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than non-metamorphosing animals of the same length. The study also
showed that in some post-treatment streams, the third year of larval life is
an immediately pre-metamorphic year when growth in length may be stable
but the animals prepare themselves physiologically for metamorphosis.
These results suggest that either growth rates, as refl ected in length, differ
within individuals of the same cohort or that there must be other factors that
are important to preparation of the animals for metamorphosis. As will be
discussed below, water temperature is an important cue for metamorphosis,
however, fi eld studies have shown that larvae in warm water creeks do
not metamorphose at a younger age than larvae from colder water creeks
(Hollett, 1998).
2.6.2 Mass, length, and condition factor
All studies to date, on a variety of species, have some record of the weight
(mass) of metamorphosing animals relative to that of animals of similar
length that did not enter metamorphosis. Although there is usually some
higher mass in animals undergoing metamorphosis in most species, the
most dramatic and potentially signifi cant, increase is observed in
P. marinus
.
Extensive laboratory studies have determined that animals at least 120
mm in length and 3.0 g in mass are likely to enter metamorphosis (Holmes
and Youson, 1994, 1997, 1998; Holmes
et al., 1994). As a result, reference is
made to the size (length and mass) of metamorphosing animals. The mass
has been correlated with the deposition of fat in strategic locations within
the body and organs of animals about to enter metamorphosis (Youson et
al., 1979). The nephric fold, fat column, muscle septa, and subcutaneous
adipose tissue are primary sites (Fig. 2). This deposition is a consequence
of lipogenesis that occurs in pre-metamorphic animals where lipids reach
a threshold level of 14% of wet body mass (Lowe
et al
.
, 1973; O'Boyle and
Beamish, 1977). Attempts at correlating length and mass through calculation
of a condition factor (CF, where CF= M/L
3
X 10
6
; M is mass [g] and L is total
length [mm]) have shown some promising results. This CF has been useful
in selecting animals about to enter metamorphosis in
P. marinus
(Fig. 3).
Results from laboratory studies (Youson
et al
.
, 1993; Holmes and Youson,
1994) have provided for a prediction in the spring for metamorphosing
animals with a CF of 1.50 for landlocked
P. marinus
of at least 120 mm long
and 3.0 g, but it seems that the anadromous
P. marinus
may metamorphose
at a smaller size (110 mm and 2.0 g).
The mass, length, and CF criteria have been applied in attempts to
select immediately pre-metamorphic animals in populations under the
conditions of their natural habitat (Hollett, 1998; Henson
et al
.
, 2003).
Larvae which fi t the above three criteria were fi tted with coded wire tags
in the summer and fall prior to the summer that they would be expected to
