Mussel acclimatization to high, variable temperatures is lost slowly upon transfer to benign conditions

Climate change is increasing the temperature variability animals face,
and thermal acclimatization allows animals to adjust adaptively to this
variability. Although the rate of heat acclimatization has received some
study, little is known about how long these adaptive changes remain
without continuing exposure to heat stress. This study explored the rate
at which field acclimatization states are lost when temperature variability
is minimized during constant submersion. California mussels (Mytilus
californianus) with different acclimatization states were collected from
high- and low-zone sites (∼12 versus ∼5°C daily temperature ranges,
respectively) and then kept submerged at 15°C for 8 weeks. Each
week, the cardiac thermal performance of mussels was measured as a
metric of acclimatization state; critical (Tcrit) and flatline (Tflat)
temperatures were recorded. Over 8 weeks of constant submersion,
the mean Tcrit of high-zone mussels decreased by 1.07°C from baseline,
but low-zone mussel mean Tcrit was unchanged. High- and low-zone
mussels mean maximum heart rate (HR) and resting HR decreased ∼12
and 35%, respectively. Tflat was unchanged in both groups. These data
suggest that Tcrit and HR are more physiologically plastic in response to
the narrowing of an animal’s daily temperature range than Tflat is, and
that an animal’s prior acclimatization state (high versus low) influences
the acclimatory capacity of Tcrit. Approximately 2 months were required
for the cardiac thermal performance of the high-zone mussels to reach
that of the low-zone mussels, suggesting that acclimatization to high
and variable temperatures may persist long enough to enable these
animals to cope with intermittent bouts of heat stress.