor are the changes of
great magnitude in all cases. As shown above, it is the spring rise from the
seasonal low that is the important water temperature cue for metamorphosis