The thermal dependence of swimming and muscle physiology in temperate and Antarctic scallops
Abstract
Swimming is important to the
ecology of many species of scallop
but the
effects
of
temperature
upon swimming are not clear.
The
ecology and natural
history
of
scallops is introduced followed by
a description
of
the
state of current
knowledge
of scallop swimming, muscle physiology and energetics.
The
effects of
temperature
and the mechanisms used
by
ectotherms
to compensate
for
such
changes over acute, seasonal and evolutionary timescales
are
discussed.
Scallops
are active molluscs, able
to
escape
from
predators using
jet
propelled
swimming.
Queen
scallops
(Aequipecten
opercularis) were acclimated
to 5,10
and
15°C in the laboratory
and collected in Autumn (13±3°C)
and
Winter (8±2°C)
in
order
to investigate
seasonal acclimatisation. The first jetting cycle of escape
responses
in these animals was recorded using
high-speed
video
(200-250fps).
Whole-animal
velocity and acceleration were
determined
while measurements of
valve movement and
jet
area allowed the calculation of muscle shortening
velocity, force
and power output.
Peak swimming speed was significantly higher at 15°C (0.37m.s⁻¹) than at 5°C
(0.28m.s⁻¹). Peak acceleration was 77% higher at 15°C (7.88m.s⁻²) than at 5°C
(4.44m.s⁻²). Mean
cyclic power output was also higher at 15°C (31.3W.kg⁻¹)
than at 5°C (23.3W.kg⁻¹). Seasonal comparison of swimming in freshly caught
animals revealed significantly greater acceleration
(x2
at
11°C)
and velocity
during jetting in Winter than in Autumn
animals
(ANCOVA). These were
associated with significant increases in peak power output (x8 at 11 °C), force
production and muscle shortening velocity. Actomyosin ATPase activity was
significantly higher (31 % at 15°C) in winter animals with peptide mapping of the
Myosin heavy chain showing no differences between groups.
Increases in muscle power output were associated with reductions in the length
of the jetting phase as a proportion of the overall cycle. As a result large
changes
in
muscle performance resulted in large short-term whole body
performance enhancement but little difference to velocity over the cycle.
Measurements of the swimming performance of the Antarctic scallop were made
from videos of escape responses.
Animals
were acclimated to +2 and
-1
°C in
the laboratory and compared to animals maintained at natural water temperature
(0±0.5°C) at the time of experimentation.
Adamussium
was very sensitive
to temperature change with
the proportion of
swimming responses
being less
common at
higher temperatures
and where an
individual
was exposed
to temperatures
above
it's
maintenance
temperature.
Analysis
of the first jetting
cycle of swimming was carried out as
described in
Chapter 2. These
analyses revealed that
over
the small
temperature
range that
the
animals can tolerate
swimming performance
is
strongly
temperature
dependent. Q₁₀s above
2 included those for thrust (3.74),
mean cyclic swimming
speed
(2.46),
mean cyclic power output
(5.71) and mean muscle
fibre
shortening velocity
(2.16).
Adamussium did not demonstrate strong phenotypic plasticity with no significant
differences in swimming of muscle performance between animals acclimated to
different temperatures. Comparison
of the relationship
between swimming
velocity and temperature in Adamussium
and other species showed
little
evidence
for
evolutionary compensation
for temperature
with all
data fitting to
a
single relationship with a
Q₁₀ of
1.96 (0-20°C). Similar
results were obtained
for
power output and
the performance of
in
vitro muscle preparations.
These
results are
discussed in the light
of field
studies revealing
the low
predator
pressure and escape performance of wild
Adamussium.
In
vivo
³¹P-Nuclear
Magnetic Resonance Spectrometry (MRS)
was used
to
measure the levels
of
ATP, Phospho-l-arginine (PLA)
and
inorganic
phosphorous (PI) in the
adductor muscle of the Antarctic
scallop,
Adamussium
colbecki, and
two temperate
species,
Aequipecten
opercularis and
Pecten
maximus.
Graded
exercise regimes
from light (1-2
contractions) to exhausting
(failing to
respond
to further
stimulation) were
imposed
upon animals of each
species.
MRS
allowed non-invasive measurement of metabolite
levels
and
intracellular
pH at
high time
resolution
(30-120s intervals) during
exercise and
throughout the
prolonged recovery period.
Significant differences
were shown
between the
magnitude and
form
of the
metabolic response with
increasing
levels
of exercise. Short-term (first 15
minutes) muscle alkalosis was
followed
by
acidosis of up
to 0.2
pH units
during the
recovery process.
Aequipecten had
significantly higher
resting muscle
PLA levels than
either
Pecten
or
Adamussium,
used a
five-fold
greater proportion of this store per
contraction and was able to
perform only
half
as many claps
(maximum
of
24)
as the
other species
before
exhaustion. All
species regenerated their PLA
store
at a similar rate
despite
widely different
environmental temperatures.
The
major results and
their impact
on our knowledge
of
biomechanics
and
it's
temperature dependence
are
discussed. Suggestions for future
research
based
upon
the
experimental
findings
and
techniques developed
are presented.
Type
Thesis, PhD Doctor of Philosophy
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